Seismic Behavior of. Unreinforced Masonry Walls. with Soft-Layer Strip Bearings

Transcription

1 Master Thesis Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings Institute of Structural Engineering (IBK) Swiss Federal Institute of Technology Zurich Supervisors: Prof. Dr. B. M. Petrov Zürich 27. January 2014

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3 Abstract A majority of residential and office buildings in Switzerland are built with load bearing masonry walls. Often, a damp proof course layer is placed between the concrete slab and the wall or into the first bed joint to prevent moisture from rising in the wall. In other cases, softlayers are used to increase the sound isolation of masonry walls or to act as slip joints. So far, very little is known about the shear behavior of masonry walls containing such softlayers as research has only been performed on a very fundamental level. In the context of a research program at the ETH Zurich, nine masonry wallettes with soft-layers in the interface joint have been tested at different pre-compression levels. The results are compared to previously performed experiments at the ETH, where the influence of different layer materials and thicknesses on the shear behavior of specimens with soft-layers in the first bed joint was investigated. The results indicate, that soft-layers have the potential to enhance the seismic performance of unreinforced masonry walls by considerably increasing the deformation capacity and the energy dissipation. While all soft-layers increased the energy dissipation of the walls, particularly soft-layers that induced horizontal sliding failure lead to a very distinct rise of energy dissipation and deformation capacity. Furthermore, walls failing in sliding show considerably less damage than walls failing in other mechanisms. The governing feature therefore seems to be the friction coefficient of a joint containing a softlayer. The experiments have shown, that the friction coefficient of rubber granulate layers is damage dependent and decreases with increasing number of load cycles. The pre-compression has a clear influence on the shear behavior. An increased axial load is on one side connected with a higher shear strength and stiffness and on the other hand with a lower deformation capacity. No influence of the pre-compression level on the energy dissipation can be found unless the sliding mechanism is induced. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings I

4 Acknowledgements I would like to express my deep gratitude to many peoples who helped me during this intensive but interesting time. First of all I would like to thank Dominik Werne, Thomas Jaggi, Pius Herzog, Patrick Morph and Christoph Gisler of the HIF laboratory team at ETH Zurich for giving me a tremendous amount of help and for always being patient when things were not going as smooth as expected. Without their help this thesis would not have been possible. Furthermore I express my gratitude to Amir Salmanpour and Milos Petrovic for advising me and lending me a helping hand in the laboratory whenever I needed it. They spent a lot of time discussing with me and giving me explanations and I am very thankful for that. I also wish to thank my supervisors Prof. Dr. Stojadinovic and Dr. Mojsilovic who gave me the opportunity to write this thesis and for providing me with constant guidance and useful informations. I have learned an incredible amount of new things during this time, that I would not want to miss. Last but not least, my loving thanks go to my family and my girlfriend Julia, who supported me during this thesis and my whole studies. II Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

5 Table of Contents Abstract... I Acknowledgements... II Table of Contents... III Table of Figures... VI List of Tables... XIII 1. Introduction Prevalence of unreinforced masonry in Switzerland and seismic hazard Application of damp proof course layers Objectives Behavior of Masonry walls with soft-layer wall bearings Masonry without soft-layers Failure modes Deformation capacity Performance under seismic loading Masonry with damp proof courses Influence on horizontal strength and deformation capacity Methods Rocking level Normalization Normalized shear strength Normalized displacement Capacity curves Bilinear idealisation of capacity curves Geometrical correction of measured data Dissipation of energy Equivalent viscous damping ratio Dissipated energy ratio Experiments on URM walls with soft-layer wall bearings in the interface joint Test program Specimens Loading pattern Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings III

6 4.2. Materials Bricks Mortar Masonry Soft-layers Test setup Measurement Force Measurement Displacement measurement Digital image correlation Crack control Results W Global behavior Energy dissipation WE Global behavior Energy dissipation Damage WE Global behavior Energy dissipation Damage WG Global behavior Energy dissipation Damage WG Global behavior Energy dissipation Damage WG Global behavior Energy dissipation Damage WG IV Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

7 Global behavior Energy dissipation Damage WG Global behavior Energy dissipation Damage WG Global behavior Energy dissipation Damage Experiments on URM walls with soft-layer wall bearings in first bed joint Test program Loading pattern Results W0R WE3R WE5R WE WG WG WG Discussion Influence of soft-layer on shear strength and deformation capacity Influence of soft-layer material Influence of soft-layer position Influence of soft-layer pre-compression Influence of soft-layer thickness Influence on stiffness Strength prediction Conclusions References Appendix Appendix A Test report of bricks Appendix B Mortar prisms test report Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings V

8 Table of Figures Figure 1.1. Risks for the population due to natural hazards (Swissbrick, 2011)... 1 Figure 1.2. Installation of a DPC in the interface joint (Mageba, 2013)... 2 Figure 2.1. Typical sliding failure in masonry shear walls... 3 Figure 2.2. Typical toe crushing failure... 7 Figure 2.3. Equilibrium of forces for flexural failure mode... 8 Figure 2.4. Typical diagonal shear tension failure in a masonry wall (Salmanpour et al., 2013)9 Figure 2.5. Failure mode and shear strength vs. axial load Figure 2.6. Comparison of ultimate drift capacities for different failure modes (Salmanpour et al., 2013) Figure 3.1. Equilibrium of forces Figure 3.2. Change of the arm of the horizontal force Figure 3.3. Change of rocking level during the rotation Figure 3.4. Construction of capacity curves Figure 3.5. Bilinear idealsation of capacity curve (Tomazevic 1999) Figure 3.6. Dissipated energy and strain energy (Chopra 2012) Figure 3.7. Input energy Figure 4.1. Horizontal loading pattern Figure 4.2. Testing of compressive strength Figure 4.3. Damp proof course layers Figure 4.4. Shear modulus G of the soft-layer materials at different temperatures (Barandun, 2013) Figure 4.5. Test setup Figure 4.6. Arrangement of potentiometers Figure 4.7. DIC speckle pattern Figure 5.1. W0.10 Horizontal force and displacement time history Figure 5.2. W Uplift vs. actuator displacement Figure 5.3. W Uplift vs. horizontal force Figure 5.4. W Crack pattern Figure 5.5. W Hysteresis loops and capacity curve Figure 5.6. W Normalized hysteresis loops Figure 5.7. W Bilinear idealisation of the capacity curve Figure 5.8. W Dissipated energy ratio VI Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

9 Figure 5.9. W Equivalent viscous damping ratio Figure WE10.10 Horizontal force and displacement time history Figure WE Uplift vs. actuator displacement Figure WE Uplift vs. horizontal force Figure WE Crack pattern Figure WE Hysteresis loops and capacity curve Figure WE Normalized hysteresis loops Figure WE Bilinear idealisation of the capacity curve Figure WE Dissipated energy ratio Figure WE Equivalent viscous damping ratio Figure WE Damage of soft-layer Figure WE3.10 Horizontal force and displacement time history Figure WE Uplift vs. actuator displacement Figure WE Uplift vs. horizontal force Figure WE Crack pattern Figure WE Hysteresis loops and capacity curve Figure WG Normalized hysteresis plots Figure WE Bilinear idealisation of the capacity curve Figure WE Dissipated energy ratio Figure WE Equivalent viscous damping ratio Figure WE Damage of soft-layer Figure WG10.5 Horizontal force and displacement time history Figure WG Hysteresis loops and capacity curve Figure WG Normalized hysteresis loops Figure WG Uplift vs. actuator displacement Figure WG Uplift vs. horizontal force Figure WG Crack pattern Figure WG Bilinear idealisation of the capacity curve Figure WG Dissipated energy ratio Figure WG Equivalent viscous damping ratio Figure WG Damage of soft-layer Figure WG10.10 Horizontal force and displacement time history Figure WG Uplift vs. actuator displacement Figure WG Uplift vs. horizontal force Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings VII

10 Figure WG Crack pattern Figure WG Hysteresis loops and capacity curve Figure WG Normalized hysteresis plots Figure WG Bilinear idealisation of the capacity curve Figure WG Dissipated energy ratio Figure WG Equivalent viscous damping ratio Figure WG Crushing of the toes Figure WG Destruction of the base Figure WG Damage of soft-layer with crushed mortar Figure WG10.15 Horizontal force and displacement time history Figure WG Uplift vs. actuator displacement Figure WG Uplift vs. horizontal force Figure WG Crack pattern Figure WG Hysteresis loops and capacity curve Figure WG Normalized hysteresis loops Figure WG Bilinear idealisation of the capacity curve Figure WG Dissipated energy ratio Figure WG Equivalent viscous damping ratio Figure WG Damage of the soft-layer Figure WG3.5 Horizontal force and displacement time history Figure WG3.5 - Uplift vs. actuator displacement Figure WG3.5 - Uplift vs. horizontal force Figure WG3.5 - Hysteresis loops and capacity curve Figure WG3.5 - Normalized hysteresis loops Figure WG3.5 - Crack pattern Figure WG3.5 - Bilinear idealisation of the capacity curve Figure WG3.5 - Dissipated energy ratio Figure WG Equivalent viscous damping ratio Figure WG3.5 - Damage of soft-layer Figure WG Horizontal force and time history Figure WG Uplift vs. actuator displacement Figure WG Uplift vs. horizontal force Figure WG Crack pattern Figure WG Hysteresis loops and capacity curve VIII Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

11 Figure WG Normalized hysteresis plots Figure WG Capacity curve and bilinear idealisation Figure WG Dissipated energy ratio Figure WG Equivalent visous damping ratio Figure WG Damage of the soft-layer Figure WG Horizontal force and dispacement history Figure WG Uplift vs. actuator displacement Figure WG Uplift vs. horizontal force Figure WG Hysteresis loops with capacity curve Figure WG Normalized hysteresis loops Figure WG Crack pattern Figure WG Capacity curve and bilinear idealisation Figure WG Dissipated energy ratio Figure WG Equivalent viscous damping ratio Figure WG Damage of the soft-layer Figure 6.1. W0R - Horizontal force and displacement time history Figure 6.2. W0R - Uplift vs. actuator displacement Figure 6.3. W0R - Uplift vs. horizontal force Figure 6.4. W0R - Hysteresis loops and capacity curve Figure 6.5. W0R - Normalized hysteresis loops Figure 6.6. W0R - Capacity curve and bilinear idealisation Figure 6.7. W0R - Crack pattern Figure 6.8. W0R - Dissipated energy ratio Figure 6.9. W0R - Equivalent viscous damping ratio Figure WE3R - Horizontal force and displcamenet history Figure WE3R - Uplift vs. actuator displacement Figure WE3R - Uplift vs. horizontal force Figure WE3R - Crack pattern Figure WE3R - Hysteresis loops and capacity curve Figure WE3R - Normalized hysteresis loops Figure WE3R - Capacity curve and bilinear idealisation Figure WE3R - Dissipated energy ratio Figure WE3R - Equivalent viscous damping ratio Figure Damage of soft-layer Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings IX

12 Figure WE5R - Horizontal force and displacement time history Figure WE5R - Uplift vs. actuator displacement Figure WE5R - Uplift vs. horizontal force Figure WE5R - Crack pattern Figure WE5R - Hysteresis loops and capacity curve Figure WE5R - Normalized hysteresis loops Figure WE5R - Capacity curve and bilinear idealisation Figure WE5R - Dissipated energy ratio Figure WE5R - Equivalent viscous damping ratio Figure WE10 - Horizontal force and displacement time history Figure WE10 - Uplift vs. actuator displacement Figure WE10 - Uplift vs. horizontal force Figure WE10 - Crack pattern Figure WE10 - Hysteresis loops and capacity curve Figure WE10 - Normalized hysteresis loops Figure WE10 - Capacity curve and vilinear idealisation Figure WE10 - Dissipated energy ratio Figure WE10 - Equivalent viscous damping ratio Figure WG3 - Horizontal force and displacement time history Figure WG3 - Uplift vs. actuator displacement Figure WG3 - Uplift vs. horizontal force Figure WG3 - Crack pattern Figure WG3 - Hysteresis loops and capacity curve Figure WG3 - Normalized hysteresis loops Figure WG3 - Capacity curve and bilinear idealisation Figure WG3 - Dissipated energy ratio Figure WG3 - Equivalent viscous damping ratio Figure WG3 - Damage of soft-layer Figure WG5 - Horizontal force and displacement time history Figure WG5 - Uplift vs. actuator displacement Figure WG5 - Uplift vs. horizontal force Figure WG5 - Crack pattern Figure WG5 - Hysteresis loops with capacity curve Figure WG5 - Normalized hysteresis loops X Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

13 Figure WG5 - Capacity curve with bilinear idealisation Figure WG5 - Dissipated energy ratio Figure WG5 - Equivalent viscous damping ratio Figure WG5 - Damage of soft layer Figure WG10 - Horizontal force and displacement time history Figure WG10 - Uplift vs. actuator displacement Figure WG10 - Uplift vs. horizontal force Figure WG10 - Crack pattern Figure WG10 - Hysteresis loops and capacity curve Figure WG10 - Normalized hysteresis loops Figure WG10 - Capacity curve and bilinear idealisation Figure WG10 - Dissipated energy ratio Figure WG10 - Equivalent viscous damping ratio Figure WG10 - Damage of soft-layer Figure 7.1. Comparison of walls with soft-layer in 1st bed joint to control specimen Figure 7.2. Comparison of specimens with soft-layer in interface joint to control specimen at 10% pre-compression level Figure 7.3. WG3.5. Global behavior and damage of soft-layer Figure 7.4. WG3 - Global behavior and damage of soft-layer Figure 7.5. WG5 - Global behavior and damage of soft-layer Figure 7.6. WG10 - Global behavior and damage of soft-layer Figure 7.7. Comparison of rocking level to sliding resistance Figure 7.8. Capacity curves and bilinear idealizations of WG3 specimens at different precompression levels Figure 7.9. Hysteresis loops and ratio of dissipated energy for WG3 specimens at different pre-compression levels Figure Capacity curves and bilinear idealizations for WG10 specimens at different precompression levels Figure Hysteresis loops and ratio of dissipated energy for WG10 specimens at different pre-compression levels Figure Capacity curve and dissipated energy ratio for solid elastomer layers in 1st bed joint at 10% pre-compression Figure Capacity curves and energy dissipation ratio for granulate layers in 1st bed joint at 10% pre-compression Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings XI

14 Figure Capacity curves and energy dissipation ratio for granulate layers in interface joint at 5% pre-compression Figure Capacity curves and energy dissipation ratio for granulate layers in interface joint at 10% pre-compression Figure Capacity curves and energy dissipation ratio for granulate layers in interface joint at 15% pre-compression Figure Influence of axial load on effective stiffness Figure Influence of layer thickness on effective stiffness Figure Strength and failure mode prediction control specimens , Figure Strength and failure mode prediction - Soft-layers in interface joint Figure Strength and failure mode prediction - Soft-layers in 1st bed joint XII Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

15 List of Tables Table 4.1. Wall specimens Table 4.2. Axial load Table 4.3. Horizontal loading pattern Table 4.4. Material parameters of the bricks Table 4.5. Material properties of the mortar Table 4.6. Results of compression tests Table 5.1. W0.10 Summary of shear parameters Table 5.2. WE10.10 Summary of shear parameters Table 5.3. WE3.10 Summary of shear parameters Table 5.4. WG10.5 Summary of shear parameters Table 5.5. WG10.10 Summary of shear parameters Table 5.6. WG10.15 Summary of shear parameters Table 5.7. WG3.5 Summary of shear parameters Table 5.8. WG Governing values Table 5.9. WG Governing values Table 6.1. Axial load Table 6.2. W0R Summary of shear parameters Table 6.3. WE3R Summary of shear parameters Table 6.4. WE5R Summary of shear parameters Table 6.5. WE10 - Governing values Table 6.6. WG3 Summary of shear parameters Table 6.7. WG5 - Governing values Table 6.8. WG10 - Governing values Table 7.1. Friction coefficients for granular soft-layers Table 7.2. Initial and effective stiffness of specimens with soft-layer in 1st bed joint Table 7.3. Initial and effective stiffness of specimens with soft-layer in interface joint Table 7.4. Strength and failure mode prediction - Soft-layers in interface joint Table 7.5. Strength and failure mode prediction - Soft-layers in 1st bed joint Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings XIII

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17 1. Introduction 1.1. Prevalence of unreinforced masonry in Switzerland and seismic hazard Masonry is a very traditional construction material in Switzerland due to its flexibility, good construction speed, thermal properties and cost effectiveness. A large number of buildings in Switzerland are built with unreinforced masonry walls in combination with concrete slabs. Since the update of the Swiss building code and the connected higher requirements regarding seismic design, most masonry buildings use a combination of masonry walls and concrete shear walls for supporting the horizontal loads form earthquakes and wind. However, there are still a large number of older buildings that only use unreinforced masonry walls for horizontal load bearing. Furthermore the majority of historic buildings is constructed with masonry. Even though the seismic hazard in Switzerland has to be classified as moderate, the monetized potential damage of earthquakes is the biggest risk originating from natural hazards in Switzerland. Figure 1.1. Risks for the population due to natural hazards (Swissbrick, 2011) The high prevalence of masonry constructions combined with the high risk originating from earthquakes and the fact, that the seismic performance of masonry is still relatively unexplored, lead to the conclusion that the behavior of unreinforced masonry walls subjected to seismic excitation has to be explored further. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 1

18 1.2. Application of damp proof course layers In many cases a damp proof course layer is placed between the concrete slab and a masonry wall or in one of the bottom bed joints of the wall to prevent moisture rising in the wall by capillary action. These layers are made of impervious materials such as rubber, elastomer, polythene, bitumen and even lead or copper. Wall bearings are also used for sound isolation and as slip joints to allow for differential movements. Damp proof courses are even inserted into existing walls to improve moisture and sound performances. Figure 1.2. Installation of a DPC in the interface joint (Mageba, 2013) 1.3. Objectives The installation of damp proof courses is a well established procedure and they are widely used in building practice. On the other hand, very little is known about the influence of these soft-layers on the shear strength and deformation capacity of masonry walls. Furthermore it would be desirable to improve the behavior of masonry walls subjected to seismic excitations by the specific application of soft-layers. A research project on the behavior of unreinforced masonry walls is underway at the ETH Zurich. Within this framework, experiments on masonry walls with damp proof courses in the first bed joint at constant axial load have recently been performed. Two different materials for the soft-layers have been investigated in these experiments. The experiments of this thesis are aimed at investigating the influence of the axial load and the position of the soft-layers in the walls. 2 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

19 2. Behavior of Masonry walls with soft-layer wall bearings 2.1. Masonry without soft-layers Failure modes The in-plane behavior of unreinforced masonry walls subjected to vertical and horizontal loads is mainly dependent on the predominant failure mechanism. From several experimental studies and from post-earthquake observations, three different failure modes can be identified for unreinforced masonry shear walls. The occurrence of the different failing mechanisms is strongly dependent on the amount of axial loading, the imposed boundary conditions as well as the aspect ratio of the wall Sliding failure Sliding of a part of the wall along the bed joints usually occurs for low axial loads and/or weak mortar and is therefore mostly observed in the top stories of a building or in historic masonry buildings. The sliding mechanism can occur either along a horizontal bed joint, usually in the bottom part of the wall where the joint is already cracked in tension due to flexure, or along the diagonal of the wall including tensile rupture of the head joints. Refer to Figure 2.1 for typical pictures of these two types of sliding failures. (a) Horizontal sliding (Javed et al. 2012) (b) Diagonal sliding Figure 2.1. Typical sliding failure in masonry shear walls Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 3

20 The behavior of a wall failing in sliding can be accurately described as a linearly elasticperfectly plastic response with a high amount of energy dissipation. The displacement capacity connected with this failure mechanism is theoretically unlimited. However, as load bearing masonry walls are usually connected to other structural elements, the ultimate displacement for sliding failure should be limited in practical applications (Salmanpour et al., 2013). A simplified approach for predicting the strength for sliding failure is based on the Mohr- Coulomb criterion for the friction in the bed joints. Basically a critical shear stress based the cohesion c and the friction coefficient can be formulated according to (Eq. 2-1). = + (Eq. 2-1) The resistance of a masonry wall failing in sliding can therefore be described as: = = + (Eq. 2-2) with lw and tw being the length and the thickness of the wall and N being the axial load. It has to be mentioned, that the parameters c and have to be understood as global strength parameters and can not be connected to local material properties as the stress distribution in the masonry is highly non-uniform (Magenes & Calvi, 1997). However, due to flexure in the wall, the neutral axis for vertical stresses is shifted and therefore not the complete length of the wall is in compression. It is widely adopted in design practice to apply (Eq. 2-2) only on the compressed length lc of the wall. Based on a neglecting of the tensile strength of the mortar joints and a linear distribution of the vertical stresses, the compressed length of the wall can be calculated according to (Eq. 2-3). = (Eq. 2-3) 4 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

21 In (Eq. 2-3) v is the shear ratio defined as: = = with = 1.0 for cantilever boundary conditions (Eq. 2-4) For the further considerations it has to be distinguished between the sliding failure along a horizontal joint and the sliding along the diagonal joint. For the sliding resistance along a horizontal joint the cohesion is usually neglected, as the joint is already cracked in tension due to flexure. Therefore (Eq. 2-2) simplifies to:, = (Eq. 2-5) The complete vertical force has to be transmitted through the compressed part of the wall. As the friction coefficient is, by definition, independent of the vertical stresses, the sliding resistance for a horizontal joint is not dependent on the length of the compressed part of the wall. For the sliding resistance of the diagonal joints (Eq. 2-2) and (Eq. 2-3) can be combined to get the sliding resistance for the compressed length of the wall:, = = (Eq. 2-6) Mann and Müller (1982) proposed a correction of the cohesion and friction coefficient in (Eq. 7-6) to account for the influence of weak head joints. They proposed to correct c and with a correction factor based on the ratio of brick length x and brick height y, according to (Eq. 2-7). = / (Eq. 2-7) Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 5

22 The sliding resistance along the diagonal joints can therefore be described as:, = (Eq. 2-8) with: = and = (Eq. 2-9) By accounting for a different friction coefficient of the soft-layer material, the sliding resistance along a horizontal bed joint containing a DPC layer can be calculated analogously to (Eq. 2-5) Flexural failure The flexural failure mode is defined as a crushing of the toes due to reaching the compressive strength of the masonry, induced by a rocking motion of the wall. The behavior of the wall can be described as nonlinear elastic with low degree of energy dissipation and negligible strength degradation. This failure mode is predominant for a high moment to shear ratios, i.e for slender walls with low axial loads (Salmanpour & Mojsilovic, 2012). Especially when the axial load is low, very large displacements can be attained in this failure mode. 6 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

23 Figure 2.2. Typical toe crushing failure Under the assumption that the wall behaves as a rigid body, the strength of the wall for the flexural failure mode can be easily determined based on a simple equilibrium of forces. The forces acting on the wall are shown in Figure 2.3, presuming a rectangular distribution of compressive stresses for the ultimate limit state. The compressive strength fx is sometimes reduced by a factor ranging from 0.85 to 1.0 to account for the transformation into a rectangular stress block. However, this does not have a big influence on the strength of the wall. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 7

24 Figure 2.3. Equilibrium of forces for flexural failure mode From the equilibrium of vertical forces the length a can be calculated: = (Eq. 2-10) A momentum equilibrium for the center point of the compressed zone renders the shear strength of the wall for the flexural failure mode: = 2 1 (Eq. 2-11) Diagonal tension failure Diagonal tension failure dominates the behavior of unreinforced masonry walls in presence of high vertical loads and/or poor quality of the bricks. This failure mode is observed very commonly in masonry buildings subjected to seismic loading and occurs predominantly in bottom stories, where high axial loads are present. This failure mode is characterized by rapid strength and stiffness degradation as well as modest energy dissipation. The displacement capacity is clearly lower than for the other two failure modes, but a non-negligible nonlinear displacement capacity has to be recognized for this failure mode anyway (Salmanpour & Mojsilovic 2012). 8 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

25 Basically the failure occurs when the tensile strength of the masonry, respectively of the bricks, is reached in the perpendicular direction to the main compression strut. Figure 2.4. Typical diagonal shear tension failure in a masonry wall (Salmanpour et al., 2013) There are two main approaches to estimate the strength connected to the diagonal tension failure mode. The first approach was developed by Turnsek and Cacovic (1970) and it treats the masonry as a homogeneous, isotropic material. Based on this simplification, the principle stresses in the center of the wall can easily be determined from the combination of axial and horizontal load. The principle tensile stress can be calculated according to (Eq. 2-12). = 2 + ( ) + 2 (Eq. 2-12) The parameter b in (Eq. 7-12) accounts for the shear stress distribution and is dependent on the aspect ratio hw/lw of the wall. A value of b=1.1 has been proposed for walls with aspect ratio of 1.0, giving good accordance with test results (Magenes & Calvi, 1997). By comparing the principal tensile stresses to the tensile strength of masonry, the strength of the wall for this failure mode can be found to be:, = 1 + (Eq. 2-13) Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 9

26 With ftu corresponding to the diagonal tensile strength of masonry. This material parameter is however impossible to determine directly. It can either be derived from diagonal compression tests or by inverting (Eq. 2-13) based on results from masonry shear tests. However, the fact that the masonry is simplified to an isotropic and homogeneous material and the difficulties in determining the diagonal shear strength parameter, make this approach questionable to apply. The other approach is formulated by Mann & Müller (1982). They propose a formulation of local shear strength based on similar considerations as above, but for the tensile strength of the bricks. The critical shear stress according to their approach is:, = (Eq. 2-14) Magenes and Calvi (1997) introduce a correction factor that accounts to the aspect ratio. This leads to the following formulation for the diagonal tensile cracking strength for masonry walls:, = 2.3(1 + ) 1 + (Eq. 2-15) This approach is more obvious from the author s point of view, as the diagonal shear cracks predominantly occur in the bricks. Therefore for the validation of the experiments conducted within the scope of this thesis, the approach from Mann & Müller (1982) with the correction of Magenes & Calvi (1997) is used Governing failure mode Based on the simplified approaches introduced in the previous chapters, for given geometry, boundary conditions and material parameters, the predicted strength can be plotted against the axial load similar to Figure 2.5. The expected strength and failure mechanism at a certain axial load corresponds to the lowest value of all these curves. 10 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

27 Figure 2.5. Failure mode and shear strength vs. axial load A comparison of the experimental results to the predicted failure mode and strength for each specimen can be found in Chapter Deformation capacity The deformation capacity of an unreinforced masonry wall depends highly on the governing failure mechanism. For the sliding and the rocking mechanism generally a very high deformation capacity can be obtained. Walls failing in diagonal shear cracking show a more brittle behavior, connected with smaller ultimate drift capacities. However a certain ductility or nonlinear deformation capacity can be observed even for this failure mode (Salmanpour et al., 2013). Salmanpour et al. (2013) have compared the results of a total of 71 shear tests on unreinforced masonry piers, performed in different institutions. The failure modes have been classified to flexural failure (F), diagonal shear failure (S), sliding failure (SL) and hybrid failure (H). Figure 2.6 shows the ultimate drift capacities for each failure mode. For the sliding failure, no drift capacity is given, as it is theoretically unlimited. The mean ultimate drift of the analyzed experimental data amounts to 1.21% for flexural failure and 0.40% for diagonal shear failure. ratio of the specimens is not considered in this comparison, as there was not sufficient data to perform a separate analysis for each of these parameters. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 11

28 Figure 2.6. Comparison of ultimate drift capacities for different failure modes (Salmanpour et al., 2013) It is prominent, that the flexural behavior shows a considerably higher deformation capacity than the diagonal shear behavior. Noticeable is also that there is a big scatter in the results. The reason for this is, that the boundary conditions of the test setups as well as the aspect ratio of the specimens have not considered in the comparison. There is not enough data, to distinguish the experiments also for these characteristics Performance under seismic loading Basically it can be said that the nature of the loading has a big influence on the strength and the deformation capacity of an unreinforced masonry wall. In general it can be observed, that the strength as well as the ductility are reduced for a wall undergoing cyclic loading (Tomazevic 1996). During an earthquake, load bearing masonry walls also often are subjected to out-of-plane loading, which reduces the strength and deformation capacity drastically Masonry with damp proof courses Influence on horizontal strength and deformation capacity Relatively little research has been performed on the shear behavior of unreinforced masonry walls containing damp proof courses. The previous investigations mainly concentrated on the assessment of the shear parameters of joints containing soft-layers with experiments performed on small masonry triplets. The results indicate, that shear force can be transmitted through the joints with DPC and that due to sliding failure along the DPC a considerable 12 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

29 energy dissipation and quasi ductile behavior could be expected for masonry walls with softlayers. Griffith & Page (1998) performed tests on masonry triplets with different types of damp proof courses placed in both joints. DPC s consisting of bitumen coated aluminum, polythene/bitumen coated aluminum and embossed polythene were used in these experiments. In some of the tests, the middle brick was made of concrete to simulate the concrete slab. After loading the specimens with a constant pre-compression, horizontal displacements were induced in monotonic, static-cyclic and dynamic manners. They concluded, that shear could be transferred through the joints containing the DPC and that a friction coefficient of 0.3 is appropriate for most of the tested materials. Similar results were reported by Zhuge and Mills (1998) and Simundic et al. (2000). Mojsilovic et al. (2010) performed static-cyclic shear tests on masonry specimens with dimensions 1.2x1.2m and built in embossed polythene membranes either in the interface joint or the first bed joint. A good behavior under cyclic loading was observed. Recently, considerably energy dissipation and large deformation capacity was observed in triplet tests with incorporated DPC s performed by Mojsilovic (2012) and in cyclic shear tests performed on masonry walls with soft-layers in the first bed joint (Mojsilovic et al., 2013). Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 13

30 3. Methods 3.1. Rocking level When subjected to a combination of horizontal and axial loading, a masonry wall is under the influence of different moments. The horizontal force generates an overturning moment, while the axial force creates a stabilizing moment. As long as the stabilizing moment is larger than the overturning moment, the wall stays in its initial position. As the horizontal force rises, the overturning moment increases. Under the assumption that the masonry wall acts as a rigid body, the wall is starting to perform a rotation as soon as the overturning moment reaches the level of the stabilizing moment. This level of horizontal force is here called the rocking level. The horizontal force should theoretically not rise over this rocking level as the wall is rotating to keep the equilibrium of forces. The rocking level can be used to interpret the behavior of the specimens. Figure 3.1. Equilibrium of forces The rocking level can be determined with a simple equilibrium of moments concerning the rotation point of the motion. However, it is difficult to determine the exact position of this rotation center, as the specimens are not perfectly rigid. For further considerations, it is assumed here, that the specimens are rotating the center axis of the compressed length of the wall in the ultimate state for the flexural failure mode (refer to Chapter ). 14 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

31 For specimens that contain a soft-layer in the first bed joint, only the part of the specimen above the layer is rotating, as no tension can be transmitted through this joint. Therefore the rotation point is located above the first course of bricks. Looking at Figure 3.1, the rocking level can be calculated according to (Eq. 8-1) = 2 (Eq. 3-1) with = (Eq. 3-2) Rocking can only occur if the sliding resistance is bigger than the rocking level. Otherwise, the specimen starts to slide first and the rocking level increases due to the increasing arm of the axial load. Therefore a critical friction coefficient can be formulated, which defines whether rocking or sliding occurs. The sliding resistance is equal to = (Eq. 3-3) By comparison of (Eq. 3-1) and (Eq. 3-3) the critical friction coefficient is found: = 2 (Eq. 3-4) For friction coefficients larger than cr, a rocking motion is induced and for friction coefficients below cr a horizontal sliding motion occurs (provided that that the wall doesn t fail in a different failure mode first). In Figure 3.2 it can be seen that the arm of the horizontal force is changing its length during the rocking motion. In pushing direction the arm increases, which leads to a decreasing rocking level. On the other hand, the arm of the horizontal force decreases in the pulling direction. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 15

32 (a) pushing direction (b) pulling direction Figure 3.2. Change of the arm of the horizontal force Figure 3.3 shows the consequences of these changes of the arm of the horizontal force. The rocking level decreases in pushing direction. In pulling direction it should actually increase. As the change of the length of the arm is of a clearly lower extent in this direction (due to geometrical reasons), the change of the rocking level is barely visible. It can also be seen, that the rocking level is higher for specimens with a soft-layer in the first bed joint. As described before, the rotation point for these walls lies above the first course of bricks. The arm of the horizontal force is therefore generally smaller than for specimens with the soft-layer in the interface joints. This leads to a higher rocking level. Figure 3.3. Change of rocking level during the rotation 16 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

33 3.2. Normalization Normalized shear strength In many of the conducted experiments a rocking motion of the specimen could be observed. In others, the wall showed a clear sliding behavior. Sometimes even both types of motions could be observed in the same cycle. To get a better understanding of the behavior of the walls, the measured horizontal force can be normalized with the theoretically calculated rocking force level (refer to Chapter 3.1). In theory it is not possible that the force gets higher than this level, as the wall reacts with an overturning motion to keep the equilibrium of forces. As the masonry wall does not behave like a perfectly rigid body, and the influence of the deformability of the soft-layers on the rocking behavior as well as the exact center of rotation are not clear, this calculated rocking level has to be understood as a guidance level. However, the normalization of the shear strength with the rocking level gives a better insight in the undergoing mechanisms Normalized displacement For the comparison of different specimens that did not achieve the same maximum displacement, the measured displacement was normalized with the maximum displacement Capacity curves The capacity curves are constructed from the force-displacement hysteresis loops according to the American Society of Civil Engineers Seismic Rehabilitation of Existing Building Guidelines (ASCE 41-06). The peak displacement, and the respective horizontal force value, of every first cycle is plotted and connected by straight lines. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 17

34 (a) Selected peaks of deformation (b) Capacity curve connecting selected peaks Figure 3.4. Construction of capacity curves 3.4. Bilinear idealisation of capacity curves One of the main goals of this experimental research is to learn more about the deformation capacity of structural masonry with damp proof courses. One of the main parameter of interest in this field is the ductility describing a structural member s ability to offer resistance in the inelastic domain of response (Paulay and Priestley 1992). The ductility factor is defined as the ratio of the total deformation at the ultimate load to the deformation at the end of the elastic range. = (Eq. 3-5) As it is difficult to derive these two deformation values from the envelope of the hysteresis gained from the experimental data, the envelope is idealized by a linear elastic, ideal plastic bilinear approximation. Different approaches to determine this bilinear idealization are mentioned in the literature and in several building codes. Most of them are based on the condition, that the dissipated energy of the envelope and the idealization are the same. In order to allow for a comparison to the database gathered by Salmanpour et al. (2013), the bilinear idealization is constructed according to Tomazevic (1999). The bilinear idealization is described by three parameters: the effective stiffness Keff, the ultimate displacement u and the ultimate shear strength Vu. The effective stiffness is defined 18 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

35 as the secant of the envelope at 70 percent of the maximum shear strength reached in the experiment. The ultimate displacement corresponds to the shear strength at 20% strength degradation compared to the maximum shear strength. Based on the equal dissipated energy approach, the ultimate shear strength can be calculated subsequently. As the envelope is constructed according to ASCE and therefore only represents the first peak of each cycle, the failure is not always captured adequately by the envelope. Therefore not all the envelopes show a strength degradation of 20%. In other cases there is a strength degradation of 20% or more, shortly after the peak, but afterwards a considerable strength plateau, or even a renewed rise of the strength can be observed. In these cases the maximum displacement is taken for the value of the ultimate displacement capacity of the bilinear idealization. However it has to be mentioned, that the ductility is strongly dependent on the method how the bilinear idealisation is calculated. Especially the elastic stiffness is subject to a big scatter when calculated with different methods and subsequently the yield deformation and the ductility show the same scatter. Figure 3.5. Bilinear idealsation of capacity curve (Tomazevic 1999) 3.5. Geometrical correction of measured data Rotation of the specimens, occurring when the walls are rocking, leads to errors in the absolute displacement measurements. Due to the fact, that the measurement devices are measuring the change in distance between a fixed point and a point on the wall, the actually Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 19

36 measured displacement does not equal the horizontal displacement of the specimen when a rotation is present. The absolute displacement measurements are therefore corrected to based on a stiff body rotation around the corner points of the walls. This is an approximation, but it improves the quality of the results nevertheless Dissipation of energy The energy dissipated by a cyclically loaded wall equals the area inside the hysteresis loops. However, the amount of dissipated energy alone is not a suitable value to compare the behavior of the different specimens, as a wall with a narrow hysteresis loops but a high maximum strength can dissipate more energy in an absolute way than a wall with wide hysteresis loops but low shear strength. To allow a comparison of the capability of the specimens to dissipate energy two ratios are introduced here Equivalent viscous damping ratio Damping in structures is usually represented by equivalent viscous damping, as the governing equation of motion is linear and therefore easy to use and amenable to analytical solutions The equivalent viscous damping ratio of a structure, also known as loss factor, is most commonly defined as the ratio of the dissipated energy in the actual structure and the dissipated energy of an equivalent viscous system (Chopra, 2012). It can therefore be calculated from the force-displacement hysteresis of an experiment under cyclic loading. 20 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

37 Figure 3.6. Dissipated energy and strain energy (Chopra 2012) The dissipated energy of the specimen ED is given by the area of the hysteresis loops. The energy dissipated by the equivalent viscous system is calculated according to (Eq. 3-6) where and are the recoverable elastic strain energies of the pushing and the pulling cycle. = 2 ( + ) (Eq. 3-6) ED should be conducted at n, where the response of the system is most sensitive to damping (Chopra, 2012). Therefore (Eq. 3-6) simplifies to = 2 ( + ) (Eq. 3-7) This leads to the (Eq. 3-8) for the equivalent damping ratio eq = 1 2 ( + ) (Eq. 3-8) However, it is debatable, if the concept of viscous damping can also be applied for the inelastic domain of the response. It has been done in some research studies, but this idealization is in general not satisfactory for large inelastic displacements. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 21

38 Viscous damping and yielding both dissipate energy in the system, but the relative effectiveness in the various regions of the response spectrum is quite different. Damping has negligible influence on the response in the displacement-sensitive and the accelerationsensitive region of the spectrum, whereas the yielding is very important for magnitude of the response in these regions. Damping is most effective in the velocity-sensitive region of the response spectrum, where the yielding is even more effective. (Chopra, 2012) Furthermore, as the energy dissipated by viscous damping is directly dependent on the velocity, and the loading speed in the experiments was very small, it can be assumed, that the energy dissipated by viscous damping is negligible. Here, the damping ratio has to be understood solely as a parameter to compare the relative dissipation of energy of the specimens. 22 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

39 Dissipated energy ratio A more direct and physically correct parameter to compare the capability of a wall to dissipate energy is the ratio of the dissipated energy to the input energy. The input energy is the work that the actuator has to invest into the system to deflect the specimen. The input energy equals the area under the hysteresis up to the maximum deformation. Figure 3.7 shows the input energy of the positive and negative part of a hysteresis loop. Figure 3.7. Input energy The dissipated energy on the other hand is the area enclosed by the hysteresis. By comparing these two parameters, the capability of a specimen to dissipate energy can be analyzed independent of the maximum strength. = ( ) + (Eq. 3-9) Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 23

40 4. Experiments on URM walls with soft-layer wall bearings in the interface joint 4.1. Test program In a prior series of tests, the influence of the material and thickness of soft-layers in the first bed joint was examined at a pre-compression level of 10% of an assumed compressive strength of 6 MPa for the masonry (refer to Chapter 6). From these experiments it was apparent, that the granular rubber material was most capable of enhancing the deformation capacity of masonry walls subjected to lateral in-plane loading. In the present thesis the influence of pre-compression and the position of the soft-layer in the wall is investigated Specimens Nine masonry specimens with nominal dimensions of 1200 x 150 x 1200 mm were built in running bond. Both the head and the bed joints were fully filled and had a nominal thickness of 10mm. The specimens were built by professional masons and stored in dry air in the laboratory for a minimum of 63 days. Eight of the specimens contained a soft-layer in the joint of the wall-slab interface. The ninth specimen had no soft-layer and acts as a control specimen. The axial pre-compression is varied between three different levels: 0.30, 0.60 and 0.90 MPa corresponding to 5, 10 and 15% of the assumed compressive strength from the experiments of Chapter 6. A solid extruded elastomer (E) and a rubber granulate (G) was used for the soft-layer materials and thicknesses of 3 and 10mm were investigated. The designation of the specimens refers to the soft-layer material (E or G), the layer thickness (3 or 10) and the pre-compression level (5, 10 or 15). The control specimen is denoted with a 0 for layer thickness and no material reference. The W in front of all specimen names stands for wall. WG3.5 therefore denotes a specimen with granular soft-layer of 3mm thickness at a pre-compression level of 5%. The soft-layers were placed at the very bottom of the wall specimens and covered with a mortar layer between the DPC and the base of the first course of bricks. The thickness of the 24 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

41 interface joint, containing the soft-layer, was kept at 10mm for the 3mm thick soft-layers. For the 10mm DPC layers the interface joint was increased to 15mm to have a minimum of 5mm of mortar above the soft-layer. Table 4.1 shows the nine specimens with the corresponding properties. Specimen Soft-layer material Layer thickness [mm] Pre-compression [MPa] W0.10 No soft-layer WE3.10 Extruded WE10.10 Extruded WG3.5 Granulate WG3.10 Granulate WG3.15 Granulate WG10.5 Granulate WG10.10 Granulate WG10.10 Granulate Table 4.1. Wall specimens The specimens tested at 10% pre-compression level can directly be compared to the previous experiments to see the influence of the position of the soft-layer. As the rubber granulate showed more promising results in the past, the influence of the pre-compression level is investigated exclusively with this material Loading pattern Axial load The vertical load was kept constant during the experiment, using a pendulum manometer to control the oil pressure in the system. Three different levels of axial load were used for different specimens to capture the influence of the amount of pre-compression on the shear behavior. In the experiments that were conducted in the end of 2012 and beginning of 2013 (refer to Chapter 6) a compressive strength of 6MPa was assumed for the masonry, and the axial load was set to 10 percent of this compressive strength, rendering a pre-compression of 0.6 MPa for all experiments. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 25

42 The experiments of this thesis were to be conducted at a pre-compression level of 5, 10 and 15% of this assumed compressive strength respectively. As the experimental evaluation of the compressive strength of the masonry showed that actually only 4.70 MPa strength can be expected, the pre-compression levels refer to higher percentages of the compressive strength. Table 4.2 shows the values of the axial load including the weight of the spreader beams, the corresponding pre-compression and the percentages of the compressive strength. Specimen Applied axial force [kn] Weight of beams [kn] Total precompression [MPa] Percentage of strength [%] W WE WE WG WG WG WG WG WG Table 4.2. Axial load Horizontal load A displacement control system, consisting of a computer controlled hydraulic actuator, was used to apply the horizontal displacement loading. The deformation was applied in form of a sinusoidal wave with 11 increasing steps of amplitude. For each amplitude step, two full cycles were conducted. For the specimens that did not fail in the 11 th cycle, a subsequent 12 th cycle with increased amplitude was performed. However in the 12 th cycle the limits of the test setup were reached, e.g. rotation capacity of the top spreader beam and the measurement range of certain LVDT s. Figure 4.1 shows the horizontal deformation loading pattern and Table 4.3 gives the according numeric values. 26 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

43 Figure 4.1. Horizontal loading pattern Amplitude step no. Target displacement [mm] Average loading speed [mm/min] Duration [min] Table 4.3. Horizontal loading pattern In the first experiment (control specimen W0.10), the actuator was controlled with the displacement measurements of a LVDT that was directly measuring the piston travel. In all subsequent experiments the actuator travel was controlled with the displacement measurement of a laser device, measuring the displacement of the steel plate on the bottom spreader beam (see Chapter 4.4.2). That way, the influence of the air-gap in the connection of the actuator and the spreader beam could be excluded from the loading of the specimen. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 27

44 4.2. Materials Bricks All specimens were built with standard perforated clay bricks with nominal dimensions of 290 x 150 x 190 mm, classified to brick type B according to Swiss code SN A total amount of 15 bricks were tested by the testing and research institute p+f Sursee. The results of these tests are summarized in Table 4.4. The complete testing report can be found in Appendix A. The bricks were tested according to SIA 266:2013, SN EN 771-1:2011. SwissModul B 15/19 Length [mm] 288 Width [mm 148 Height [mm 190 Form factor [-] 1.21 Void area [%] 45 Compressive strength [MPa] 27.8 Normalized compressive strength [MPa] 33.6 Transverse tensile strength [MPa] 7.7 Mass of a Brick [g] 7288 Gross density [kg/m 3 ] 905 Moisture suction [kg/(m 2 *min)] 2.7 Table 4.4. Material parameters of the bricks Direct tensile strength of the bricks In the presence of high axial loads or strong mortar the shear failure of a masonry wall may be initiated by tensile failure of the bricks (Magens & Calvi, 1997). This failure mode is mainly influenced by the tensile strength of the bricks. As there are numerous difficulties connected with the direct testing of the tensile strength of masonry bricks, the common procedure to determine this parameter is by the means of indirect methods like the bending test or the splitting test. However it is debatable how to convert the results of such tests to the direct tensile strength. A special test procedure to measure the tensile strength of bricks directly was developed at the ETH Zurich and a series of tests on Swiss clay bricks were conducted (Mojsilovic, 2011). From these experiments a direct tensile strength of ft=1.24 MPa for the bricks was derived. However it has to be mentioned, that the tensile strength obtained from these tests show a large scatter. 28 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

45 The ratio of the tensile strength to the compressive strength was calculated to This allows for a transformation of the results to bricks with different compressive strength. With an average compressive strength of fb=27.8 MPa for the bricks used in the experiments of this thesis. a tensile strength of ft=1.08 MPa can be calculated Mortar A standard cement mortar with a characteristic compressive strength of fm=15 MPa was used. At the time of building the specimens, six mortar prisms with nominal dimensions of 160 x 40 x 40 mm were casted. The specimens were built on two consecutive days. Three of the mortar prisms were cast on the first day and the rest on the second day of building. The prisms were stored next to the specimens until the day of testing. The first batch of mortar prisms was tested after 58 days, the second batch after 61 days. They were tested according to EN 196-1:2005 at the laboratories of the ETH Zurich by the author of this thesis. The prisms were tested according to the following test protocol: 1. The bending strength of Prism 1 was determined with a standard three-point bending test. 2. On each of the two resulting halves of Prism 1, the compressive strength was determined with a force controlled compression test. 3. With the assumption of having approximately the same compressive strength for all mortar prisms, the elastic modulus of Prisms 2 to 5 was determined at 30% of the initially measured compressive strength. 4. The bending strength of Prisms 2 to 5 was determined analogue to Prism The compressive strength of both halves of Prisms 2 to 5 was determined. This procedure gives a total of 6 results for the bending strength, 5 for the elastic modulus and 12 values for the compressive strength. Unfortunately Prism 3 broke during the test for the elastic modulus, due to a controlling problem of the testing machine. Therefore no material parameters could be gained from this prism. Table 4.5 shows the results of the mortar tests. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 29

46 Prism Date of casting Compressive strength [MPa] Bending strength [MPa] Elastic modulus [MPa] Average Table 4.5. Material properties of the mortar Masonry All specimens were built in running bond masonry with fully filled head and bed joints with a nominal thickness of 10mm Compressive strength For the determination of the compressive strength of the masonry three additional specimens with nominal dimensions of 590 x 150 x 1000mm were built at the same time and with the same batch of bricks and mortar as the specimens for the main tests. They were air dried at the same location as the main specimens and tested after 54 days according to EN :1998. The compression tests were conducted on a general purpose testing machine. The specimens were placed centrically between two steel beams. A layer of very soft plywood was placed between the wall and the beams on top and on the bottom to guarantee a uniform load distribution (refer to Figure 4.2.a). The axial force was linearly increased in a deformation controlled manner up to failure. According to the EN 1052, the failure of the specimen should occur 15 to 30 minutes after the beginning of the test. With an expected ultimate strain of approximately 0.2%, the loading speed was selected to be 0.2mm/min. As the plywood layers are very deformable, the loading speed was initially increased to 2mm/min up to 100kN axial force to compress the wood. The deformations of the specimens were measured with three potentiometers (refer to Figure 4.2.b) to calculate the elastic modulus at 30% of the ultimate compressive strength. 30 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

47 (a) Test setup (b) Potentiometers (c) Typical failure Figure 4.2. Testing of compressive strength Table 4.6 shows the results from the compression tests. Note that the time to failure was not in the stated time frame for all three specimens, as it was difficult to predict the failure load exactly. However the difference to the time window are not very big, and therefore the results can be used nevertheless. Specimen Time to failure Compressive strength Elastic modulus [min] [MPa] [MPa] P P P Average Table 4.6. Results of compression tests Shear parameters of mortar joints The shear strength of a mortar joint can be described with a Mohr-Coulomb friction law. Mojsilovic (2012) has performed shear tests on masonry triplets, consisting of three bricks and two mortar joints, with very similar materials. He found a cohesion value of c = 0.3MPa and a friction coefficient of = 0.87 for mortar joints containing no DPC. These values are used for the strength predictions of the experiments of this thesis. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 31

48 Soft-layers Two different materials were used for the soft-layers. Namely an extruded elastomer, designated from here on with the abbreviation E, and a rubber granulate (G). Both materials are products used in practice and are provided from the supplier Mageba SA. Thicknesses of 3, 5 and 10 mm were tested in the earlier tests, while later only 3 mm and 10 mm layers were used. (a) extruded elastomer soft-layer (b) rubber granulate soft-layer Figure 4.3. Damp proof course layers From the supplier there is very little available information about the mechanical properties of the soft-layer materials. Only a maximum compressive strength of 5 MPa is provided. The shear modulus has been determined for the two materials by the means of a dynamic material analysis (DMA), performed at the Department of Materials of ETH Zurich. In a DMA, a sinusoidal load pattern is applied to viscoelastic materials. From the amplitude of the applied force and deformation and the recorded phase lag in the force-displacement time history the elasticity modulus E* can be determined. E* consists of a real and a imaginary part. The real part corresponds to the storage modulus E, stating the amount of stored mechanical energy in the system. The imaginary part is the loss modulus E, which shows how much energy has been dissipated by the material. From these two parameters, the shear modulus can be calculated (Barandun, 2013). Figure 4.4 shows the results from this analysis. The shear modulus for both materials was calculated from measurements at different temperatures. The temperatures in the laboratory where the masonry experiments are performed can be expected to be around 20 C. But one can see that the shear modulus varies significantly in the typical temperature range that buildings have to go through. 32 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

49 At room temperatures a shear modulus of G = 2.0 MPa can be used for the rubber granulate and G = 5.6 MPa for the extruded elastomer respectively. Figure 4.4. Shear modulus G of the soft-layer materials at different temperatures (Barandun, 2013) Shear parameters of joints containing soft-layers The shear strength of a joint containing a DPC can be calculated with a Mohr-Coulomb friction law, as described in Chapter The values of the friction coefficient are dependent on the material of the soft-layer. Mojsilovic (2012) has found a friction coefficient for extruded elastomer based soft layers of = 0.71 from tests performed on masonry triplets. This value can be used for the prediction of the sliding resistance of the specimens of the present experiments that were containing an extruded elastomer DPC. For the rubber granulate layers no appropriate value could be found in the literature Test setup Figure 4.5 shows the test setup. The setup applies cantilever boundary conditions to the wall: the wall is fixed at the base and the rotation on the top is free. The axial load is applied with two hydraulic cylinders (2), each with a total stroke of 100mm. The cylinders are placed between the support frame (1) and the upper spreader beam (3), with the pistons at half the stroke in the initial position, giving the upper spreader beam sufficient space to rotate. The axial load of both cylinders is kept constant during the experiment. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 33

50 To allow a free horizontal movement of the bottom spreader beam (4), a roller bearing (5) is placed between the top and bottom spreader beam. The vertical force is transmitted through eleven hardened steel rollers. The specimen (6) is placed between the bottom spreader beam (4) and a steel baseplate (7) and connected with a layer of fast hardening cement mortar to ensure a uniform distribution of the axial load. The surface of the steel plates is treated with a mixture of epoxy and sand to simulate a concrete surface. The baseplate is tied down to the strong floor (8) at the northern end of the plate and at the center axis of the specimen. At the southern end, the horizontal movement of the baseplate is blocked by shifting it against a reaction body. In previously performed experiments on this test setup (Barandun, 2013), there were problems with unintended movements of the baseplate. The additional tie-downs at the center axis and the shifting of the southern end prevent the movement of the baseplate now successfully. The cyclic horizontal displacement is induced by the means of a hydraulic actuator (12), which is fixed to the reaction wall (13). (a) Schematic drawing of test setup (b) Test-setup with built-in specimen Figure 4.5. Test setup 34 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

51 4.4. Measurement Force Measurement Axial force The two cylinders applying the axial force are connected to the same hydraulic system. The oil pressure in this system was measured with two redundant pressure gauges and transformed to the value of the vertical force by multiplication of the measured pressure with the area of the pistons of the vertical cylinders Horizontal force The horizontal force was directly measured with a load cell fixed between the actuator and the bottom spreader beam Displacement measurement The displacements and deformations of the masonry specimens as well as of crucial elements of the test setup were measured with a total amount of 19 potentiometers. Additionally, the deformations of the specimens were measured with an optical measurement system using digital image correlation software Deformation measurement with potentiometers Figure 4.6 shows the arrangement of the LVDT s measuring the displacements and deformations of the specimen. The imposed horizontal displacement was measured at different positions. POT2 was measuring the piston travel directly at the cylinder. POT16 measured the movement of the steel plate under the bottom spreader beam and POT15 the movement of the top brick on the southern side of the specimen. In the first experiment (W0.10), there was a considerably big asymmetric difference between the measurements of the cylinder travel and the displacement of the bottom spreader beam. Responsible for this was an air-gap between the cylinder and the spreader beam. Despite all attempts to reduce this air-gap, the difference consisted also in the next experiment. Therefore it was decided to control the actuator directly over a laser device, measuring the displacement of the steel plate under the spreader beam. That way it could be guaranteed, that the wall was actually deflected to the desired amplitude in both directions. The vertical deformations of the specimen were measured with two potentiometers (POT6 and POT7). POT5 and POT6 measured the horizontal deformation, POT8 and POT9 the Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 35

52 diagonal deformation. The uplift of the interface joint between the specimen and the baseplate was measured with POT17 and POT19. Furthermore the absolute displacement of the wall was measured at the bottom (POT10 and POT11) and the top (POT15). The displacement of the baseplate was measured with POT12 and tilting of the spreader beam was measured with a tilt sensor on the spreader beam. Figure 4.6. Arrangement of potentiometers Digital image correlation Digital image correlation is a method that uses tracking and image registration techniques to measure changes in images. That way it is possible to optically measure full field displacements and deformations of an object. This method requires to apply a random speckle pattern to the surface of the specimen and to take images of the specimen in certain time or deformation steps. By the means of a computer script and an electronical trigger, the camera was commanded to take pictures in predefined deformation steps. A total of 496 pictures was taken, given that the specimen endured all cycles up to 40 mm amplitude. The DIC software can measure the changes between any of these images and calculate the deformation field of the whole specimen. 36 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

53 Camera and Camera position A Nikon D800 with a AF-S NIKKOR :2.8 G ED lens was used to take the images. The aperture and the exposure time were set to 5.6 and 1/250 seconds respectively. Due to the unfortunate position of the support frame columns, an up front positioning of the camera was not possible. The camera therefore had to be positioned with a small angle to the center axis of the specimen. The perspective distortion of the image that occurs due to this angle is later removed in the post processing Speckle pattern The size of one pixel in a picture is dependent on the resolution of the image and the distance of the camera to the specimen. For the setup described in Chapter , the pixel size amounts to approximately 1/3 of a millimeter. For best results, the speckles on the specimen should not be smaller than 3 pixels. There is no definite upper boundary, but with bigger speckles the resolution of this measurement method decreases. The speckles used for these experiments had a diameter of 3.2 mm, which equals a size of approximately 10 pixels. The pattern was randomly generated with a computer script and then transferred to a cardboard stencil with a laser cutter. A coverage of 40% was selected. The face of the wall was first painted with white color. After the drying process, the speckle pattern was applied by the means of the cardboard stencils and an airbrush. This procedure was used for the first six specimens. For the last three, an adhesive foil stencil, produced with a cutter plotter was used to increase the quality of the pattern. The paint was applied with a classical aerosol spray. (a) Specimen with applied pattern (b) Close-up of speckle pattern Figure 4.7. DIC speckle pattern Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 37

54 Calibration process For reduction of the lens distortion, several calibration pictures of a checkerboard are taken at the beginning of each experiment. With a camera calibration toolbox based on Matlab, the inner camera parameters can be evaluated and the lens distortion can be removed from the pictures. The perspective distortion can be removed with a 2D transformation of the images. A picture of the specimen with a checkerboard pattern glued to the wall is taken in the beginning of the experiment. In the real world the outer corners of the checkerboard form a square with known dimensions. Due to the perspective distortion, the square has the form of an arbitrary quadrangle without right angles in the image. By transforming the image so that the checkerboard again forms a square, the perspective distortion can be removed Crack control The visible cracks were subsequently marked and photographed after every full displacement cycle. Furthermore the digital image correlation processing will also show the formation of the cracks. 38 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

55 5. Results 5.1. W Global behavior Figure 5.1 shows the time history of the jack travel and the corresponding shear strength of specimen W0.10. It can be seen that the specimen reaches the maximum strength in the 10mm cycle, showed considerable strength degradation in the 15mm cycles and failed in the first pushing cycle with amplitude of 20mm. Figure 5.1. W0.10 Horizontal force and displacement time history The initial behavior of the wall was mainly flexural with a clear rocking motion. The plots of Figure 5.2 and Figure 5.3 show that the positive deflections of the uplift sensors, corresponding to an opening of the base joint, are larger than the negative measurements, indicating a rocking motion. Also a clear force plateau can be seen in Figure 5.3 for the northern uplift measurement. It is clearly visible, that the rocking motion was more distinct in the pushing direction, which opens the joint on the northern side. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 39

56 Figure 5.2. W Uplift vs. actuator displacement Figure 5.3. W Uplift vs. horizontal force It has to be mentioned here that the uplift sensors were susceptible to falling off at large displacements or big sliding movements of the specimens. Therefore the uplift measurements are not available for the whole experiment. But they can give good information about the initial movement of the specimen. Horizontal cracking along the interface joint and cracking of both toes are the consequence of this rocking behavior. From the crack pattern in Figure 5.4 it can be seen that the diagonal sliding resistance of the masonry was reached before the toes completely failed in compression and the wall failed in a diagonal sliding mode. The diagonal cracks follow the bed and the head joints. The tensile strength of the bricks is not reached. Figure 5.4. W Crack pattern 40 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

57 The hysteresis loops, shown in Figure 5.5, feature a distinct S-shape in the smaller amplitude cycles corresponding to the initial rocking motion. A modest energy dissipation is associated with this behavior. The formation of the diagonal cracks leads to a widening of the hysteresis loops and a distinct and fast strength degradation. Figure 5.5. W Hysteresis loops and capacity curve Figure 5.6. W Normalized hysteresis loops The bilinear idealization of Figure 5.7 shows that the ultimate level, corresponding to 20% strength degradation, is reached at a smaller deformation in the pushing direction than in the pulling direction. The wall reaches an ultimate deformation of 12.4 mm, corresponding to an ultimate drift of 1,01% on the pushing side. On the pulling side, the ultimate displacement is not reached, as the strength degradation is below the level of 20% defined for the failure criterion. The maximum recorded deformation in the pulling direction was 15.0 mm. The drift for this displacement amounts to 1.22%. These values are given here for the sake of completeness. However, as the cyclic performance of the walls is assessed, the failure in one direction corresponds to the failure of the entire wall. The effective stiffness corresponds well with the initial stiffness, indicating a moderate stiffness degradation due to the cyclic nature of the loading in the elastic phase. A ductility of =7.25 on the pushing side and 8.26 on the pulling side is reached. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 41

58 Figure 5.7. W Bilinear idealisation of the capacity curve W0.10 Pushing Pulling K0 [kn/mm] Keff [kn/mm] Keff/K0 [%] Vmax [kn] Vu [kn] u/hw [%] m [-] Table 5.1. W0.10 Summary of shear parameters Energy dissipation From Figure 5.8 it can be seen, that in the first small cycles the ratio of dissipated energy to input energy rises and then remains at a level of approximately 60%. The increase of the diagonal cracks connected to the failure mechanism leads to an increase of the dissipated energy ratio to almost 80%. The equivalent viscous damping coefficient, plotted in Figure 5.9 shows a distinct plateau, corresponding to the rocking motion of the specimen. As the diagonal crack opens and the hysteresis loops get wider, the damping coefficient increases clearly. 42 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

59 Figure 5.8. W Dissipated energy ratio Figure 5.9. W Equivalent viscous damping ratio Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 43

60 5.2. WE Global behavior Also specimen WE10.10 reaches the maximum strength in the 10 mm amplitude cycle. In the pushing direction on the 15 mm cycle the failure occurs with a rapid strength degradation in the second pushing cycle. On the pulling side only a negligible strength degradation was observed up to the failure of the wall. Figure WE10.10 Horizontal force and displacement time history The specimen shows a distinct rocking motion up to the failure due to a combination of toe crushing and diagonal sliding which is connected with a fast strength degradation. The rocking motion can clearly be identified from the uplift plots in Figure 5.11 and Figure The diagonal crack follows the head and bed joints and is accompanied by cracking of the southern toe of the wall. A horizontal crack runs above the soft-layer on the complete length of the wall. However it originates from the rocking of the specimen on the soft-layer and not from a sliding motion of the wall. The crack pattern is very similar to the pattern of the control specimen. 44 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

61 Figure WE Uplift vs. actuator displacement Figure WE Uplift vs. horizontal force Figure WE Crack pattern The hysteresis loops show the characteristic S-shape of the rocking behavior, widened due to the cracking of the wall. The failure occurred on the second pushing cycle with a 15mm amplitude and is therefore not captured by the envelope. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 45

62 Figure WE Hysteresis loops and capacity curve Figure WE Normalized hysteresis loops Up to the very sudden failure, no strength degradation can be observed in both directions. The ultimate displacements therefore correspond with the maximum displacements. A drift of 1.20% in the pushing direction and 0.92% in the pulling direction was reached. As the failure occurred in the pushing direction the value from this direction is governing. The corresponding ductilities are 6.32 and 5.05 respectively. The effective stiffness amounts to Keff = 16.1 kn/mm for the pushing direction and Keff = 17,5 kn/mm for the pulling direction. The initial stiffness of the wall is slightly higher than the effective stiffness, indicating some stiffness degradation in the elastic phase. Figure WE Bilinear idealisation of the capacity curve 46 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

63 WE10.10 Pushing Pulling K0 [kn/mm] Keff [kn/mm] Keff/K0 [%] Vmax [kn] Vu [kn] u/hw [%] [-] Table 5.2. WE10.10 Summary of shear parameters Energy dissipation Slightly more energy is dissipated compared to the control specimen. The energy dissipation ratio shows a plateau at 70% and increases as the failure occurs. The equivalent damping ratio shows the characteristic plateau of the rocking motion at a slightly higher level than the control specimen. The diagonal cracking connected to the failure mechanism of the wall results in a increasing damping ratio on the second cycles. However the damping ratio before the failure is slightly higher than 20%. Figure WE Dissipated energy ratio Figure WE Equivalent viscous damping ratio Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 47

64 Damage The soft-layer shows no traces of damage or deterioration at all. However the wall is damaged to a similar amount as the control specimen, indicating no pronounced influence of the softlayer on the damage of the wall. Figure WE Damage of soft-layer 5.3. WE Global behavior Specimen WE3.10 failed in the last pulling cycle with an amplitude of 30 mm. The maximum strength was reached in the 10 mm cycle and from there the strength continuously decreased until the failure occurred. Figure WE3.10 Horizontal force and displacement time history A clear rocking motion can be identified from the plots in Figure 5.21 and Figure The rocking motion seems to be more distinct in the pulling direction. However the shape of the measurements from the uplift sensor on the northern side indicate, that the sensor was not working properly. 48 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

65 Figure WE Uplift vs. actuator displacement Figure WE Uplift vs. horizontal force The crack pattern in Figure 5.23 shows diagonal cracks along the joints on both directions and some additional cracking in the base of the wall. Again a horizontal crack above the soft-layer is the result from the rocking in both directions and not due to a sliding motion. Figure WE Crack pattern The hysteresis loops have a distinct rocking shape that is widened due to the diagonal cracking in the bigger amplitude cycles. The strength degrades faster in pulling direction. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 49

66 Figure WE Hysteresis loops and capacity curve Figure WG Normalized hysteresis plots This can also be seen in the bilinear idealisation of Figure A strength degradation of 20% is already reached at a displacement of 16.9mm on the pulling side, while de failure criterion is not reached in pushing direction. The governing drift and ductility are u/hw=1.37% and =8.15. In the pushing direction a clearly higher deformation capacity, with a drift of 2.46% and a ductility of can be achieved. The effective stiffness of 21,3 kn/mm (pulling) and 22,6 kn/mm (pushing) corresponds very well with the initial stiffness of the wall. Almost no stiffness degradation happens in the elastic range. Figure WE Bilinear idealisation of the capacity curve 50 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

67 WE3.10 Pushing Pulling K0 [kn/mm] Keff [kn/mm] Keff/K0 [%] Vmax [kn] Vu [kn] u/hw [%] [-] Table 5.3. WE3.10 Summary of shear parameters Energy dissipation The equivalent damping ratio, shown in Figure 5.28, rises almost constantly in the smaller cycles and remains at a value of approximately 20% until failure. This is very similar to the specimens WE10.10 and the control specimen W0.10 and results from a rocking behavior of the wall. The ratio of dissipated energy to input energy shows a plateau at 60% and rises for the first cycles to approximately 75% as the cracking of the wall increases. Figure WE Dissipated energy ratio Figure WE Equivalent viscous damping ratio Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 51

68 Damage The soft-layer shows only some minor traces of deterioration in the corner regions. Otherwise the layer is undamaged. Figure WE Damage of soft-layer 52 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

69 5.4. WG Global behavior Specimen WG10.5 did not experience a failure in the 11 th cycle with an amplitude of 40mm. Therefore the additional 50 mm cycle was conducted. The wall reaches the maximum strength in the 20mm amplitude cycle and shows very little strength degradation in pushing direction until the failure mechanism is initiated. The force is considerably larger in pulling direction and drops to a lower level in the 30mm amplitude where it remains for the larger cycles. Figure WG10.5 Horizontal force and displacement time history Figure 5.31 shows clearly that the behavior of specimen WG10.5 was characterized by a very asymmetric behavior. In direction of pushing forces a distinct rocking motion up to failure correlates with S-shaped hysteresis loops and an almost constant force plateau. In pulling direction the hysteresis loops are, after initially resembling the S-shape, significantly wider and the force is considerably larger. This implies a sliding motion of the wall in pulling direction. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 53

70 Figure WG Hysteresis loops and capacity curve Figure WG Normalized hysteresis loops The uplift sensors were measuring up to a displacement of 15mm in pulling direction and 20mm in pushing direction. The shape of the uplift plots of Figure 5.33 and Figure 5.34 indicate that up to this displacement an almost symmetrical rocking motion took place. This correlates with the time history plot in Figure 5.30, where the drop of the horizontal force, which indicates the start of the sliding motion, happens in the 30 mm cycle. Figure WG Uplift vs. actuator displacement Figure WG Uplift vs. horizontal force The crack pattern shows that the failure was initiated by compression failure at the southern toe in the pushing cycle. Diagonal cracking is only present in the direction that correlates with a pushing force. It seems like the sliding in pulling direction protects the wall from cracking. 54 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

71 Figure WG Crack pattern Figure 5.36 shows the capacity curve and its bilinear idealisation. Up to the very sudden failure, no strength degradation is visible on the pushing side. On the pulling side the force drops down to a plateau that is kept up to failure. Figure WG Bilinear idealisation of the capacity curve Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 55

72 WG10.5 Pushing Pulling K0 [kn/mm] Keff [kn/mm] Keff/K0 [%] Vmax [kn] Vu [kn] u/hw [%] [-] Table 5.4. WG10.5 Summary of shear parameters Energy dissipation A ratio between 60 and 70% of the input energy was dissipated by the wall. In first cycles more energy seems to be dissipated, as the ratio is clearly higher. The damping ratio calculated for the first cycles is higher than for the specimens described before. It almost reaches an equivalent damping ratio of 30%. When calculated with the measurements of the second cycles the damping ratio remains at the established plateau at 20% of critical damping. Figure WG Dissipated energy ratio Figure WG Equivalent viscous damping ratio Damage The soft-layer is clearly damaged in both corner regions. The southern corner (left corner in Figure 5.39), corresponding to the rotation point for the rocking motion, is damaged more severely. In this region also the mortar joint above the soft-layer is seriously destroyed. On 56 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

73 the corner where the wall was sliding instead of rocking, almost no crushed mortar can be found and the soft-layer is less damaged. Figure WG Damage of soft-layer 5.5. WG Global behavior Specimen WG10.10 reached the maximum shear strength in the 15 mm amplitude cycle and continuously degraded from there. The failure occurred in the first pulling cycle with an amplitude of 40 mm after the wall successfully resisted one pushing cycle on this amplitude. Figure WG10.10 Horizontal force and displacement time history A distinct rocking behavior can be derived from Figure 5.41 and Figure This also corresponds well with the cracking pattern in Figure The wall showed serious damage of both toes, before in the last cycle almost the complete bottom course of bricks was destroyed. Additionally diagonal cracks along the joints formed in both directions. The failure however was initiated by the failure of the base of the wall. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 57

74 Figure WG Uplift vs. actuator displacement Figure WG Uplift vs. horizontal force Figure WG Crack pattern The hysteresis loops again are S-shaped in the beginning and get wider with the propagation of the cracking of the wall. After reaching the maximum strength a distinct strength degradation in both directions was observed. 58 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

75 Figure WG Hysteresis loops and capacity curve Figure WG Normalized hysteresis plots From the bilinear idealisation of the capacity curve in Figure 5.46 it can be seen that the criterion of 20% strength degradation was reached in both directions. The governing side is the pulling side with an ultimate drift of 2.20% and a ductility of In pushing direction the drift of 2.58% and ductility of 4.15 was only marginally larger. The effective stiffness is clearly lower than the initial stiffness indicating a distinct stiffness degradation in the elastic region. Figure WG Bilinear idealisation of the capacity curve Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 59

76 WG10.10 Pushing Pulling K0 [kn/mm] Keff [kn/mm] Keff/K0 [%] Vmax [kn] Vu [kn] u/hw [%] [-] Table 5.5. WG10.10 Summary of shear parameters Energy dissipation Between 50 and 70% of the input energy is dissipated. The equivalent viscous damping ratio is slowly rising to a plateau at 20%, comparable to most of the previous specimens. First and second cycles are similar and show only a slight difference in damping. Figure WG Dissipated energy ratio Figure WG Equivalent viscous damping ratio Damage After the initial failure of both toes in the 30 mm amplitude cycle visible in Figure 5.49 the wall continued to rock on the inner bricks of the base course, leading to a collapse of the complete base of the wall. 60 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

77 Figure WG Crushing of the toes Figure WG Destruction of the base The mortar layer above the soft-layer was crushed on the complete length of the wall due to the above mentioned behavior of the wall. The soft-layer, however was only damaged on the northern end of the wall and shows otherwise almost no signs of deterioration. Figure WG Damage of soft-layer with crushed mortar 5.6. WG Global behavior Figure 5.52 shows the time history of the jack travel and the corresponding shear strength of specimen WG The maximum strength is reached in the 15 mm amplitude and continuous strength degradation was observed until the failure occurred in the first pulling cycle with an amplitude of 30 mm. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 61

78 Figure WG10.15 Horizontal force and displacement time history The uplift sensors failed early in this experiment, as distinct cracking of the wall caused the attachment points of the LVDT s to fall off. The useable measurements do not show a clear difference between the positive and negative uplift component, indicating that only modest rocking took place. Figure WG Uplift vs. actuator displacement Figure WG Uplift vs. horizontal force From the crack pattern it can be seen that beside the diagonal cracks along the joints, splitting of the bricks and serious damage at the base of the wall occurred. This is a typical crack pattern for high vertical loads leading to a compression dominated failure. 62 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

79 Figure WG Crack pattern From the hysteresis loops from Figure 5.56 it is also evident that no distinct rocking, or sliding motion was taking place in this experiment. The stiffness and strength of the wall are decreasing due to massive cracking of the wall until a combined failure mode consisting of splitting of the bricks and failing of the base occurred. Figure WG Hysteresis loops and capacity curve Figure WG Normalized hysteresis loops The effective stiffness of the bilinear idealisation in Figure 5.58 shows a big difference to the initial stiffness, corresponding to the distinct stiffness degradation from cycle to cycle. The failure occurred on the pulling side, giving an ultimate drift of 1.59% and a ductility of Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 63

80 The failure criterion is also reached in the pushing direction with an ultimate drift of 2.22% and a corresponding ductility of Figure WG Bilinear idealisation of the capacity curve WG10.15 Pushing Pulling K0 [kn/mm] Keff [kn/mm] Keff/K0 [%] Vmax [kn] Vu [kn] u/hw [%] [-] Table 5.6. WG10.15 Summary of shear parameters Energy dissipation The high axial load leads to relatively modest energy dissipation. The equivalent damping ration barely reaches the 20% mark in the last cycle. 64 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

81 Figure WG Dissipated energy ratio Figure WG Equivalent viscous damping ratio Damage As mentioned before, the wall is massively cracking along the joints and also the tensile strength of the bricks is reached, leading to splitting of the bricks. The base of the wall fails in compression. It can be seen in the picture of Figure 5.61 that the soft-layer generally endured a low amount of damage. Only at the northern end, some traces of deterioration and plastic deformations can be recognized. Figure WG Damage of the soft-layer 5.7. WG Global behavior This specimen did not fail in the standard cycles up to amplitude of 40 mm. Therefore the additional 12 th cycle with an amplitude of 50 mm was conducted. However, the specimen did also not fail in this cycle. The experiment could not be continued with bigger amplitudes because the limits of the test setup were reached. The rotation of the spreader beams reached the critical limit where the stroke of the pistons could no longer guarantee a free rotation of the beams. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 65

82 Figure WG3.5 Horizontal force and displacement time history Specimen WG3.5 showed a very interesting behavior. From the plots of Figure 5.63 and Figure 5.64 it can be seen that a very distinct rocking motion dominated the behavior in the initial cycles. Figure WG3.5 - Uplift vs. actuator displacement Figure WG3.5 - Uplift vs. horizontal force Starting in the cycle with an amplitude of 15 mm, the motion of the specimen can be described as followed: As the deformation increased, the specimen at first started to perform the rocking motion as before. When the compressed length of the wall reached a critical value, the sliding resistance was overcome and the motion changed from the rocking rotation to a sliding motion. The force dropped to a lower level and the uplift of the wall decreased to zero. 66 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

83 This behavior could be observed in both directions for the cycles up to the amplitude of 40 mm. This behavior can be nicely seen in the hysteresis plot of Figure In the 50 mm cycle the behavior in pushing direction changed back to a rocking motion, connected with a very large rotation of the top of the wall. This lead to a jamming of the vertical cylinders. This produced the unusual peak visible at a deformation of approximately 50 mm in Figure Figure WG3.5 - Hysteresis loops and capacity curve Figure WG3.5 - Normalized hysteresis loops The wall showed the typical diagonal cracking along the joints and some additional cracks at the base of the wall (refer to Figure 5.67). However the cracks were not very wide and the wall did not loose its strength. The change to the rocking behavior in the 50 mm cycle lead to a crushing of the southern toe. The force was not dropping though, indicating that the toe still had some residual strength. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 67

84 Figure WG3.5 - Crack pattern The capacity curve in Figure 5.65 features a definite drop of the shear strength after the peak, followed by a force plateau in pulling direction and a plateau with a renewed rise of strength in pushing direction. This is caused by the change of motion types and the change from static to kinematic friction once sliding is induced. Even though the strength reduces to a value below 80% of the maximum strength this can not be defined as the failure of the wall in this case. Therefore the bilinear idealisation (refer to Figure 5.68) was constructed with the ultimate deformation equal the maximum deformation in both directions. The combination of a relative big effective stiffness of Keff = 17.7kN/mm in both directions and a big ultimate deformation leads to a very high ductility of for the pushing and for the pulling side with respective ultimate drifts of 4.24% and 3.65%. The effective stiffness matches the initial stiffness very well, which corresponds to the low amount of damage in the wall. 68 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

85 Figure WG3.5 - Bilinear idealisation of the capacity curve WG3.5 Pushing Pulling K0 [kn/mm] Keff [kn/mm] Keff/K0 [%] Vmax [kn] Vu [kn] u/hw [%] [-] Table 5.7. WG3.5 Summary of shear parameters Energy dissipation From the plot of the dissipated energy ratio in Figure 5.69 and the equivalent viscous damping ratio in Figure 5.70 the changes of the mechanisms are clearly visible. After an initially modest ratio of dissipated energy of 40%, with the change to the sliding mechanism in the cycles with 15 mm amplitude a sudden increase of energy dissipation can be observed. Over 90% of the input energy are dissipated with the sliding mechanism. The damping ratio also distinctively increases at a displacement of 15 mm and comes down to below 20% again in the 50 mm cycle. There is a considerable difference of the first and the second cycle concerning energy dissipation after approximately 25 mm displacement. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 69

86 Figure WG3.5 - Dissipated energy ratio Figure WG Equivalent viscous damping ratio Damage As mentioned before, the wall shows a very low degree of cracking. On the other hand, the soft-layer is heavily destroyed in the corner regions. Furthermore it is torn apart at two locations in the center. Otherwise the center region is only modestly destroyed. Figure WG3.5 - Damage of soft-layer 5.8. WG Global behavior Specimen WG3.10 reached the maximum force in the 10 mm amplitude cycle and showed a very modest strength degradation in the pushing direction and almost no degradation in pulling direction. The failure occurred on the first pushing cycle with amplitude 30 mm. 70 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

87 Figure WG Horizontal force and time history From the shape of Figure 5.73 and Figure 5.74 a clear rocking mechanism can be identified. Figure WG Uplift vs. actuator displacement Figure WG Uplift vs. horizontal force The wall failed in a combination of crushing of the southern toe and sliding along the diagonal joints. In the crack pattern of Figure 5.75 it can be seen that some modest splitting of bricks occurred along the diagonal crack. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 71

88 Figure WG Crack pattern The hysteresis loops are S-shaped with modest energy dissipation in the smaller cycles and get wider at bigger displacements due to formation of diagonal cracks. The failure is connected with a sudden loss in strength. Figure WG Hysteresis loops and capacity curve Figure WG Normalized hysteresis plots From Figure 5.78 it can be seen that there was close to no strength degradation until the sudden failure of the wall. The ultimate deformations therefore correspond with the maximum deformations. An ultimate drift of 1.64% with a ductility of 6.64 was reached in 72 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

89 pushing direction. In the pulling direction, values of 1.56% for the drift and 6.19 for the ductility can be attained. The initial stiffness is slightly bigger than the effective stiffness. Figure WG Capacity curve and bilinear idealisation WG3.10 Pushing Pulling K0 [kn/mm] Keff [kn/mm] Keff/K0 [%] Vmax [kn] Vu [kn] u/hw [%] [-] Table 5.8. WG Governing values Energy dissipation The dissipated energy ratio is almost constantly kept at a modest level of 50%. The equivalent viscous damping ratio stays at a low level below 20% of critical damping throughout the whole experiment. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 73

90 Figure WG Dissipated energy ratio Figure WG Equivalent visous damping ratio Damage The wall shows serious diagonal cracking and damage of the toes, whereas the soft layer is almost not damaged (see Figure 5.81). Only a slight plastic deformation with a cut on the northern end of the layer is visible. Figure WG Damage of the soft-layer 5.9. WG Global behavior WG3.15 reached the maximum force in the 10 mm amplitude cycle and failed during the last pulling cycle with an amplitude of 15 mm. The specimen featured almost no strength degradation until it failed. 74 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

91 Figure WG Horizontal force and dispacement history The uplift plots of Figure 5.83 and Figure 5.84 indicate a distinct rocking motion for the initial cycles. The heavy cracking of the wall lead again to a early failure of the uplift sensors. Figure WG Uplift vs. actuator displacement Figure WG Uplift vs. horizontal force However from the shape of the hysteresis loops it is visible that the specimen maintained this rocking behavior until the failure occurred. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 75

92 Figure WG Hysteresis loops with capacity curve Figure WG Normalized hysteresis loops The slightly wider hysteresis loops in pulling direction, visible in Figure 5.85, corresponds to the more distinctive cracking along the diagonal subject to tension stresses during the pulling cycles. It can be seen in Figure 5.87 that the tensile stress of the bricks is reached in several locations along the diagonals in both directions. This failure mode corresponds well to the high level of axial load applied in this experiment. Figure WG Crack pattern Almost no strength degradation occurred up to the failure. The ultimate drift denotes to 1.20% in pushing direction and 1.22% in pulling direction. The corresponding ductilities are 4.51 and Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

93 The effective stiffness is, with a value of 22.9 kn/mm compared to 18.9 kn/mm, larger on the pulling side than the pushing side. This explains why a slightly higher force was reached in this direction and why the main failure mechanism corresponds to a pulling force. Figure WG Capacity curve and bilinear idealisation WG3.15 Pushing Pulling K0 [kn/mm] Keff [kn/mm] Keff/K0 [%] Vmax [kn] Vu [kn] u/hw [%] [-] Table 5.9. WG Governing values Energy dissipation The amount of dissipated energy is almost constantly at 50% of the input energy for the first cycles and for the second cycles even lower. The equivalent damping ratio remains at a level below 20% throughout the experiment. The initially high values for the damping ratio origin from the big influence of measuring and controlling inaccuracies for the very small cycles and do not have a physical meanings. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 77

94 Figure WG Dissipated energy ratio Figure WG Equivalent viscous damping ratio Damage As it was expected for a high axial load, the soft-layer shows no signs of damage. The mortar layer below the soft-layer is also intact. The wall on the other side is heavily cracked. Figure WG Damage of the soft-layer 78 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

95 6. Experiments on URM walls with soft-layer wall bearings in first bed joint The results from the conducted experiments for this thesis (Chapter 1) are compared to the results of the tests performed in 2012 and early 2013 by Barandun & Vögeli. These tests were conducted at the ETH Zurich in the fall of 2012 and spring of 2013 with similar materials and on the same test setup (apart from a few improvements). The following chapter shortly summarizes the test procedure and gives the results in the same manner as previously done for the recent experiments to allow for the best possible comparison Test program A total of 7 specimens were tested. Each of them, except the control specimen contained a soft-layer in the first bed joint. Soft-layers from two materials (extruded elastomer (E) and rubber granulate (G)) were used in thicknesses of 3mm, 5mm and 10mm. All specimens were tested at a pre-compression level of 10% of an assumed 6MPa compressive strength for the masonry. In the first three experiments there were problems with movements of the baseplate and inconstant vertical load. Therefore these experiments had to be repeated in spring of The mentioned specimens are designated with an R in the end of the name Loading pattern Axial load The axial load was kept constant during the experiments. Table 6.1 shows the axial load, the pre-compression and the effective percentage of the compressive strength for each specimen. Specimen Applied axial force [kn] Weight of spreader beams [kn] Total precompression [MPa] Percentage of strength [%] W0R WE3R WE5R WE WG WG WG Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 79

96 Table 6.1. Axial load Horizontal load The same deformation controlled load pattern was applied as for the recent experiments described in Chapter Results W0R Global behavior Specimen W0R is the control specimen of this series of tests, containing no soft-layer. This was one of the three experiments that had to be repeated in spring 2013 due to problems with varying axial load. In the repetition the vertical load could be kept constant. The maximum shear strength was reached in the 15 mm amplitude cycle and was kept with almost no strength degradation until the failure in the first pushing cycle with an amplitude of 30 mm. The vertical forces were higher in direction of pulling force. Figure 6.1. W0R - Horizontal force and displacement time history The uplift sensors were fully functional until the end of the experiment and show a clear rocking behavior with larger positive uplift components and a clear force plateau in Figure 6.3. It has to be mentioned here, that the bottom course of bricks was blocked with fixed steel 80 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

97 blocks in this series of experiments because the soft-layers were placed in the first bed joint instead of the interface joint at the very bottom of the wall. Therefore the wall was rocking about a rotation point at the base of the second course of bricks and also the uplift sensors measure the opening of the first bed joint. Figure 6.2. W0R - Uplift vs. actuator displacement Figure 6.3. W0R - Uplift vs. horizontal force The hysteresis loops have a very distinct S-shape with an almost perfect accordance of first and second cycles. The area of the hysteresis loops is small and the strength is kept at a plateau after the initial rise. Figure 6.4. W0R - Hysteresis loops and capacity curve Figure 6.5. W0R - Normalized hysteresis loops As no strength degradation occurred, the ultimate deformation corresponds to the maximum deformation in both directions. The failure occurred in the pushing direction leading to a Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 81

98 bigger ultimate drift of 2.20% in this direction. On the pulling side a drift of 1.64% was reached. The wall showed a relatively low effective stiffness in both directions. Keff amounts to 10.3 kn/mm for pushing and 8.5 kn/mm for pulling forces. However it is clear from Figure 6.6 that, especially in pushing direction, the initial stiffness was considerably higher than the effective stiffness. From these values, the ductility can be calculated to 6.05 in pushing and 3.46 in pulling direction. Figure 6.6. W0R - Capacity curve and bilinear idealisation W0R Pushing Pulling K0 [kn/mm] Keff [kn/mm] Keff/K0 [%] Vmax [kn] Vu [kn] u/hw [%] [-] Table 6.2. W0R Summary of shear parameters The wall exhibited relatively thin cracks along the diagonals in both directions and in additional joints. The failure mode can be clearly classified as a toe crushing failure on the southern end of the wall, corresponding to the pushing direction. 82 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

99 Figure 6.7. W0R - Crack pattern Energy dissipation The energy dissipation remained at a relatively low level throughout the experiment. Only 30 to 40% of the input energy are dissipated. The equivalent viscous damping ratio rises to a level of approximately 10% of critical damping and remains there. There is no big difference between first and second cycles. Figure 6.8. W0R - Dissipated energy ratio Figure 6.9. W0R - Equivalent viscous damping ratio Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 83

100 Damage A large number of cracks can be seen in the crack pattern in Figure 6.7. However, almost all cracks were very thin and did not lead to a big strength reduction of the wall WE3R Global behavior Also specimen WE3R had to be repeated. The wall features a 3mm thick extruded elastomer based soft-layer and was subjected to an average axial load of kn. A distinct strength degradation can be observed after the wall reached the maximum shear strength in the first 15 mm cycle. The experiment was ended after the both 20 mm amplitude cycles were conducted. Figure WE3R - Horizontal force and displcamenet history The LVDT measuring the uplift of the wall on the southern end, failed very early in the experiment. However the sensor monitoring the uplift on the northern side was fully functional until the end of the experiment. It can be clearly seen from Figure 6.11 that the rocking motion lead to a degradation of the bottom brick in this corner. Hence a clear stiffness reduction and increase in negative uplift can be observed. 84 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

101 Figure WE3R - Uplift vs. actuator displacement Figure WE3R - Uplift vs. horizontal force A clear orientation of cracks along the joints in direction of both diagonals is visible in the crack pattern from Figure However, these diagonal cracks are not very wide and the wall is otherwise intact in the upper part. The bricks above, but especially below the soft-layer, are ruptured in various positions. The failure can be classified as a crushing of the toes. Figure WE3R - Crack pattern The S-shaped hysteresis is very symmetric and the area is a bit wider than for the control specimen, indicating energy dissipation due to cracking. No sliding motion is observed in this experiment. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 85

102 Figure WE3R - Hysteresis loops and capacity curve Figure WE3R - Normalized hysteresis loops From the comparison of the capacity curve and its bilinear idealisation in Figure 6.16 it can be seen, that only in the pulling direction a strength degradation of 20% and more was reached. The pushing direction shows a slightly lower degradation in the last cycles. The governing ultimate drift and ductility in pulling direction are 1.57% and The effective stiffness is with 7.8 kn/mm respectively 8.4 kn/mm similar to the control specimen. In pushing direction a drift of 1.63% and a ductility of 4.72 can be calculated. Figure WE3R - Capacity curve and bilinear idealisation 86 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

103 WE3R Pushing Pulling K0 [kn/mm] Keff [kn/mm] Keff/K0 [%] Vmax [kn] Vu [kn] u/hw [%] [-] Table 6.3. WE3R Summary of shear parameters Energy dissipation The dissipation rate of energy is constantly rising throughout the experiment and reaches a maximum value of approximately 55%. The damping ratio is slightly higher than for the control specimen. Figure WE3R - Dissipated energy ratio Figure WE3R - Equivalent viscous damping ratio Damage Apart from the diagonal cracks, the upper part of the wall is almost not damaged (refer to Figure 6.13). The courses of bricks above and below the soft-layer show numerous vertical cracks in the bricks, indicating high tensile stresses in lateral direction. The soft layer shows almost no signs of damage apart from some plastic deformations at the edges. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 87

104 Figure Damage of soft-layer WE5R Global behavior WE5R marks the last specimen of the repeated experiments. The wall was vertically loaded to kn. The behavior is very similar to the previous specimen. The maximum strength is reached in the 15 mm amplitude cycles and shows a degradation of strength up to the failure at the end of the 20 mm cycles. Figure WE5R - Horizontal force and displacement time history Again a rocking motion in combination with a degradation of the bottom bricks can be derived from the uplift plots in Figure 6.21 and Figure The uplift sensors were fully functional until the end of the experiment. 88 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

105 Figure WE5R - Uplift vs. actuator displacement Figure WE5R - Uplift vs. horizontal force Also the crack pattern looks very similar to the WG3R specimen. There are cracks along the joints of the diagonals in both directions and otherwise no cracking in the upper part of the wall. The rupture of bricks in the first two courses is more distinct and seems to increase with increasing thickness of the solid elastomer layer. The toes are intensively crushed, especially on the northern side that corresponds with the failure. Figure WE5R - Crack pattern The hysteresis loops appear less S-shaped and more similar to a classical shear failure. However the failure of the wall was marked by the failure of the southern toe, and the diagonal cracks are not very wide. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 89

106 Figure WE5R - Hysteresis loops and capacity curve Figure WE5R - Normalized hysteresis loops The ultimate drifts of 1.62% (pushing) and 1.64% (pulling) and the effective stiffness of Keff = 7.5 in both directions, correspond well to experiment WE3R. Up to the failure the strength degradation was below 20%. Figure WE5R - Capacity curve and bilinear idealisation 90 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

107 WE5R Pushing Pulling K0 [kn/mm] Keff [kn/mm] Keff/K0 [%] Vmax [kn] Vu [kn] u/hw [%] [-] Table 6.4. WE5R Summary of shear parameters Energy dissipation The damping ratio and dissipated energy ratio are similar to experiment WE3R. An increasing percentage of input energy is dissipated up to between 50 and 60%. The damping ratio ranges between 10 and 20% of critical damping. The difference between the first and second cycles ratio s increases to the end of the experiment, indicating degradation. Figure WE5R - Dissipated energy ratio Figure WE5R - Equivalent viscous damping ratio Damage Again there was almost no damage to the soft-layer and the wall was mainly cracked in diagonal directions. Additionally a more intensive rupture of the bottom two courses of bricks occurred. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 91

108 WE Global behavior Specimen WE10 reached the maximum strength in the cycles with 10 mm amplitude and showed no considerable strength degradation until the failure occurred. The wall failed in the first pulling cycle with an amplitude of 20 mm. Figure WE10 - Horizontal force and displacement time history The uplift sensors show a certain rocking motion and a clear degradation of the bearing course of bricks. 92 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

109 Figure WE10 - Uplift vs. actuator displacement Figure WE10 - Uplift vs. horizontal force Form the crack pattern in Figure 6.32 it is evident, that extensive damage of the bottom course of bricks occurred before intensive cracking of the wall could develop. The failure was induced by failing of the toes. Figure WE10 - Crack pattern Figure 6.33 shows the hysteresis loops and the capacity curve. The untypical peak between 15 and 20 mm on the pushing side can be explained with an increase of axial load induced by the limited stroke of the vertical cylinders. This peak has not been considered for the construction of the capacity curve. The shape of the hysteresis loops is only slightly S-shaped. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 93

110 Figure WE10 - Hysteresis loops and capacity curve Figure WE10 - Normalized hysteresis loops As there was no strength degradation up to the failure, the maximum values of deformation determine the ultimate drift of 1.22% in both directions. As the effective stiffness is higher in pulling direction, a different ductility results for the two directions. Figure WE10 - Capacity curve and vilinear idealisation 94 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

111 WE10 Pushing Pulling K0 [kn/mm] Keff [kn/mm] Keff/K0 [%] Vmax [kn] Vu [kn] u/hw [%] [-] Table 6.5. WE10 - Governing values Energy dissipation The dissipated energy ratio and the damping ratio are located in the same range as for the other WE specimens. Figure WE10 - Dissipated energy ratio Figure WE10 - Equivalent viscous damping ratio Damage The rupture of the bricks in the bottom course is very distinct in this experiment and leads to an early failure. This observation substantiates the presumption, that the lateral strain originating from the deformation of the solid elastomer soft-layers lead to a rupture of the adjacent bricks. The soft-layer is not damaged at all. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 95

112 WG Global behavior WG3 was the first specimen with a rubber granulate soft-layer. It was pre-loaded with an axial load of kn. The specimen failed in the first pulling cycle with an amplitude of 30 mm after undergoing a series of sliding cycles with a degrading horizontal strength. Figure WG3 - Horizontal force and displacement time history Although the specimen was mainly sliding on the soft-layer, the uplift plots from Figure 6.39 Figure 6.40 indicate signs of a faint rocking motion in the smaller cycles, more distinctly visible in the pulling direction. 96 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

113 Figure WG3 - Uplift vs. actuator displacement Figure WG3 - Uplift vs. horizontal force From Figure 6.41 it can be seen, that the wall shows diagonal cracks along the joints of the diagonals in both directions. These are not very wide though and the failure, occurred due to the damage of the bottom bricks in the southern corner. Figure WG3 - Crack pattern The hysteresis has a clear sliding shape, with an initially higher force corresponding to the static sliding resistance. As soon as the sliding is initiated the resistance drops to the kinematic sliding strength level. The area of the hysteresis is very large. Especially in pushing direction a decrease of the sliding resistance can be observed in every cycle. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 97

114 Figure WG3 - Hysteresis loops and capacity curve Figure WG3 - Normalized hysteresis loops Due to the decreasing of the sliding resistance, the failure criterion of 20% strength degradation is reached very early. The wall therefore features a relatively low ultimate drift of 1.35% in pushing direction and 1.68 in pulling direction, even though much larger displacement cycles could be conducted until the break-down of the wall. The effective stiffness matches the initial stiffness very well and also the yield point is accurately displayed by the bilinear idealisation. Figure WG3 - Capacity curve and bilinear idealisation 98 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

115 WG3 Pushing Pulling K0 [kn/mm] Keff [kn/mm] Keff/K0 [%] Vmax [kn] Vu [kn] u/hw [%] [-] Table 6.6. WG3 Summary of shear parameters Energy dissipation The energy dissipation is very large compared to the WE specimens and the control specimen. The ratio from dissipate energy to input energy rises early to 90% and even higher. The damping ratio shows a distinctive rise starting at 5 mm displacement and goes up to almost 60% of critical damping. Figure WG3 - Dissipated energy ratio Figure WG3 - Equivalent viscous damping ratio Damage The upper part of the wall is not damaged intensively. The failure of the bottom bricks induced the failure. The soft-layer and the mortar layer above it are completely destroyed. The joint is filled with a mixture of mortar dust and granulate particles. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 99

116 Figure WG3 - Damage of soft-layer WG Global behavior The behavior of the wall with a 5 mm granular soft-layer was similar to the 3mm specimen. The failure occurred at the end of the 30 mm cycles and the maximum strength was reached in the 10 mm cycle. Figure WG5 - Horizontal force and displacement time history The rocking motion was very limited, seen in Figure 6.49 and Figure The wall was mainly featuring a sliding behavior. The decreasing stiffness in direction of compression in the uplift plots indicate a progressing degradation of the bottom course of bricks. 100 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

117 Figure WG5 - Uplift vs. actuator displacement Figure WG5 - Uplift vs. horizontal force The horizontal joint containing the soft-layer is cracked over the complete length, corresponding to the sliding motion of the wall. Cracks in the upper part of the wall appear only in the cycles with big deformations and stay very faint. The failure occurs due to splitting of the bottom bricks. Figure WG5 - Crack pattern The decreasing of the sliding level was less distinct for this wall than for the WG3 specimen. The big hysteresis areas indicate a strong energy dissipation connected with the sliding of the wall. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 101

118 Figure WG5 - Hysteresis loops with capacity curve Figure WG5 - Normalized hysteresis loops Bigger ultimate deformations are connected with the slower degradation of shear strength. The ultimate drift amounts to 1.93% in pulling direction and 2.05 in pushing direction. The initial stiffness and the yield point correspond well to the bilinear idealisation shown in Figure Figure WG5 - Capacity curve with bilinear idealisation 102 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

119 WG5 Pushing Pulling K0 [kn/mm] Keff [kn/mm] Keff/K0 [%] Vmax [kn] Vu [kn] u/hw [%] [-] Table 6.7. WG5 - Governing values Energy dissipation The energy dissipation is again large, connected with the sliding. The maximum ratio of dissipated energy of 90% is reached later than in the WG3 experiment. The damping ratio rises to 45% of critical damping. Figure WG5 - Dissipated energy ratio Figure WG5 - Equivalent viscous damping ratio Damage The soft-layer is completely damaged in the outer regions on both sides. The joint in these areas is filled with the mixture of granulate particles and mortar dust. The middle part of the soft-layer is only damaged on the surface. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 103

120 Figure WG5 - Damage of soft layer WG Global behavior Specimen WG10 failed in the second pushing cycle with an amplitude of 40 mm. The maximum strength was reached in the 15mm cycle connected with a flexural behavior and was not considerably decreasing until the wall changed to a sliding motion in the 30 mm cycle. Figure WG10 - Horizontal force and displacement time history The uplift plots show that the initial rocking motion was more distinct in pulling direction. Also a clear degradation of the bearing bottom layer of bricks can be observed. 104 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

121 Figure WG10 - Uplift vs. actuator displacement Figure WG10 - Uplift vs. horizontal force The wall is almost not cracked in the upper part of the specimen. The northern toe however, shows serious damage. Figure WG10 - Crack pattern From the hysteresis loops in Figure 6.62 it can be seen, that the wall was featuring a slight rocking motion in the beginning and then changed to the sliding behavior connected with a distinct energy dissipation. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 105

122 Figure WG10 - Hysteresis loops and capacity curve Figure WG10 - Normalized hysteresis loops The strength degradation was more distinctive in pulling direction leading to a smaller ultimate drift of 2.36% and a ductility of In pushing direction the values are 2.44% and 3.64 respectively. Figure WG10 - Capacity curve and bilinear idealisation 106 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

123 WG10 Pushing Pulling K0 [kn/mm] Keff [kn/mm] Keff/K0 [%] Vmax [kn] Vu [kn] u/hw [%] [-] Table 6.8. WG10 - Governing values Energy dissipation The rocking motion up to a displacement of 20 mm is connected with a modest energy dissipation below 50% and a damping ratio below 20%. As soon as the sliding starts, the dissipated energy ratio and the damping ratio show a fast rise and goes up to as much as 90% and 60 % respectively. Figure WG10 - Dissipated energy ratio Figure WG10 - Equivalent viscous damping ratio Damage, strength and stiffness degradation and deterioration The soft layer is heavily destroyed in the northern half. The southern part is mostly damaged on the surface and more distinctive in the corner region. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 107

124 Figure WG10 - Damage of soft-layer 108 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

125 7. Discussion 7.1. Influence of soft-layer on shear strength and deformation capacity Figure 7.1 shows the comparison of the walls with a soft-layer in the 1 st bed joint to the control specimen W0R. It can be seen that in general a slightly lower shear strength is reached for both types of soft-layer materials. For the extruded elastomer layers this can be explained with lateral tensile stresses induced into the masonry bricks due to the deformation of the soft-layer (this will be discussed in the following chapters). The lower strength of the rubber granulate specimens can be explained with a different failure mode connected to a lower shear strength. The displacement capacity is not influenced distinctively for the extruded elastomer layers. The sliding failure mode of the rubber granulate specimens is connected with a clearly larger displacement capacity than the control specimen. Concerning the energy dissipation it can be clearly noted, that both types of soft-layers lead to higher energy dissipation compared to the specimen without soft-layer. In the cases where a change of failure mode to a sliding mechanism occurred the increase of energy dissipation is considerably larger. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 109

126 (a) Capacity curves - extruded elastomer layers (b) Capacity curves - rubber granulate layers (c) Dissipated energy ratio extruded elastomer layers (d) Dissipated energy ratio rubber granulate layers Figure 7.1. Comparison of walls with soft-layer in 1st bed joint to control specimen In Figure 7.2 the comparison of the specimens with the soft-layer in the interface joint to the control specimen can be seen. The influence of the soft layers is less distinct here. In general similar shear strength but a higher displacement capacity is reached. No big influence on the energy dissipation can be found. 110 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

127 (a) Capacity curves extruded elastomer layers (b) Capacity curves rubber granulate layers (c) Dissipated energy ratio extruded elastomer layers (d) Dissipated energy ratio rubber granulate layers Figure 7.2. Comparison of specimens with soft-layer in interface joint to control specimen at 10% pre-compression level The main effect of a damp proof course layer on the shear strength and ultimate deformation capacity of a masonry wall is based on its influence on the failure mechanism. Basically a soft-layer exhibiting the appropriate parameters can provide a weak horizontal joint where the sliding mechanism is induced before any other failure mode occurs. Connected with this sliding failure mode a big deformation capacity and large energy dissipation can be achieved, which are both favorable characteristics in case of an earthquake. In this chapter the influence of different parameters of the soft-layers on their capability to change the global behavior of the wall to a more desirable behavior for seismic excitations are investigated, based on the results of the experiments described in Chapter 4 and 6. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 111

128 Influence of soft-layer material The main material-connected parameter of the soft-layer is the friction coefficient for a joint containing said soft-layer. The failure modes of masonry shear walls are described in Chapter 2.1 and in Figure 2.5 the predicted shear strength for a masonry wall in dependence of the axial load is plotted for each failure mode. For a certain level of axial load, the governing failure mode is the one connected with the lowest shear strength. For walls containing a DPC, an additional sliding failure mechanism along the bed joint containing the soft-layer has to be considered. As described in Chapter 2.1, the strength connected to a horizontal sliding mechanism along a joint containing a DPC can be calculated based on a Mohr-Coulomb criterion. There is no physical cohesion between the soft-layer and the mortar. Only after applying the axial load a certain small apparent cohesion due to the deformability of the soft-layer, can be observed (Mojsilovic, 2012). However for practical applications this cohesion can be neglected. Three experiments featured an almost pure horizontal sliding motion of the specimen. Two additional specimens showed a partial horizontal sliding behavior. All of these specimens contained a granular soft-layer. No specimens with an extruded elastomer soft-layer showed any sliding behavior. It can be concluded from these observations, that joints containing a granular soft-layer feature a lower friction coefficient than joints containing a solid elastomer layer. Based on these considerations, a friction coefficient = for a bed-joint containing a granular soft layer can be derived from the experiments where the specimen performed a sliding motion. The calculated values are listed in Table 7.1. It has to be mentioned here, that corresponds to the static friction coefficient. As soon as the sliding motion is induced, the kinematic friction coefficient applies and the force drops to a slightly lower level. 112 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

129 Specimen N VSL (pushing) VSL (pulling) (pushing) (pulling) [kn] [kn] [kn] [-] [-] WG WG WG WG WG Table 7.1. Friction coefficients for granular soft-layers From these values for the friction coefficient a mean value of 0.44 can be calculated. Note that specimen WG10.5 marks a discordant value, which is not considered for the calculation of the mean value. This value can be compared to the critical friction coefficient cr, defined in Chapter The critical friction coefficient for specimens with soft-layers in the first bed joint is calculated as follows: = = = 155 (Eq. 7-1) = 2 = = 0.43 (Eq. 7-2) Respectively for specimens with soft-layers in the interface joint: = = = 83 (Eq. 7-3) = 2 = = 0.40 (Eq. 7-4) It can be seen, that the mean measured friction coefficient is very close to the calculated critical friction coefficient. This indicates, that the rocking and sliding mechanism are very Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 113

130 close to each other for the present aspect ratios and axial loads. For walls with more squat geometry it can be expected, that the sliding mechanism is clearly the predominant one. As no sliding occurred for the extruded elastomer soft-layers, it is not possible to calculate a friction coefficient based on the results of the concluded experiments. However, Mojsilovic (2012) has performed masonry triplet shear tests with very similar materials and received a value of = 0.71 for elastomer based DPC. Based on these values, the granular soft-layers feature a 38% lower sliding resistance than the extruded elastomer layers. Beside the determination of a general friction coefficient for walls with granulate soft-layers, the behavior of certain specimens allow to draw some additional conclusions. Especially specimen WG3.5 showed a very interesting behavior. In the initial, smaller cycles the behavior was dominated by a distinct rocking motion. Starting in the 15mm cycle, the wall changed to a combined rocking and sliding behavior. With rising displacement, it started to rotate and as the compressed length of the wall reached a critical minimum, the wall started to slide. This obvious dependence of the sliding resistance on the contact length can not be explained with a Mohr-Coulomb friction law without cohesion. Also specimens WG3, WG5 and WG10, that featured an almost pure sliding behavior show a very distinct degradation of the strength level after each cycle, even though the sliding resistance should stay constant. This is especially the case for the 3mm granulate specimen. A possible explanation for these observations can be found, by looking at the damage of the soft-layers of these specimens. The soft-layer of specimen WG3.5, shown in Figure 7.3, is heavily damaged in both corner regions. Apart from 2 ruptures, the middle part of the layer is only lightly damaged. The joint in this area is filled with a mixture of rubber and mortar particles. 114 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

131 Figure 7.3. WG3.5. Global behavior and damage of soft-layer Figure 7.4, Figure 7.5 and Figure 7.6 show the same comparisons for specimens WG3, WG5 and WG10. Almost the complete soft-layer of specimen WG3 is deteriorated. Specimen WG5 features also large areas of completely decomposed granulate. The intact parts of the layer are heavily damaged on the surface. The soft-layer of specimen WG10 is also damaged on the complete surface and heavily deteriorated in the northern half, corresponding to the pulling direction, where the strength deterioration is more distinct. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 115

132 Figure 7.4. WG3 - Global behavior and damage of soft-layer Figure 7.5. WG5 - Global behavior and damage of soft-layer 116 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

133 Figure 7.6. WG10 - Global behavior and damage of soft-layer It can be concluded from these considerations, that the friction coefficient of the granulate DPC s is highly dependent on the degree of damage of the soft-layers. The decomposed particles of the granulate act similar to a roller bearing and reduce the friction coefficient in certain areas or in the complete joint. The sliding motion of the wall leads to further damage of the soft-layer, which leads to a lower friction coefficient in every cycle. Besides the friction coefficient, another material related parameter can influence the shear strength of the walls. In the experiments WE3R, WE5R and WE10 a slightly lower shear strength than the control specimen was measured (refer to Figure 7.1), even though these specimens showed the same flexural dominated behavior as the control specimen. It is assumed that the lateral deformation of the extruded elastomer soft-layers induce lateral tensile stresses into the adjacent bricks, which leads to a premature failure in compression Influence of soft-layer position Two different positions of the soft-layers in the wall have been investigated. The soft-layer can either be placed in the interface joint of the concrete slab and the masonry wall or in the first bed joint of the wall. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 117

134 The main influence of the position of the DPC is on the rocking force level. Positioning the soft-layer in the first bed joint leads to a smaller arm of the horizontal force due the fact, that in rocking mode the upper part of the wall is rocking on the bottom course of bricks. This increases the force that is necessary to induce a rocking motion of the wall. Figure 7.7 shows the rocking level and the sliding resistance for a granulate soft-layer ( = 0.44) for both positions of the DPC. The influence of the position on the governing failure mechanism is clearly visible. (a) Soft-layer in interface joint (b) Soft-layer in 1 st bed joint Figure 7.7. Comparison of rocking level to sliding resistance This explains why the specimens at 10% pre-compression with the granulate soft-layer in the 1 st bed joint showed a clear sliding behavior, while the specimens with the same precompression level but a different position of the soft-layer were mainly rocking. Another effect the position of the soft-layers can have is on the deterioration of the layers. Soft-layers placed on the concrete slab showed clearly lower degrees of degradation than softlayers placed directly on the masonry bricks. This can be explained with the rougher surface and the voids in the bricks. Putting the soft-layer between two layers of mortar could reduce the strength degradation for cases where the soft-layer is applied in the first bed joint Influence of soft-layer pre-compression The level of pre-compression has a strong influence on the governing failure mode and therefore on the strength, deformation capacity. But also for the same failure mode, there is 118 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

135 an influence of axial force. In general it can be said, that a wall with a high axial load has a higher shear strength but a lower deformation capacity. The axial load was only varied in the experiments with the soft-layers in the interface joints. Figure 7.8 shows the capacity curves and the corresponding bilinear idealizations for the 3mm granulate soft-layer specimens. A clear influence of the pre-compression level on the ultimate shear strength and the deformation capacity can be observed. (a) Capacity curves (b) Bilinear idealisations Figure 7.8. Capacity curves and bilinear idealizations of WG3 specimens at different pre-compression levels Although the width of the hysteresis loops seems to correlate to the axial load as well (Figure 7.9 (a)), Figure 7.9 (b) shows, that the ratio of dissipated energy to input energy can only be increased, when a sliding mechanism is induced. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 119

136 (a) Hysteresis loops (b) Dissipated energy ratio Figure 7.9. Hysteresis loops and ratio of dissipated energy for WG3 specimens at different pre-compression levels The 10 mm granulate specimens show the same dependence of axial load for the shear strength and deformation capacity. (a) Capacity curves (b) Bilinear idealisations Figure Capacity curves and bilinear idealizations for WG10 specimens at different pre-compression levels Figure 7.11 (b) seconds the conclusion, that the ratio of dissipated energy to input energy is in general uninfluenced by the axial load, unless a sliding motion occurs. Specimen WG10.5 shows a slightly higher ratio in the cycles where it was sliding in pulling direction. 120 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

137 (a) Hysteresis loops (b) Dissipated energy ratio Figure Hysteresis loops and ratio of dissipated energy for WG10 specimens at different pre-compression levels Influence of soft-layer thickness Figure 7.12 shows the capacity curves and the ratio of dissipated energy to input energy for the solid elastomer soft-layers in the 1 st bed joint. No significant influence of the soft-layer thickness can be found either for the shear strength, deformation capacity nor for the energy dissipation. (a) Capacity curves (b) Dissipated energy ratio Figure Capacity curve and dissipated energy ratio for solid elastomer layers in 1st bed joint at 10% precompression The same applies for the strength and deformation capacity for granulate soft-layers in the 1 st bed joint (seen in Figure 7.13). These were the specimens that showed more or less pure sliding behavior. An influence of the layer thickness can be found for the energy dissipation. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 121

138 Even though all specimens reach a similar maximum energy dissipation ratio of approximately 90%, it can be seen from Figure 7.13.b that the thinner the layer, the earlier the maximum energy dissipation ratio is reached. This corresponds well with the assumption, that the friction coefficient is damage dependent. (a) Capacity curves (b) Dissipated energy ratio Figure Capacity curves and energy dissipation ratio for granulate layers in 1st bed joint at 10% precompression Figure 7.14, Figure 7.15 and Figure 7.16 show the capacity curves and energy dissipation ratios for the granulate layers in the interface joint. It can be seen that in general the walls with a thinner soft-layer have a slightly higher shear strength but lower displacement capacity and energy dissipation ratio. This does not apply for specimens with a sliding behavior, seen in Figure (a) Capacity curves (b) Dissipated energy ratio 122 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

139 Figure Capacity curves and energy dissipation ratio for granulate layers in interface joint at 5% precompression (a) Capacity curves (b) Dissipated energy ratio Figure Capacity curves and energy dissipation ratio for granulate layers in interface joint at 10% precompression (a) Capacity curves (b) Dissipated energy ratio Figure Capacity curves and energy dissipation ratio for granulate layers in interface joint at 15% precompression 7.2. Influence on stiffness A clear influence of the soft-layers on the effective stiffness of the walls could be found in the experiments. Table 7.2 and Table 7.3 give the initial and effective stiffness of the specimens. Following conclusions can be drawn: Specimens containing a soft-layer have a lower stiffness than the control specimens With increasing thickness of the layers, the stiffness decreases (refer to Figure 7.18) Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 123

140 Specimens with granulate soft-layers have a lower stiffness than specimens with an extruded elastomer soft-layer The stiffness decreases with increasing axial load (refer to Figure 7.17) Pushing Pulling Specimen K0 Keff Keff/K0 K0 Keff Keff/K0 W0R WE3R WE5R WE WG WG WG Table 7.2. Initial and effective stiffness of specimens with soft-layer in 1st bed joint Pushing Pulling Specimen K0 Keff Keff/K0 K0 Keff Keff/K0 W WE WE WG WG WG WG WG WG Table 7.3. Initial and effective stiffness of specimens with soft-layer in interface joint The influence of the axial load can be seen in Figure The stiffness is clearly increasing with increasing axial load for the 10mm granulate layers. The 5% pre-compression specimen with a 3mm granulate layer is marking an exception to the rule, showing a higher stiffness than the 10% specimen. 124 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

141 (a) 10 mm granulate layer in interface joint (b) 3mm granulate layer in interface joint Figure Influence of axial load on effective stiffness The influence of the layer thickness is clearly visible non-regarding the material of the soft layers. (a) Extruded elastomer layers in 1 st bed joint (b) Rubber granulate layers in 1 st bed joint Figure Influence of layer thickness on effective stiffness 7.3. Strength prediction Based on the friction coefficients for soft-layer joints derived in Chapter 7.1.1, the strength estimations described in Chapter can be completed with an additional failure mechanism for the horizontal sliding along the joint containing a DPC. In this chapter, the experimental results are compared to the predicted failure modes and shear strength s. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 125

142 (a) Control specimen W0.10 (b) Control specimen W0R Figure Strength and failure mode prediction control specimens (a) Extruded elastomer soft-layers (b) Rubber granulate soft-layers, Figure Strength and failure mode prediction - Soft-layers in interface joint 126 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

143 (a) Extruded elastomer soft-layers (b) Rubber granulate soft-layers Figure Strength and failure mode prediction - Soft-layers in 1st bed joint Figure 7.19,, Figure 7.20 and Figure 7.21 show the measured strength of the specimens and compare them to the strength and failure mode prediction. The results of this comparison are summarized in Table 7.4 for the specimens with the soft-layer in the interface joint and in Table 7.5 for the specimens with soft-layers in the first bed joint. The failure modes are labeled with the following abbreviations: F for flexural failure (toe crushing), S for diagonal shear cracking of the bricks, SLD for sliding along the diagonal joint and SLH for sliding along a horizontal joint (containing the soft-layer). If a specimen failed in a combination of two failure modes, both modes are noted, starting with the one that occurred predominantly. The measured shear strength value corresponds to the loading direction where the failure occurred. Specimen Axial load [kn] Predicted failure mode Observed failure mode Predicted strength VP [kn] Measured strength VM [kn] W F SLD/F WE F F WE F F/SLD WG F F/SLH WG F F/SLD WG S S WG F F/SLH WG F F WG S S VP/VM [%] Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 127

144 Table 7.4. Strength and failure mode prediction - Soft-layers in interface joint Specimen Axial load [kn] Predicted failure mode Observed failure mode Predicted strength VP [kn] Measured strength VM [kn] VP/VM [%] W0R F F WE3R F F WE5R F F WE F F WG SLH SLH WG SLH SLH WG SLH SLH Table 7.5. Strength and failure mode prediction - Soft-layers in 1st bed joint In general it can be concluded, that the failure mode can be predicted fairly well with the applied methods described in Chapter The strength however, is clearly underestimated for the specimens with the soft-layer in the interface joint. The predicted strength by average only amounts to 83% of the measured strength for these specimens. On the other hand, the specimens with a DPC in the 1 st bed joint are generally well predicted or overestimated (refer to Table 7.5). This can be explained with the following two considerations: The coefficient of friction for joints with a granulate soft-layer was determined based on the measured strength of these specimens. Therefore it is clear that specimens failing in sliding along the soft-layer should be predicted fairly well. The bottom course of bricks in the specimens with extruded elastomer soft-layers showed heavy damage. The assumption has been stated earlier, that the lateral stresses due to the deformation of the elastomer induce additional lateral tensile stresses into the bricks of the adjacent courses. Therefore they might show a lower compressive strength and fail earlier in walls with a flexural behavior. 128 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

145 8. Conclusions From the discussion of the results of the preformed experiments, the following conclusions can be drawn: Shear strength, deformation capacity and energy dissipation of unreinforced masonry walls are mainly dependent on the failure mode. The sliding failure mode is connected with the largest deformation capacity and energy dissipation. It is therefore desired for walls subjected to seismic excitation. Soft-layers in general, have the potential to enhance the seismic performance of unreinforced masonry walls to a more desirable, quasi-ductile behavior with large deformation capacities. The main influence is the ability to provide a weak layer, where a horizontal sliding mechanism can occur before the wall fails in a different mechanism. A Mohr-Coulomb friction law with zero cohesion can be applied to calculate the sliding resistance of masonry walls along bed joints containing a damp proof course layer. Based on the performed experiments a friction coefficient of = 0.44 is proposed for rubber granulate soft-layers. Extruded elastomer layers have a higher friction coefficient. It cannot be determined from the experiments at hand, as the specimens with elastomer layers did not show a sliding behavior. The friction coefficient for granulate layers is damage dependent. With an increasing number of cycles, the decomposition of the soft-layer increases and the friction coefficient decreases. The granulate particles act as a roller bearing in the joint. The energy dissipation is higher for specimens containing soft-layers, non-regarding of the governing failure mechanism. However, layers that induce the sliding failure mode can increase the energy dissipation to a far higher degree. Walls failing in sliding along the soft-layer show considerably less damage due to the energy dissipation by sliding. Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 129

146 The effective stiffness of walls with soft-layers is lower than for conventional masonry walls. The stiffness decreases with increasing layer thickness and decreasing shear modulus of the layer material. The main influence of the position of the soft-layer in the wall is the change of the rocking level. The higher the soft layer is located in the wall, the higher is the resistance against rocking and therefore the friction coefficient of the joint containing a DPC can be higher to still induce the desired sliding mechanism. Placing granular soft-layers directly on bricks enhances the deterioration of the softlayers and therefore the decrease of the sliding resistance. A sandwich application, with mortar layers beneath and on above the soft-layer could enhance the performance of granulate soft-layers. Besides the very direct influence of the pre-compression level on the governing failure mode, an increased axial load is connected with higher strength and stiffness of the walls. No influence of the pre-compression level on the energy dissipation can be found if the failure mode is not changed. Unless sliding governs the behavior of the wall, specimens with thinner soft-layers show a higher shear strength but a lower deformation capacity and energy dissipation. 130 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

147 9. References American Society of Civil Engineers (ASCE), Seismic Rehabilitation of Existing Buildings (ASCE/SEI 41-06), Reston, V A. Barandun, A Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings. Master Thesis (unpublished). Chopra, A. K Dynamics of Structures: Theory and Applications to Earthquake Engineering. Fourth edition. Prentice Hall, Upper Saddle River, NJ. Griffith, M.C. & Page A.W On the seismic capacity of typical DPC and slip joints in unreinforced masonry buildings. Australian Journal of Structural Engineering. Javed, M., Alam, B. & Ali, S Seismic Resistance & Failure Modes in Unreinforced Masonry Shear Walls. International Journal of Civil, Structural, Environmental and Infrastructure Engineering Research and Development. Mageba Magenes, G. & Calvi, G. M In-plane Seismic Response of Brick Masonry Walls. Earthquake Engineering and Structural Dynamics, Vol. 26, pp Mann, W. & Müller, H Failure of shear-stressed masonry An enlarged theory, tests and application to shear walls. Proceedings British Ceramic Society, No proof course membrane: Assessment of shear strength parameters. Construction and Building Materials. 1. Tensile strength of clay blocks: an experimental study. Construction and Building Materials. -proof course membrane subjected to cyclic shear: An experimental study. Construction and Building Materials. Walls with soft-layer wall bearings. Deformation capacity of unreinforced masonry walls subjected to in-plane loading: a state-of-the-art review. International Journal of Advanced Structural Engineering. Capacity of Structural Masonry. 12 th Canadian Masonry Symposium Vancouver, British Columbia Simundic, G., Page, A.W. & Chen, Q The cyclic and long term behavior of slip joints in load-bearing masonry construction. Proceedings of the 12 th International Brick/Block Masonry Conference, Madrid, pp Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 131

148 Swissbrick, Erdbebensicherer Entwurf. Earthquake-resistant design of masonry buildings. Imperial College Press. ar resistance of masonry walls and Eurocode 6: shear versus tensile strength of masonry. Materials and Structures. Experimental Simulation. Journal of Structural Engineering, Turnsek, F. & Cacovic, F Some Experimental Results on the Strength of Brick Masonry Walls. 2 nd International Brick Conference Zhuge, Y. & Mills, J The behavior of masonry walls containing a damp proof course under cyclic loads. Proceedings of the 2 nd Australasian Structural Engineering Conference, Auckland. 132 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

149 10. Appendix Appendix A Test report of bricks Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings 133

150 134 Seismic Behavior of Unreinforced Masonry Walls with Soft-Layer Strip Bearings

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