Seismic isolation, strengthening of walls with CFRP strips and heritage masonry buildings

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1 Structural Analysis of Historic Construction D Ayala & Fodde (eds) 2008 Taylor & Francis Group, London, ISBN Seismic isolation, strengthening of walls with CFRP strips and heritage masonry buildings M. Tomaževič, I. Klemenc & P. Weiss Slovenian National Building and Civil Engineering Institute, Ljubljana, Slovenia ABSTRACT: The efficiency of improving the seismic resistance of heritage masonry buildings by means of seismic isolation and strengthening of structure with CFRP laminate strips has been investigated. Five models of a simple two-storey brick masonry building with wooden floors without wall ties have been tested on the shaking table. Besides control model, two models, isolated by either damp proof course or seismic isolators, have been tested. Models four and five have been strengthened with CFRP laminate strips, simulating the wall ties placed horizontally and vertically at floor levels and corners of the building, respectively. One of the CFRP strengthened models has been placed on seismic isolators. Tests have shown that the damp proof course, unless adequately designed, cannot be considered as seismic isolation. The isolators have also not improved the behavior in the case of the building without wall ties. However, both models confined with CFRP strips exhibited significantly improved seismic behavior. They did not collapse even when the accelerations of the shaking table exceeded the accelerations, measured when tested the control models without wall ties, by three times. 1 INTRODUCTION Besides traditional technologies, such as the tying of walls with steel ties, strengthening the walls by injecting cementitious grouts and applying reinforced cement coating, which have been used in the last decades, new materials and technologies have been developed and proposed also for upgrading the seismic resistance of heritage buildings. Among them, seismic isolation in different forms (e.g. Bailey and Allen, 1988; Page, 1995; Sarrazin et al., 1996; Zhou and Miao, 1996,) and strengthening the walls with fiber reinforced polymer laminates have been already studied (e.g. Schwegler, 1995;Triantafillou and Fardis, 1997; Hamilton and Dolan, 1998; Gayevoy and Lissel, 2004; Trantafillolu, 2001). Although the requirements of preservation of cultural heritage limit the application of such materials and technologies, the methods are convenient because they require minimum intervention in the existing structural system by providing substantial improvement in seismic behavior at the same time. Recently, experiments to investigate some aspects of seismic isolation and possibility of tying the walls of heritage masonry buildings with CFRP laminate strips instead of steel ties, have been also carried out at Slovenian National Building and Civil Engineering Institute in Ljubljana. Experiments and test results will be presented and discussed in this contribution. 2 RESEARCH PROGRAM AND DESCRIPTION OF TESTS 2.1 Research program Typically, the walls of heritage masonry buildings are not tied with wall ties, what results into the separation of walls during earthquakes and subsequent out-ofplane collapse. Consequently, when retrofitting the building for seismic loads, the tying of walls with steel ties represents the basic measure to ensure the structural integrity and utilize the resistance capacity of structural walls. In this regard, one of the objectives of the study was to investigate the possibility of omitting the installation of wall ties by placing the structure on seismic isolators. Besides, the possibility of using a simple PVC damp proof course as isolating device, has also been studied. Last but not least, the idea to replace the usual steel ties with CFRP laminate strips, placed both horizontally at the level of floors and vertically at the corners of the building, has been investigated. 2.2 Description of models Taking into consideration the payload capacity of the simple uniaxial shaking table, installed in the structural laboratory of the institute, experiences and available materials, the tests have been carried out on models constructed at 1:4 reduced scale. Five models with basically the same structural layout have 789

2 Figure 2. Typical model during construction. Figure 1. Scheme of laying the bricks and position of wooden joists and lintels. been constructed and tested, prepared for testing in five different ways. Since the main objective of the study was limited at obtaining basic information about the possibility of application and efficiency of tested strengthening method, a single room brick masonry house with wooden floors without wall ties has been tested. The models maintained the basic structural characteristics of typical buildings, such as storey height, span between the structural walls and openings size, with outer dimensions adjusted to the size of the platform of the shaking table. The scheme of laying the bricks and placing the wooden joists is shown in Figure 1, whereas a typical model under construction is shown in Figure 2. Model M1 represented the control model with wooden floors without wall ties. It has been constructed directly on the r.c. foundation slab, fixed to the moveable platform of the shaking table, without any specific measures to improve the seismic behavior. Model M2 was similar, however, a simple PVC sheet has been placed as a damp-proof course in the bedjoint between the second and third course of masonry units. Model M3, also similar to model M1, has been isolated with isolators, placed between the foundation slab and the model s upper structure. The effect of tying the walls with CFRP laminate strips has been studied on Models M4 and M5. The strips which simulated horizontal and vertical ties (confining elements) have been bonded to masonry on the outer side of the walls. Horizontal strips have been placed at the level of wooden floors. They have been connected together at the corners of the building with steel anchor plates. Vertical strips, placed at the corners, have been glued at the bottom of the walls on steel angle profiles, bolted into the model s r.c. foundation Figure 3. Position of CFRP laminate strips used to confine the models and strengthening of walls. Transverse section. slab. In addition, the piers between the openings have been strengthened with diagonally placed CFRP laminate strips without any special provision for anchoring at the ends. The position and dimensions of CFRP strips are shown in Figures 3 and 4. In the same figure, the main dimensions of the models are also indicated. Whereas model M4 has been built directly on the foundation slab as was the case of control model M1, model M5 has been placed on seismic isolators as was the case of model M3. Whereas model M3 has been placed on 790

3 Figure 5. Calibration test of seismic isolator. had mechanical characteristic similar to the prototype ones. The following mean values have been obtained: Figure 4. Position of CFRP laminate strips used to confine the models and strengthening of walls. Longitudinal section. six isolators, only four isolators have been used in the case of model M5 in order to further shift the natural frequency of vibration of the isolated model from the predominant frequency of the model earthquake. Since physical modeling of CFRP laminates and bonding properties turned out to be rather complicated if impossible, the technique of simple modeling has been used so that the strength characteristics of brick masonry remained close to the prototype values. Tests of model masonry walls indicated that similarity in non-linear behavior and failure mechanism at the simulated conditions of prototype loading has been fulfilled just as well. At the same time, the distribution of lateral stiffnesses and masses along the height of the models was in the proportions usual for typical prototype buildings, hence fulfilling also the conditions of similarity of dynamic behavior. Since all models have been tested in equal loading conditions, their seismic behavior can be directly compared. However, when calculating the values of physical quantities measured on the models to prototype, model scale factors for the case of the simple model similarity should be taken into consideration. The mechanical properties of model masonry, determined by compression tests (3 specimens) as well as cyclic lateral resistance tests at different levels of precompression (7 specimens) indicate that the models have been constructed with model masonry, which compressive strength: f = 6,1 MPa, modulus of elasticity: E = 1864 MPa, tensile strength: ft = 0,23 MPa, shear modulus: G = 678 MPa, and ductility factor: µ = 3,9. As a damp-proofing element, a commercially available PVC sheet, 2 mm thick and cut to fit the dimensions of the cross section of the walls, has been used. The PVC sheet has been placed in the mortar bed joint between the second and the third course of units of the walls in the ground floor of model M2. The sliding mechanism and friction characteristics of the dampproof course in relation to vertical stresses in the walls have been determined by testing. The isolators were 92 mm in diameter and 100 mm high. They have been manufactured of vulcanized rubber, 8 mm thick. In order to keep adequate stiffness in vertical direction, ten pieces of 2 mm thick steel sheets have been uniformly distributed along the height of each isolator. Before placing and fixing the isolators in the position, the deformability characteristics of each isolator have been determined by calibration tests (Figure 5). The measured average lateral stiffness at lateral displacement d = 10 mm was KH = 0,0325 kn/mm, at displacement d = 30 mm, KH = 0,0284 kn/mm, whereas the average stiffness in vertical direction amounted to Kv = 1,64 kn/mm. To confine the structure and tie the walls with horizontally and vertically placed CFRP laminate strips, as well as to strengthen the walls with diagonally placed strips, readily available CFRP laminate, 1,2 mm thick, has been cut to 2 and 3 cm wide strips. The strips have been glued on the masonry according to the instructions provided by the manufacturer. The tensile 791

4 Table 1. Maximum accelerations and displacements of the shaking table, measured during individual test runs (average values for the groups of models). Models M1, M2 and M3 Models M4 and M5 Duration Test run amax (g) dmax (mm) amax (g) dmax (mm) 12 s R005 R025 R050 R075 R100 R150 R200 R300 R350 0,028 0,131 0,386 0,505 0,688 0,795 3,480 7,116 10,827 14,620 0,027 0,109 0,239 0,380 0,483 0,727 1,015 2,682 3,555 0,600 2,854 5,660 8,509 11,322 17,034 22,721 34,002 39,492 Figure 6. Typical response spectra of shaking table motion. strength of material, Sika CarboDur S, in the direction of fibers amounts to 3000 MPa, and the modulus of elasticity to MPa. Before gluing the strips, the surface of masonry has been thoroughly cleaned and penetrated with primer. Original epoxy adhesive material, SikaDur, has been used to glue the strips on the masonry. 2.3 Seismic load and instrumentation of models The shape of the ground acceleration time history, used to control the shaking table motion, corresponded to the 24 seconds long strong phase of the N-S component of the ground acceleration record, recorded at Petrovac during the Montenegro earthquake of April 15, Maximum measured ground acceleration was 0,43 g. The actual model earthquake, prepared for testing the so called complete models, was 12 seconds long, and had the same maximum ground acceleration as was the case of the actual acceleration record. The shaking table motion during test run R100 represented such an earthquake (100% of intensity). Since the models have been made of materials with strength characteristics similar to the prototype (simple models), the actual model earthquake represented 48 sec long earthquake (St = SL = 4) with maximum ground acceleration 0,11 g (Sa = 1/SL = 0, 25). Shaking table displacements in each successive test run have been scaled from 5% to 350% of those of the model earthquake (test runs R005 to R350, respectively). All models have been tested with the same sequence of seismic excitations with increased intensitiy of motion in each successive test run, the characteristics of the model earthquake did not influence the observations. Maximum accelerations and displacements of the shaking table motion obtained in Figure 7. Instrumentation of models. each test run are given in Table 1, whereas the typical response spectra are shown in Figure 6. All models have been instrumented with a set of displacement transducers and accelerometers (Figure 7), fixed to the models at the level of floors. The missing live load at the levels of floors has been modelled by means of concrete blocks of adequate mass, which have been fixed to wooden joists with steel bolts so that the in-plane rigidity of floors has not been significantly affected. In order to prevent damage to instruments and shaking table at the moment of collapse, concrete blocks have been loosely hanged on the crane.all models have been oriented so that the direction of shaking table motion coincided with longer dimension of the 792

5 Figure 8. Mechanism of collapse of non-strengthened control model M1. Figure 9. Mechanism of collapse of non-strengthened model M2 with damp proof course. model. In other words, seismic loads acted in the direction of load-bearing walls, pierced with window and door openings. 3 TEST RESULTS 3.1 Failure mechanism The control model M1 exhibited typical behaviour of old masonry buildings with wooden floors without wall ties: in the beginning of tests when subjected to low intensity earthquake ground motion, the behaviour was monolithic. However, with increased intensity of shaking, vertical cracks developed in the upper part of the model. As a result of separation of walls, the upper storey of the model disintegrated in the subsequent test runs and collapsed (Figure 8). The tests of model M2 have shown that the dampproof course in the form of a simple PVC sheet placed in the mortar in the bed joint cannot be considered as seismic isolating device. Although the compressive stresses in the walls with installed damp-proof course were low, the measurements have indicated that neither sliding along the damp-proof course took place nor rocking motion of the upper part of the building has been observed. The walls in the upper storey disintegrated and the storey collapsed at the same intensity of excitation as was the case of control model M1 (Figure 9). Although improved behaviour of model M3, placed on rubber seimic isolators, has been expected, model M3 exhibited practically the same poor behaviour as Figure 10. Mechanism of collapse of non-strengthened and isolated model M3. non-isolated models M1 and M2. However, a slight difference in the sequence of damage propagation has been observed. Whereas damage propagated gradually in dependence on intensity of motion in subsequent test runs in the case of models M1 and M2, the collapse of model M3 was sudden, without cracks occuring during the previous test runs (Figure 10). 793

6 Figure 11. Damage to CFRP laminate strengthened, nonisolated model M4 at the end of shaking table tests. Figure 13. Detached anchor plate caused rocking of model M4. Vertical strip buckled at the bottom of the model. Figure 12. Unsignificant damage to CFRP laminate strengthened, isolated model M5 at the end of shaking table test. The seismic behaviour of both models strengthened with CFRP laminate strips, however, was significantly improved. They did not suffer severe damage or collapse even when subjected to ground motion with accelerations, which by more than three times exceeded the accelerations measured during the testing of non-strengthened models (Figures 11 and 12). Since the capacity of shaking table has been reached and the output motion already distorted, tests had to be terminated at that point. In the case of non-isolated model M4 the anchor bolts, by means of which steel anchor angles of vertical strips have been fixed to the foundation slab, pulled out (Figure 13) and the model started rocking on the foundation slab. Consequently, masonry crushed at the corners and severe cracks occurred in the lintel parts of the walls. In the case of model M5 on seismic isolators, one of the isolators detached (Figure 14). However, almost no damage has been observed in the model s walls. It has to be noted, that also in the case of model M4 no structural damage has been observed before the pulling out of anchor bolts. By comparing the results of tests of CFRP laminate strengthened and non-strengthened model walls, it seems that, in the particular case studied, this is the result of confining the model structure with horizontal and vertical CFRP strips, and not the result of diagonally placed strips on the wall piers. However, additional measurements should have been carried out in order to confirm this observation. The changes in dynamic characteristic of the tested models, measured before the tests and after each subsequent test run, are presented in Table 2. The values of the first natural frequency of vibration f and coefficient of equivalent viscous damping ζ (in % of critical damping), have been determined by hitting the model with impact hammer and analyzing the 794

7 Figure 14. Detachment of isolator at the end of tests of model M5. Table 2. First natural frequency of vibration f (in s 1 ) and coefficient of equivalent viscous damping ζ (in % of critical damping) measured on the models before the beginning of shaking table tests and after characteristic test runs. Model M1 M2 M4 M5 f (s 1 ) ζ(%) f (s 1 ) ζ(%) f (s 1 ) ζ(%) f (s 1 ) ζ(%) Before test R50 R75 R150 R300 15,6 13,5 19,0 5,5 21,2 3,8 2,2 11,6 12,3 16,7 15,3 13,4 20,6 5,0 2,1 10,2 12,3 15,5 13,9 13,8 19,5 9,5 2,1 10,1 18,9 10,2 2,1 10,0 12,6 8,9 2,1 10,3 Figure 15. Response of control model M1 during test run R075. measured response. Fourier analysis of acceleration and displacement records has been used to obtaine these data. Unfortunately, dynamic characteristics of isolated, non-strengthened model M3 have not been measured. As expected, a trend of degradation of the first natural frequency of vibration and increase in values of coefficient of equivalent viscous damping can be observed with increased intensity of excitation in all cases, except in the case of isolated model M5, which has not suffered damage during testing. The differences in initial values, especially in the case of models M1 and M2 can be mainly attributed to relatively large scattering of deformability characteristic of used model masonry materials. The increase in stiffness in the case of model M4, confined with CFRP laminate strips, however, can be attributed to the effect of confining the model structure horizontally and vertically with rigid elements. No changes in the first natural frequency of vibration and damping in the case of isolated model M5 are the result of seismic isolators which reduced the response and prevented structural damage. As an indication of efficiency of confining the structures wth CFRP laminate strips, measured top acceleration and relative first storey displacement responses of two of the tested models are shown in Figures 15 and 16. As a measure of intensity of excitation, shaking table acceleration time history is also plotted in each figure. In the figures, the responses of control model M1 and CFRP laminate strengthened model M4 to seismic excitation of the same intensity are compared (test runs R75 and R300, respectively (see Table 1 for comparison of maximum shaking table accelerations). Maximum values of the base shear evaluated on the basis of the measured acceleration responses of the models during each test run are compared in Table 3. Base shear is given in terms of the base shear coefficient BSC, i.e. the ratio between the base shear BS developed in the model during shaking and the weight of the model above the base W : BSC = BS/W. Base shear has been calculated as the sum of products of masses, concentrated at the levels of floors mi and measured average maximum values of accelerations at the same level ai : BS = mi ai. Since all models were equal, the mass concentrated at the second floor was taken as m2 = 287 kg, mass at the first floor m1 = 448,4 kg, whereas the total mass of 795

8 the case of model M4 without isolators developed in the model at full intensity of shaking table motion. 4 CONCLUSIONS Figure 16. Response of CFRP laminate strengthened model M4 during test run R300. Table 3. Maximum base shear coefficient evaluated on the basis of the measured response of the models during characteristic test runs BSC = m i a max,i /W. Model R50 R75 R100 R150 R200 R300 M1 0,245 0,662 0,306 M2 0,225 0,453 0,511 M3 0,305 0,434 0,492 M4 0,228 0,380 0,488 0,739 1,050 2,032 M5 0,108 0,322 0,415 0,564 0,702 0,895 Experiments have shown that a simple damp-proof course in the form of PVC sheet installed in the mortar bed joint cannot be considered as seismic isolation. Experiments have also shown that seismic isolation alone is not enough to improve the seismic behaviour of heritage masonry buildings without wall ties. However, the shaking table tests of models confined with horizontal and vertical CFRP laminate strips indicated significantly improved seismic behaviour. The CFRP laminate confined models did not collapse even when subjected to ground accelerations which by more than three-times exceeded accelerations causing the collapse of the models without wall ties. The experiments indicated the possibility of replacing the commonly used steel ties by CFRP laminate strips. Placed also vertically, CFRP laminate strips additionally strengthen the structure, if properly anchored to foundation system at the ends. The experiments also confirmed the long known fact that seismic isolation of rigid masonry structures represents an efficient way to reduce seismic loads in the case of the usual short periodic earthquakes. However, the isolation does not permit that the usual meaures of seismic strengthening, such as the tying of walls with wall ties, be omitted. The experiments indicated the efficiency of contemporary technical solutions. However, they also pointed out that technological problems need to be resolved before wider application of such methods to heritage masonry buildings. In this regard, efficient interaction between the materials with extremely different mechanical characteristics, such as CFRP laminates and masonry is critical. the model above the foundation slab, on the basis of which the weight of the model W was calculated, was m tot = 856,8 kg. From the results of tests, summarized In Table 3, it can be clearly seen that no basic difference in the resistance has seen obtained in the case of the models without wall ties. Whereas control model M1 attained its maximum resistance during test run R75, models M2 and M3 attained the maximum of somewhat smaller value at the same intensity of motion during test run R100. Significant improvement in resistance (by four times) has been obtained in the case of CFRP laminate confined model M4. In the case of CFRP confined and isolated model M5, noticeable reduction of seismic forces can be observed. As a result of isolation, more than two times lower baser shear than in ACKNOWLEDGEMENTS The research presented in the paper has been carried out within the framework of research project L and research program P2-0274, financed by the Ministry of High Education, Science and Technology of the Republic of Slovenia and co-financed by rubber industry, Sava Company Ltd., program Construmat from Kranj, Slovenia. CFRP laminates and bonding materials have been given at disposition free of costs by Sika AG, Slovenian branch from Trzin, Slovenia. REFERENCES Bailey, J.S.,Allen, E.W. (1988). Seismic isolation retrofitting: Salt Lake City and County building. APT Bulletin, 20 (2). 796

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