Shaking table tests on the large scale model of Mustafa Pasha Mosque without and with FRP

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1 Structural Analysis of Historic Construction D Ayala & Fodde (eds) 2008 Taylor & Francis Group, London, ISBN Shaking table tests on the large scale model of Mustafa Pasha Mosque without and with FRP L. Krstevska, Lj. Tashkov & K. Gramatikov University St. Cyril and Methodius, Skopje, Republic of Macedonia R. Landolfo, O. Mammana, F. Portioli & F.M. Mazzolani University of Naples Federico II, Naples, Italy ABSTRACT: Shaking table tests were carried out on a large scale model of Mustafa Pasha Mosque in Skopje within the activities of Sixth Framework Program PROHITECH Earthquake protection of historical buildings by reversible mixed technologies. The main aim of experimental investigation was to evaluate the seismic stability of the monument after applying a reversible technology for strengthening. Beside the experimental shaking table testing of the model, three-dimensional finite element analyses were carried out using the general purpose software package ANSYS. The numerical models were calibrated on the basis of mechanical properties of masonry obtained from compression and shear tests on wall specimens built with same technique used for the original prototype. The comparison between experimental and numerical results allowed the reliability of the implemented models to be evaluated. Finally the effectiveness of strengthening and the evolution of crack patterns observed during each phase of the testing programme has been analyzed on the basis of the obtained results. 1 INTRODUCTION Shaking table testing of the large scale model of the Mustafa Pasha mosque in Skopje, (15th century) was performed in the Laboratory of the Institute of Earthquake Engineering and Engineering Seismology in Skopje in the period November-December The main objective of the tests was to investigate experimentally the effectiveness of the proposed reversible technology for strengthening of this type of historical monuments. The performed testing was a part of the activities within the Sixth Framework Programme PROHITECH Earthquake Protection of Historical Buildings by Reversible Mixed Technologies. In order to predict the response of the large scale model during the shaking table tests performed at the IZIIS Laboratory in Skopje, a finite element model was implemented. The main aim of numerical investigations was to evaluate the peak ground accelerations to be assigned to the shaking table during the test phases of experimental programme that were devoted to the assessment of the seismic strength of both the original and strengthened structure. In particular, it was important to control the damage on the original Minaret and on the Mosque in order to prevent collapse before the application of FRP strengthening. Moreover, the numerical models were used to determine suitable FRP strengthening schemes based on the calculated tensile stress distributions. To assess the seismic capacity of the structure and the effectiveness of FRP reinforcement, nonlinear pushover analyses were carried out by means of the implemented finite element model. In the following, a detailed description of the implemented models and of the corresponding obtained results is also reported. 2 EXPERIMENTAL INVESTIGATION The model of the mosque was constructed to a length scale of 1:6 according to the gravity forces neglected modeling principle, using the same materials as those of the prototype: stone, bricks and lime mortar (Gramatikov et al. 2007). The experimental testing was performed on the bi-axial seismic shaking table at the IZIIS Laboratory. Following the main objective, the testing was performed in three phases: 1) Testing of the original model for low intensity level, to provoke small damage; 2) Testing of repaired model and strengthened minaret with FRP, until total collapse of the minaret; 3) Testing of strengthened model with FRP and carbon fiber bars until collapse. 383

2 Figure 2. Damage to the minaret (horizontal crack) and of the mosque after phase 1. Figure 1. Dimensions and instrumentation of the large scale model of Mustafa Pasha Mosque. To investigate models response during seismic action different types of transducers were placed at characteristic points accelerometers, linear potentiometers and LVDTs, as shown in Figure 1 (Krstevska et al. 2007). 2.1 Phase 1 Testing of the original model Before starting the seismic testing, the dynamic characteristics of the model were defined by means of ambient vibration method as well as by low intensity random excitation within the frequency range of Hz. The accelerogram of the Montenegro earthquake of 1979 (Petrovac N-S component) was selected as a representative earthquake excitation. The earthquake time histories were scaled (compressed) 6 times, according to the similitude requirements and several tests were performed with intensity of g. Under input intensity of 2% g, the first horizontal crack appeared at the base of the minaret. In the next tests performed under intensities of up to 10% g, damage occurred on the mosque, too. The reason of this was the frequency content of the applied excitation, which was close to the natural frequencies of both the minaret and the mosque. Figure 3. Characteristic response time histories at the top of the model for input intensity 10%g, original model. The damaged model is presented in Figure 2, while some representative response parameters for input intensity of 10% g are given in Figure Strengthening of the minaret After performing the seismic tests on the original model of the mosque, the damaged model was repaired by crack injection and the minaret was strengthened by application of CFRP wrap, cut to have the corresponding width and applied upon a layer of epoxy glue (Fig. 4). The obtained strips with a width of 15 cm were 384

3 Figure 5. Collapse of the upper part of the minaret. Figure 4. Repaired model and strengthened minaret after phase 1 testing. placed on four sides along the length of the minaret in vertical direction for the purpose of its stiffening. To confine the structure, strips with a width of 10 cm were placed at four levels along the height of the minaret in horizontal direction, while a strip with a width of 20 cm was placed at its base. 2.3 Phase 2 Testing of repaired model and strengthened minaret Before the seismic tests, the dominant frequencies of the model were checked by random excitation. For the minaret dominant frequency was 4.7 Hz, while for the mosque two frequencies were dominating: f = 7.4 Hz and f = 9.6 Hz. During this testing phase, 11 tests were performed with input acceleration between 0.2 g to 1.5 g. The first cracks on the minaret occurred under input intensity of 0.34 g, which indicated that the applied strengthening enabled stiffening of the minaret and increasing of its bending resistance. The initial cracks in the mosque appeared under input intensity 0.42 g. During the next tests, the cracks developed and at input level of 0.49 g, the upper part of the minaret totally collapsed (Fig. 5). After reaching the max input acceleration of 1.5 g, the mosque model was heavily damaged and the testing was stopped. There were cracks in the walls and also in the dome (Fig. 6). Figure 6. Damaged model after phase 2 testing. The characteristic response parameters during the seismic tests are given in Table 1, while the representative time histories for the final test with an input intensity of 1.5g are given in Figure Strengthening of the model After the final test of the model in phase 2 the minaret was removed and the mosque rebuild on damaged parts. Then the model was strengthened. The main adopted principle in strengthening of the model was that the methodology to be applied be reversible and invisible. Hence, the cracks in the damaged model were not repaired by injection, but a concept was adopted that the model be strengthened to the conditions after the preceding tests. 385

4 Table 1. Performed seismic tests, phase 2. Test No Inp. acc (g) Top acc. CH1 Top acc. CH3 Input displ. (mm) Top displ. (dome) Figure 8. The strengthened model ready for testing, phase 3. course was formed whereby the tensile resistance of the wall was improved and synchronous behaviour of the bearing walls was achieved. Formation of a horizontal belt course around the tambour by applying a CFRP wrap with a width of 10 cm. Formation of a horizontal belt course at the base of the dome by use of a CFRP wrap with a width of 50 cm. The formation of these horizontal belt courses enabled better integrity of the tambour and the dome base and prevented opening of the dome which was the most common reason for occurrence and prolongation of cracks in the bearing walls. The strengthened model is presented on Figure Figure 7. Response during the test with intensity 1.5 g, phase 2. The strengthening consisted of incorporation of horizontal belt courses for the purpose of increasing the integrity of the structure at those levels and providing as better as possible synchronous behaviour of the bearing walls: Incorporation of carbon rods in two longitudinal mortar joints around the four walls at two levels: the level above the openings and at the top of the bearing walls, immediately below the tambour. With the incorporation of these carbon rods, a horizontal belt Phase 3 Testing of strengthened mosque model Before the seismic tests, the dominant frequencies of the model were checked. The resonant frequency of 9.2 Hz was obtained and it was compared to the frequency of 8.6 Hz measured after the last test of testing in phase 2. This pointed out that strengthening had increased the resonant frequency of the model for about 8%, meaning that the stiffness of the model hadn t been recovered completely compared to the initial state. During this phase, 24 tests were performed under input acceleration between 0.15 g to 1.5 g. The accelerogram of the Petrovac earthquake (N-S component) was scaled by 6 (compressed) in the first 15 tests. During the tests performed under input intensities between 0.15 g and 0.40 g. the model s behaviour was stable, without provocation of large cracks. In the next 6 tests carried out under input acceleration of g, the dome experienced sliding at a visible horizontal crack at its base. To provoke more intensive response of the model, in the next tests, a time scaling 386

5 Table 2. Performed seismic tests, phase 3. Input Scal. Input acc Top acc. displac. Top displ. factor (g) (dome) (mm) (dome) Figure 9. Damage of the model after phase 3 tests accomplishment. factor of 3 was used, producing an input acceleration of g. In this series of tests, many new cracks appeared in the walls as well as in the dome, decreasing the dominant frequency of the model to f = 4.4 Hz. This value was more than twice lower compared to the initially measured frequency of 9.2 Hz, thus indicating a pre-collapse state of the model. The next two tests were performed by a scaling factor of 2, under an input acceleration g. Progressive cracks appeared, but still without collapse. The final test was performed by a scaling factor of 1, under an input acceleration of 0.35 g. Heavy damage occurred in many places on the dome and around the openings as well as big cracks and inclination of one corner of the model, including damage to the strengthening FRP belt at the lower level of one of the walls, after which the testing was stopped. The damaged model is presented in Fig. 9.The characteristic response parameters during the seismic tests are given in Table 2. The representative time histories in the final tests are given in Fig. 10. As a common conclusion after the performed experimental testing of the mosque model in phase 3, it might be said that the model s behaviour was evidently different in respect to that of the original model. Under tests of moderate intensity, the existing cracks were activated but during the subsequent more intensive tests, the failure mechanism was transferred to the lower zone of the bearing walls, in the direction of the excitation, where typical diagonal cracks occurred due to shear stress. 3 FE ANALYSIS 3.1 Types of analysis, geometric modeling and meshing The finite element analysis of the large scale model was carried out using the ANSYS software. Push-over analyses were performed for the evaluation of structural capacity, both for original and reinforced model (Landolfo et al. 2007). The numerical model was generated by importing in the FE program a three dimensional solid model of the Mosque that was implemented in a computer aided design system. In order to properly evaluate the structural interactions among the different parts, the implemented geometrical model reproduces all the main elements of the building accurately, including the openings and the pendentives connecting the walls with the dome. Two separate FE models were implemented for the Minaret and the Mosque, respectively (Fig. 11). In the last case, the symmetry of the model along the vertical plane parallel to the direction of the input displacement was considered, in order to save CPU time for solving non-linear equations. To model the reinforcement, the outer surfaces of the Mosque and Minaret were divided into different areas corresponding both to the FRP sheets and bars. The properties of overlapping areas were set up in order to match the meshes corresponding to masonry and FRP reinforcement elements. The whole masonry structure was discretized with tetrahedral 3D solid elements considering a mesh size of mm in the case of Mosque model. SOLID45 elements were used for masonry and SHELL181 elements were employed to model the FRP sheets. The shells corresponding to the FRP reinforcement were directly overlapped to the masonry bricks 387

6 Table 3. Masonry material properties assumed in numerical simulations of Minaret and Mosque. Minaret Mosque γ [kn/m3 ] E [MPa] ν c [MPa] ϕ [ ] δ [ ] and no interface elements were considered. The parts of the Mosque model reinforced with bars were modelled with elastic solid elements through the whole thickness of the shear walls with Young s modulus equal to that of FRP material. 3.2 Material modeling Figure 10. Response of the model during the last test with intensity 0.35 g, scaling factor 1, phase 3. The material properties used for the two finite element models are reported in Table 3. In particular, the elastic parameters are referred to the values that have been calibrated on the basis of first random vibration tests performed on the original undamaged structure. The strength properties were determined on the basis of both compression and shear experimental tests carried out on masonry wall samples. As far as structural masonry, the elastic-perfectly plastic Drucker-Prager material model was considered. In the case of Mosque, cohesion c and angle of internal friction ϕ were calibrated in order to obtain values of 0.05 MPa and 1.0 MPa for tensile (ft ) and compressive strengths (fc ). For Minaret, the considered values for ft and fc were 0.1 and 1.0 MPa. In order to assign an associative flow rule for plastic strains, the assumed dilatancy angle δ was equal to φ. With regard to composites, an elastic material model was considered. In particular, the adopted Young s modulus for sheets is equal to 240 GPa and the considered equivalent thickness is 1.0 mm, according to the nominal mechanical properties provided by the manufacturer. 3.3 Load modeling and boundary conditions With regard to the load modelling approach implemented in pushover analyses for the evaluation of seismic capacity, a uniform and a linear acceleration distribution along the horizontal direction was applied to the FE models of Mosque and Minaret, respectively. As far as the boundary conditions are concerned, full restraints were assumed at the base of the structure in the performed analyses. 3.4 Results of the non-linear pushover numerical analyses Figure 11. The FE model of the Mosque and Minaret implemented for non-linear pushover analysis. The assessment of the ultimate seismic strength of both the original and reinforced large scale model was 388

7 Figure 12. Distribution of first principal plastic strains on the original large scale model of the Minaret at collapse load. carried out by means of nonlinear pushover analyses performed on the implemented FE model. On the basis of obtained numerical results, the evolution of damage distribution till collapse load on the investigated prototype has to be ascribed to the attainment of tensile or shear strength, while the compressive resistance is never exceeded. The collapse loads corresponding to the original and reinforced prototype were determined by checking the attainment of the maximum plastic strain calculated for masonry on the basis of the FE models calibrated against shear tests. With regard to the Minaret, the results of numerical analysis show that the reinforcement increases the ultimate strength in terms of peak acceleration at the top from the value of 0.3 g corresponding to the original model to 1.2 g and the collapse mechanism shifts to the upper part without strengthening (Figs 12, 13). As far as the Mosque is concerned and according to the implemented numerical model, the original prototype collapses with a mixed pier/spandrel mechanism of the vertical bearing structures, that is typical of weakly coupled perforated walls (Fig 14). On the basis of the numerical results, the evolution of collapse mechanism can be divided in different phases. According to the numerical model, the first diagonal tension cracks occur in the shear walls, namely in the spandrels between the first and second row of openings from the basement. In this phase, the damage also develops at the base of the walls perpendicular to the direction of ground motion, owing to bending stresses induced by Figure 13. Distribution of first principal plastic strains on the reinfroced large scale model of the Minaret at collapse load. Figure 14. Distribution of first principal plastic strains on the original large scale model of the Mosque at collapse load. out plane horizontal loads. In the second step, the damage extends to the upper spandrels, between the second and third row of opening in the shear walls parallel to the direction of seismic loads. In particular, it develops up to the dome, among the openings in the supporting polygonal drum and the ones in the shear walls. Finally, the seismic strength of the structure is attained when the central wall at the base of the Mosque and the lateral piers collapse for shear and bending mechanisms, respectively. The study of collapse mechanism, obtained from numerical analysis on both original and 389

8 1.8 Top Acc. [g] original minaret Exp. FE strengthened minaret Exp. FE Relative Displ. [mm] Figure 15. Distribution of first principal plastic strains on the reinforced large scale model of the Mosque at collapse load. Figure 16. Comparison between experimental and numerical response for the original and strengthened Minaret model. reinforced Mosque, allowed the effectiveness of FRP reinforcement to be analyzed. The wraps around the dome and the top of shear walls fully prevent the propagation of cracks from the bottom part to the drum, as shown in Figure 15. The role played by FRP bars in the shear walls is to stiffen and strengthen the spandrels forming a sort of reinforced masonry beams at different levels able to distribute the seismic action among the piers. According to the numerical model of the reinforced Mosque, in this case the collapse mechanism turns from a mixed into a weak piers/strong spandrel type. 4 COMPARISON BETWEEN EXPERIMENTAL AND NUMERICAL RESULTS The results of FE model were compared with experimental tests. The distribution of first principal plastic strains predicted by the numerical model fits well the crack patterns observed on the large scale model of the Mosque during shaking table tests. As shown in Figures 6 and 9, in the case of the original Mosque cracks formed on the spandrels between the openings up to the tambour, while in the reinforced model cracks on the bearing walls were observed at the base of the structure, according to the predicted damage pattern. With this regard, it should be noted, however, that sliding of the dome at tambour opening level has been observed before the collapse of the shear walls. This collapse mechanisms is not predicted by numerical results and can be ascribed to a different quality of masonry at the base of the dome from other parts of the Mosque. Top Acc. [g] original mosque Exp. FE strengthened mosque Exp. FE Relative Displ. [mm] Figure 17. Comparison between experimental and numerical response for the original and strengthened Mosque model. The comparison between experimental and numerical responses in terms of acceleration and relative displacement at the top is shown in Figures 16 and 17. Also in this case, the numerical and experimental results are in good agreement. In the case of Minaret, it should be noted that the experimental value of 0.2 g on the original model corresponds to first observed cracks and to the attainment of first plastic strains in the numerical model. However, during the testing the original Minaret did not collapse for values of peak acceleration at the top up to 0.7 g and the structure exhibited a rigid block behaviour with rocking. Also in the case of the strengthened Minaret the experimental peak accelerations at the top are higher 390

9 than numerical ones. Considering that a non-linear static FE analysis was performed, this discrepancy can be ascribed to cyclic loading induced by seismic excitations and to the rigid block-type behaviour of the Minaret observed during testing. 5 CONCLUSIONS In this study, a numerical and experimental investigation on the seismic strength of the masonry large scale model of the Mustafa Pasha Mosque reinforced with FRP has been presented. The experimental testing of Mustafa pasha large scale model was performed to investigate the seismic stability of the monument after applying a reversible strengthening methodology, both for the minaret and for the mosque. The strengthening applied to the minaret enabled stiffening and increasing of its bending resistance. The mosque model s behaviour after strengthening was evidently different in respect to that of the original model. Under tests of moderate intensity, the existing cracks were activated but during the subsequent more intensive tests, the failure mechanism was transferred to the lower zone of the bearing walls, in the direction of the excitation, where typical diagonal cracks occurred due to shear stress. In order to support the experimental test set-up, to design the FRP reinforcements and to analyze their effects on the prototype, a finite element analysis was performed. In particular, the implemented finite element model allowed the evolution of partial and global collapse mechanisms observed during the tests to be analyzed. The results of numerical investigation have been compared with the experimental outcome. Generally, a good agreement between the behaviour predicted by numerical models and test results was observed, both in terms of ultimate seismic capacity and collapse mechanisms. Further studies are in progress, mainly devoted to refine the calibration of FE model by implementing non linear time history analysis with other types of material models for masonry, such as smeared crack. The calibrated non-linear numerical models will represent a reliable tool both for the design optimization of FRPs and for the evaluation of the effectiveness of other types of strengthening on the original Mosque. ACKNOWLEDGMENTS It is significantly acknowledged the financial support of the European Commission (grant No. INCO-CT ), for funding the research project PRO- HITECH (Earthquake PROtection of HIstorical Buildings by Reversible MixedTECHnologies), which is the main framework of the experimental and numerical activity presented in this paper. REFERENCES Gramatikov, K., Taskov, Lj., Krstevska, L Final report on Shaking table testing of Mustafa-Pasha mosque model. FP6-PROHITECH Project, Workpackage 7-Experimental analysis. University Sts.Cyril and Methodius, Skopje, Macedonia. Krstevska, L., Taskov, Lj., Gramatikov, K., Landolfo, R., Mammana, O., Portioli, F., Mazzolani, F.M Experimental and numerical investigations on the Mustafa Pasha Mosque large scale model. In Proceedings of Cost Action C26Workshop Urban habitat constructions under catastrophic events, Prague , Landolfo, R., Portioli, F., Mammana, O., Mazzolani, F.M Finite element and limit analysis of the large scale model of Mustafa Pasha Mosque in Skopje strengthened with FRP. In Proceedings of the First Asia-Pacific Conference on FRP in Structures (APFIS 2007). Hong Kong, China, 1214 Dec