Energy Dissipation Properties of Infilled RC Frames Retrofitted with CFRP *

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1 Energy Dissipation Properties of Infilled RC Frames Retrofitted with CFRP * Principal Investigator: Assist. Prof. Dr. E. Yuksel, Istanbul Technical University, Istanbul. Ph.D. Student: H. Ozkaynak, Istanbul Technical University, Istanbul. Research Collaborators Assoc. Prof. Dr. C. Yalcin, Bogazici University, Istanbul. Prof. Dr. O. Buyukozturk, Massachusetts Institute of Technology, Cambridge, MA, U.S.A. Dr. A.A. Dindar, Istanbul Kultur University, Istanbul. Introduction Research Sponsored by The Scientific and Technological Research Council of Turkey Project Number: 106M050 Retrofitting and strengthening of existing reinforced concrete (RC) structures against earthquake effects has become a subject of focus during the last decade. Devastating earthquakes which occurs in recent years have shown that non-engineered concrete frames are particularly vulnerable to seismic action and are a major cause of loss of lives. Vulnerability of the existing low-rise structures with infilled RC frames which are not well designed and/or which lack construction quality has motivated us to initiate this research project. The main objective of this study is to determine the best energy dissipative retrofitting technique for an effective strengthening of the infilled RC frames by the use of Carbon Fiber Reinforced Polymers sheets. Infill walls in RC frames are essential stiffness, strength and damping sources and they increase the energy dissipation capacities of the overall structure. For this study, CFRP strips where applied to the infilled frames to convert the frame structure into a load resisting composite system with increased stiffness and strength capacities, and especially improved damping and energy dissipation properties. Alternative retrofitting schemes, namely diamond cross braced and cross braced configurations, have been evaluated in order to preserve the weak beam column joint region while keeping the integrity of the system. Methodology To investigate the beneficial effects of Carbon Fiber Reinforced Polymers on the seismic response of infilled RC frames, a comprehensive experimental schedule has been carried out in Structural and Earthquake Engineering Laboratory of Istanbul Technical University twenty two-1/3 scaled infilled RC 2D frames were tested both using the testing techniques of quasistatic and pseudo-dynamic. The lateral force representing ground motion was applied to the specimen in its own plane. Two separate inertia forces corresponding to the masses exerted on higher and lower stories of a mid-rise RC building were used in the pseudo dynamic tests and also two different drift based cyclic loading pattern were used in quasi-static tests. The test set-up is seen in Fig. 1. * Note: In this online article, we summarize the recently completed work which is expected to constitute a part of the Ph.D. thesis of H. Ozkaynak. Publications from this work are currently pending; any information used from this article should be properly referenced

2 Fig. 1 General View from the test set-up Descriptions of 4 different test specimens used in QS, low-inertia mass PsD and high-inertia mass PsD experiments is shown in Fig (a) Bare Frame (b) Infilled Wall (c) Cross-Braced (d) Cross Diamond-Braced Fig. 2: General geometry of the specimens - 2 -

3 Besides the cross braced retrofitting scheme, the effectiveness of the cross diamond-bracing system which is newly used in the literature, has been experimentally evaluated. [Yuksel E, et. al]. The seismic performance enhancement is evaluated in terms of the PGA level of the input acceleration record, maximum inter-storey drift, and energy dissipation capacity, variation of strength and stiffness and the observed damages. Summary of Test Results in Behavior The behaviors of two CFRP retrofitting schemes applied on the infilled RC frames, namely, cross-braced, and cross diamond-braced were experimentally investigated, and compared with the reference bare and infilled frame specimens. All the retrofitted specimens showed increased strength and rigidity when compared with the reference specimens. Except the cross diamond-braced frame, the post peak behavior of all retrofitted specimens showed a sudden drop in their lateral load levels. The cross diamond-braced frame exhibited the best behavior when compared with the others in terms of improved strength and the post peak behavior. The results of the tested specimens showed that the cross-braced and the cross diamondbraced frame specimens exhibited considerably less damage than the other reference specimens when compared at the same story drift levels. The energy dissipation capacities especially of the cross-braced retrofitted frames turned out to be more than those of the barely infilled frame, thus reducing the seismic demand imposed on the frames. With the reduced seismic demands the lateral forces induced to the structure decreases significantly and this ensures reduced stresses at the weak beam column joints of the existing structure. The use of this retrofitting technique is effective in limiting the crack widths with increased ductility in the behavior (Fig. 3). Fig. 3 The performances of the infilled, cross-braced, cross diamond braced frames respectively at 3% drift The test results showed a significant increase in the yield and ultimate strength capacities of the frames with a decrease in relative story drifts, especially in the cross-braced and the cross diamond-braced type of retrofitting schemes. The cross diamond-braced type of retrofitting scheme, which was positioned on the infill wall and outside the beam-column connection regions of RC frame, showed a better behavior than the cross braced retrofitting schemes. This scheme not only prevented the brittle shear failures of the infill wall, but also prevented the transfer of additional forces to the weak and brittle beam-column connections

4 PGA CRACK PATTERN STRAINING ACTION 0.40 g d1'=1.2 k2'=1.2 e1'=1.2 j1>3.5 a1'=0.1 c1'=0.1 l2'>3.5 j1'=0.1 n2'=0.1 f1'=0.1 b1'>3.0 g1'=0.1 m2'=0.4 h1'=4.0 a1>> n2=0.3 o2=0.2 k1'<=0.1 d1=0.4 m2>> h1=0.8 j2>3.5 b1=0.6 i2=0.4 e1<=0.1 c1=1.0 f1=0.8 g1>> PUSH PULL b3'= g e3'=2.0 d3'=3.0 b3=1.4 a1'=0.1 c3'=0.2 c3=0.2 a1=2.0 PUSH PULL Fig. 4 Damage Propogations comparison between barely infilled and diamond-cross bracing strengthening technique. Test Results with Various Parameters Load-displacement relationships For each specimen type, the force-displacement envelope obtained from QS testing are drawn with the maximum restoring force and its corresponding displacement couples are displayed for various inertial mass conditions and PGA levels as seen in Fig. 5. (a) Bare frame (b) Infilled frame - 4 -

5 (c) Cross-Braced frame (d) Cross Diamond-Braced frame Fig. 5 Restoring force-lateral top displacements including drift-based performance limits according to FEMA356 The chosen loading pattern for QS testing and its corresponding response turned out to be comparable with the response obtained by the PsD testing. A significant increase in strength from 40 kn to 150 kn was determined in the retrofitted specimens, especially in cross diamond-braced retrofitting scheme, when compared to the results of bare and infilled wall specimens. Another important observation was that the scattered coordinates of maximum restoring force and its corresponding displacement couples in bare frame specimen were concentrated within the ascending branch of response curve and in Immediate Occupancy region in the retrofitted specimens. Here, the cross diamond-braced retrofitting scheme behaved better than the cross braced retrofitting scheme and survived PGA=0.6g intensity level with M2 condition. Stiffness For each specimen type, the stiffness envelope which defined as the slope of the line drawn at peak to peak response coordinates, are drawn with the maximum restoring force and corresponding displacement couples of PsD tests for various inertial mass conditions and PGA levels as shown in Fig. 6. (a) Bare frame (b) Infilled frame - 5 -

6 (c) Cross-Braced frame (d) Cross Diamond-Braced frame Fig. 6 Restoring force-lateral top displacement envelopes The stiffness values calculated for QS and PsD tests are consistent with each other. Also the observed stiffness values calculated for retrofitted specimens are well above the nonretrofitted ones. It was observed that the stiffness values corresponding to various PGA levels and mass conditions accumulated within the IO region for retrofitted specimens whereas these coordinates are scattered in bare and infilled frame specimens. This indicates the effectiveness of retrofitted specimens in terms of improved stiffness. Energy dissipation capacity The values of the cumulative energy dissipation were calculated as the enclosed area of hysteretic restoring force etc. The energy dissipation capacity of the retrofitted specimens increased significantly when compared with the non-retrofitted specimens as given in Fig. 7. Cross-braced retrofitted frame dissipated 4.6 times more energy than the bare frame. For the diamond cross-braced retrofitted frame this value was increased to 5.2. When the retrofitted frames are compared with the infilled frame, the priors dissipated 1.7 times more than the latter. (a) Bare frame (b) Infilled frame - 6 -

7 (c) Cross-Braced frame (d) Cross Diamond-Braced frame Fig. 7: Restoring force-lateral top displacement envelopes As seen in Fig. 7, although the dissipated energy values obtained from QS and PsD tests are close to each other and follow a similar trend, it was observed that the PsD energy values are slightly over the QS energy values. During testing, for the same level of drift, more damage was observed in PsD tests than QS tests. This may also explain less energy dissipation in QS tests given that damage and energy dissipation is directly proportional to each other. Also, the higher number of zero crossing in the loading pattern of PsD test might result more accumulated energy dissipation and thus more damage than that of QS test. The infilled frames with or without retrofitting, regardless of the PGA levels, had no effect on the overall performance in low mass inertia condition (M1). However, in higher mass condition (M2) the significance of retrofitting is apparent and from bare frame to retrofitted frames the cumulative energy couples move towards lesser dissipated energy levels indicating gained higher stiffness and strength. Concluding Remarks: 1. The maximum restoring forces and their corresponding drifts that were obtained from PsD tests showed a close behavior pattern, regardless of the level of inertial masses, compared with QS tests. 2. Although general behavior of any specimen could be obtained in QS test using a common drift-based loading protocol, it is not possible to predict the restoring force levels corresponding to PGA levels. However, in PsD tests, the maximum strengths for low (M 1 ) and high (M 2 ) inertial masses were observed at PGA=0.6g and 0.4g, respectively. 3. Only the flexural cracks for M 1 case on columns at the end of the PGA=0.6g PsD test were observed, while for M 2 case, in addition to flexural cracks, separation of wall from the frame as well as diagonal cracks on the wall were observed at the end of the PGA=0.4g PsD test. This shows the effect of inertial masses on the damage magnitude. Even though similar damages were observed during the QS tests, it is not possible to determine which level of PGA s created these damages. 4. When both test methods are compared for the same drift ratios, relatively more damage were observed during PsD tests compared with QS tests. On the other hand, in PsD tests, similar damages were observed for different inertial mass cases. It may be concluded that - 7 -

8 since the number of the reverse cycles in the PsD loading is greater than that of QS loading caused more damages in PsD tests. 5. The cumulative energy dissipation is found to be comparatively less in QS tests for the varying drift ratios due to the greater number of reverse cycles used in PsD tests. 6. Another outcome of this study was that the results from the different loading patterns in stiffness and energy dissipation values were similar to each other. Energy terms may be overestimated with the PsD results. This may lead to the conclusion that the QS loading pattern is appropriate. This aspect should be further studied. 7. The observed initial stiffness and critical damping ratios had a good convergence in both low and high mass inertia mass conditions. Also, there is an increase in the critical damping ratios in the infilled and retrofitted infilled frames. 8. From the PsD results in damage it is seen that for the same drift ratios PsD type loading may result in more damage in primary and secondary elements