Seismic strengthening of reinforced concrete frames by precast concrete panels

Transcription

1 Magazine of Concrete Research, 2011, 63(5), doi: /macr Paper Paper received 02/02/2010; last revised 15/04/2010; accepted 28/04/2010 Published online ahead of print 08/04/2011 Thomas Telford Ltd & 2011 Seismic strengthening of reinforced concrete frames by precast concrete M. Baran Department of Civil Engineering, Kirikkale University, Yahsihan/Kirikkale, Turkey D. Okuyucu Department of Civil Engineering, Structural Mechanics Laboratory, Middle East Technical University, Ankara, Turkey M. Susoy OM Engineering Services, Inc., Orlando, Florida, USA T. Tankut Department of Civil Engineering, Structural Mechanics Laboratory, Middle East Technical University, Ankara, Turkey An innovative occupant-friendly retrofitting technique has been developed for reinforced concrete (RC)-framed structures which constitute the major portion of the existing building stock. The idea is to convert the existing hollow brick infill wall into a load-carrying system acting as a cast-in-place concrete shear wall by reinforcing it with relatively thin high-strength precast concrete epoxy bonded to the plastered infill wall and epoxy connected to the frame members. In this study, results of 11 one-third scale, one-bay, one-storey RC frames, tested by the authors, having the deficiencies commonly observed in residential buildings in Turkey, tested under reverse-cyclic lateral loading until failure are given in detail. It was clearly observed that the seismic performance indicators such as strength, lateral stiffness and energy dissipation capacity of the strengthened frames were significantly improved by the proposed technique. Notation A c gross cross-sectional area of the section A st total cross-sectional area of the longitudinal reinforcements in the section f c 9 specified compressive strength of the concrete f y yield strength of reinforcing bars h height of the first storey N axial load applied to each column N 0 axial load capacity of the column section ä first-storey level displacement at a specified load level ö diameter of the bar Introduction Most of the existing reinforced concrete (RC) residential buildings in Turkey are seismically deficient. Therefore, a huge existing In the proposed technique, the basic idea is to convert the existing building stock awaits seismic vulnerability assessment followed by hollow brick infill wall into a load-carrying system acting as a seismic retrofitting since they endanger public safety in a possible cast-in-place concrete shear wall by reinforcing it with relatively future earthquake. Adding new shear walls to RC frames is a thin high-strength precast concrete to be epoxy bonded to common and reliable method of system improvement studied by the plastered infill wall and epoxy connected to the surrounding various researchers (Altin et al., 1992; Sonuvar et al., 2004; Turk frame members. One single piece of precast concrete panel would et al., 2003). By constructing a cast-in-place RC infill wall, in definitely be unmanageable, too large to go through doors and too some cases in the place of a partitioning wall, the building gains heavy to be carried manually. Therefore, it has to consist of considerable strength and lateral stiffness increases. Many buildings individual of manageable size and weight, and has to be in Turkey were repaired or strengthened with this method, assembled on the wall by connecting the together. The especially after major earthquakes such as the 1992 Erzincan and would be readily available and easily transported and 1999 Kocaeli earthquakes. Moreover, there are some drawbacks of assembled to their place at the site. cast-in-place infill wall strengthening. The application of this method requires heavy construction work, so it is necessary to A total of nearly 40 one-bay, one- or two-storey RC frames were evacuate the building. The workmanship in this rehabilitation method is difficult and time consuming. Cast-in-place RC infill wall application is naturally very suitable for post-quake strengthening applications of damaged buildings that are already evacuated. Therefore, a very challenging aspect of the problem is how to introduce an economical, structurally effective and practically applicable pre-quake strengthening intervention without evacuating the building, even without causing more disturbances to the occupants than a painting job. The experimental study presented in this paper concerns the development of an innovative occupantfriendly retrofitting technique (Baran, 2005; Baran et al., 2003; Duvarci, 2003; Susoy, 2004; Tankut et al., 2005), suitable for the RC framed structures with hollow brick masonry infill. 321

2 quasi-statically tested in this comprehensive research. Most of the frames tested were two-storey specimens, even with varying aspect ratios. From the tests, similar and compatible results were obtained for both one-storey and two-storey frames, which indicated that both frame types could equally be acceptable as test units and onestorey frames would be used for taking advantage of simplicity in specimen construction, testing procedure and limitations stemming from the test set-up and laboratory. Hence, the application and success of the proposed technique is presented in this paper by presenting the test results of one-bay, one-storey frames. Experimental investigation Test specimens Test frames The test specimens used in this study are in one-third scale, onestorey, one-bay RC frames with hollow brick masonry infills. These frames reflect typical characteristics and common deficiencies of the structural frames of RC buildings in Turkey. These weaknesses include low concrete strength, use of plain bars, insufficient lapped-splice length, insufficient anchorages, poor confinement and strong beam weak column combination. The frames were infilled with hollow brick and plastered at both faces with columns fixed to the rigid foundation beams. Dimensions and reinforcement of the test frames are illustrated in Figure 1. Reinforcement pattern of frames with continuous and lappedsplice longitudinal reinforcement are given in Figure 2. The frames were infilled with a special production of one-third scaled hollow brick infill covered with scaled layer of plaster at both faces. Realising the critical consequences that workmanship may have on the performance of the test frames, ordinary workmanship was employed while infilling the frames with hollow brick infill and covering with plaster. The frames are grouped in pairs and briefly indicated by letters. 100 mm 1300 mm 100 mm 150 mm 150 mm 150 mm 150 mm 750 mm 400 mm 15 mm 15 mm 70 mm 15 mm 15 mm 60 mm 60 mm 15 mm 1 15 mm 4 φ8 φ4/ φ8 φ4/ mm Column 15 mm Beam Figure 1. Dimension and reinforcement of frames 322

3 300 mm 250 mm 160 mm 3 φ8 Stirrup φ4/100 mm Stirrup φ4/100 mm 2 φ8 2 φ8 Stirrup φ8/150 mm 5 φ16 3 φ8 Stirrup φ4/100 mm Stirrup φ4/100 mm 2 φ8 2 φ8 Stirrup φ8/150 mm 5 φ16 high strength of concrete (30 50 MPa) to be used in, mm panel thickness can reasonably be proposed for the actual practice. Since the usual floor height is about m and the usual beam depth is around mm, a panel arrangement with three layers sounds sensible and leads to a panel size around mm in vertical direction, horizontal size being around mm. Panels used in the present study had the dimensions of one-third scale of that used in the actual practice and panel types are illustrated in Figure 3. In the first two strengthened frame tests (CA4 and CB4), extreme care and attention was paid to the panel connection. Shear keys, welded connections, fixing reinforcing steel bars to each other and to epoxy anchored dowels by welding, were provided. However, in the following tests, the epoxy mortar used in connecting the proved to be so successful that both shear keys and welded connections appeared to be redundant. This is why no shear keys and welded connections were used for the in the studies conducted (Baran, 2005; Duvarci, 2003; Okuyucu and Tankut, 2009; Susoy, 2004). Panel arrangements for all types of are illustrated in Figures 4 7. The anchorage dowels were epoxy fixed to the frame members, prior to drilling holes in the existing frame members for epoxy anchored dowels considering the panel dimensions. Then, gaps between the neighbouring and between the and the frame members were filled with epoxy mortar. No surface finishing was normally required since the had sufficiently smooth surfaces. 300 mm 250 mm Figure 2. Reinforcement pattern of frames with continuous and lapped-splice longitudinal reinforcement The legend of each pair starts with the letter C to indicate that the frame has continuous column longitudinal reinforcement or L to indicate that the frame has lapped-splices in column longitudinal reinforcement. The second letter A, B, C or D indicates the type of the panel used to strengthen the frame. The third letter indicates the number of the sides on which dowels are used. If it exists, the last number 2 indicates that the specimen is duplicate and tested under lower axial load. Infill wall strengthening In the scope of the present study, four different types of precast Test set-up and instrumentation concrete were designed and tested to observe their Figure 8 gives a general view of the test set-up. As seen in this contribution on overall frame behaviour as infill strengthening figure, tests were performed in front of a reaction wall. Test elements. The factor dominating the design of precast is frames were subjected to reverse-cyclic lateral loading resembling weight: each piece to be used in actual practice should not exceed the seismic effects and the resulting deformations were measured kg so that it can be handled by two workers. The other at numerous locations to obtain data needed for a comprehensive important factor is the panel thickness. Considering the relatively analytical seismic performance evaluation of the specimens. Material properties A low-strength concrete was intentionally used in the test frames to represent the concrete commonly used in existing building structures. On the other hand, relatively strong concrete was preferred for the precast to provide the required loadcarrying capacity by using relatively thin layers of concrete, minimising the panel weight. Ordinary cement-lime mortar was used for the plaster, imitating the usual practice. For the same reason, mild steel plain bars were used as reinforcement in test frames. Typical properties of reinforcing bars used in this study are listed in Table 1 and the average compressive strength values for frame, panel concrete and plaster determined on the testing day are tabulated in Table 2. Epoxy mortar having a compressive strength of 65 MPa was used in panel joints and between the and the plaster on the wall. According to the manufacturer s manual, its tensile strength was nearly 20 MPa, which is obviously beyond the strength required in concrete applications. 323

4 100 mm 76 mm 100 mm 10 mm 10 mm 69 mm φ3/55 φ4 10 mm 70 mm 63 mm 61 mm Type A 69 mm 70 mm φ3/50 99 mm 78 mm 99 mm 10 mm φ4 81 mm 109 mm 101 mm 111 mm 100 mm 111 mm 80 mm φ4 φ4 65 mm φ3 φ3 φ3/50 φ3/50 65 mm φ4 φ4 80 mm 111 mm 100 mm 111 mm 101 mm 109 mm 81 mm Type B mm Type C φ3/50 mesh 740 mm 105 mm φ3 φ3 Type D φ3/50 mesh Figure 3. Dimension and reinforcement of panel types 324

5 /3 /250 mm /3 80 mm Foundation beam anchorages bars Figure 4. Panel arrangement for type A (specimen CA4) /250 mm φ8/245 mm φ8/3 φ8/3 150 mm 100 mm φ8/245 mm Foundation beam and column and beam anchorages φ8 bars Figure 6. Panel arrangement for type C (specimen CC4, CC4-2 and LC4) 80 mm /105 mm /105 mm Foundation beam Reversed cyclic lateral loading was applied by using a doubleacting hydraulic jack. A load cell was connected between the anchorages bars hydraulic jack and the test frame to measure the magnitude of the Figure 5. Panel arrangement for type B (specimen CB4) applied lateral load. The lateral loading system had pin connections at both ends to eliminate any accidental eccentricity mainly Loading and supporting system An RC universal base, serving as a rigid foundation for the frames and enabling various support configurations, has been prestressed to the strong testing floor of the Structural Mechanics Laboratory. Each test frame was cast together with a rigid foundation beam, which could be suitably bolted down to the universal base. The quasi-static test loading consists of reversecyclic lateral loading applied at floor level, besides constant vertical load on both columns. The vertical load application gear consisted of a hydraulic jack and a load cell placed between a spreader beam and a cross-beam at the top, which was pulled down by two prestressing cables attached to the universal base on either side of the specimen. Having been supported as a simple beam with supports at the column heads as shown in Figure 8, the spreader beam divided the load developing in the ram and transferred the two equal components to the two columns. The load was continuously monitored and readjusted during the test. 325

6 Not present in specimen CD2 Not present in specimen CD2 150 mm 100 mm φ8/105 mm 10 mm 100 mm 150 mm φ8/105 mm Foundation beam anchorages φ8 bars 30 mm Figure 7. Panel arrangement for type D (specimen CD2, CD4 and LD4) Bar type Property Location f y : MPa ö3 Plain Mesh steel for panel reinforcement 670 ö4 Plain Stirrup for beam and column 220 Panel reinforcement ö4 Plain Stirrup for beam and column (for specimen CR-2) 268 ö6 Deformed Dowel for frame-to-panel connection 378 ö8 Plain Beam and column longitudinal bars 330 ö8 Plain Anchorage bar between adjacent (for specimen CR-2) 405 ö8 Deformed Anchorage bar between adjacent 330 Stirrup for foundation beam ö8 Deformed Anchorage bar between adjacent (for specimens CC4-2 and CD2) 518 ö16 Deformed Foundation beam longitudinal bar 420 Table 1. Properties of reinforcing bars 326

7 Specimen designation Frame concrete: MPa Panel concrete: MPa Mortar: MPa CR CR LR CA CB CC CD CC CD LC LD Table 2. Frame concrete, panel concrete and mortar strengths of the frames in the vertical direction and tolerating a small rotation in the horizontal direction normal to the testing plane. At the floor level, a clamp made of four steel bars connected to two loading plates at both ends was loosely attached to the test frame. Before every test, the clamp was carefully controlled to be loose so as not to cause any external prestressing on the beam. In this way, a horizontal push was applied to the test unit through steel loading pads without inducing any undesirable tension in the beam in case of pulling the frame from a loading point anchored into the beam end. The infill wall, which has a smaller thickness than the beam width, was placed eccentrically on the exterior side of the beam to reflect common practice. Thus, the contribution of the infill makes the frame behaviour somewhat unsymmetrical, and the load applied in the plane of symmetry creates warping, which may lead to significant undesirable out-of-plane deformations, especially towards the end of the test. A rather rigid external steel guide frame attached to the universal base, was used to prevent any outof-plane deformations. Four guide bars, two on each side, with roller ends, were attached to the guide frame, and they gently touched the test frame beam, smoothly allowing in-plane sway. Increasing load cycles were applied to the specimen so far as the frame was capable of resisting higher loads; and beyond that, deformation controlled loading was performed with increasing displacement cycles, usually in steps of multiples of yield displacement. Deformation measurement system All deformations were measured by displacement transducers, using either linear variable differential transformers (LVDTs) or electronically recordable dial gauges as shown in Figure 9. Sway displacement was measured at the first floor level. Three measurements were normally taken at this level to ensure a reliable collection of these very important data, even in the case of one or two transducers unexpectedly going wrong. Infill wall shear deformation was determined on the basis of displacement measurements along the diagonals. Displacement measurements taken at the bottom of both columns are meant for computation of rotations of the entire frame, when the infill wall remains intact and the overall behaviour of the test unit resembles cantilever behaviour. However, they also provide data for monitoring the critical column section deformations steel yielding in the tension side column, concrete crushing in the compression side column and so on. Although it was heavily prestressed to the strong testing floor, the Crossbeam Prestressing cable Ram Foundation beam Spreader beam Load cell Ram Pins Strong wall Universal base Figure 8. Loading and supporting system 327

8 δ 6 8 cm δ 3 δ cm 13 cm rigid body rotations and displacements of the universal base were monitored using four dial gauges during the tests to enable corrections in the critical measurements in the case of an unexpected movement. Experimental results δ 1 δ 2 13 cm Foundation beam Universal base δ1, δ2: electrical dial gauges with 50 mm strokes δ3, δ4: electrical dial gauges with strokes δ5, δ6: mechanical dial gauges with strokes : LVDTs with 200 mm strokes Figure 9. Instrumentation 1 General Photographs of test specimens at the end of tests are given in Figure 10 and a summary of the test results is presented in Table 3. Test results are evaluated considering the seismic performance indicators such as lateral strength, initial stiffness, energy dissipation and interstorey drift ratios. δ 4 of specimen CD4 had dowel connections to all frame members, and owing to the strip shape of the, it had a greater number of dowels to the foundation and beam. According to the test results, intense dowels and panel shapes provided this specimen an infill of relatively higher load capacity. Stronger infill caused this specimen to reach higher lateral load resistance than other specimens, but upon crushing of the infill, relatively weak frame members could not carry this excessive load and performed frame action. The more ductile behaviour of specimens CC4-2 and CD2 with respect to the specimens CC4 and CD4 can be attributed to the lower axial load on the columns causing less damage on the infill and relatively weak frame members. Response envelopes of specimens with lap-spliced longitudinal column steel are provided in Figure 12. The strengthened lappedspliced specimens show similar strength increase as the specimens with continuous longitudinal column steel with respect to the reference specimen. When compared with specimens having continuous longitudinal column steel, corresponding lappedspliced specimens had relatively lower lateral strength. Lower lateral load capacity is attributed to lower frame capacity owing to bar slip and additional deformations at the lapped-splice regions. Behaviour of lapped-spliced specimens was observed to be closer to typical frame behaviour. Lower axial load level applied to specimens led to widening of diagonal cracks on the infill. As a result of this, larger diagonal cracks decreased the capacity of the infill and increased deformations. The superior capacity of specimens strengthened with type D (CD4, CD2 and LD4) over specimens strengthened with type C (CC4, CC4-2 and LC4) is very significant. The most influential factor can be stated as the number of dowel bars. At the foundation level and beam level, five dowels were used for type C dowels, whereas 13 dowels were employed between of type D, owing to different geometry of the. The strip shape of type D can also be stated to appear more effective for load transfer and better connection. Response envelopes of the specimens with continuous longitudinal column reinforcement are plotted together in Figure 11. In this figure, the overall behaviour of the six strengthened specimens with respect to the two reference specimens are displayed. Response envelopes for specimens CA4, CB4, CC4-2 Initial stiffness values of test specimens are tabulated in Table 3. and CD2 follow a very similar trend. They exhibit more than The values show that initial stiffness values of the specimens with twice the lateral strength of the reference specimens, and they continuous longitudinal column steel increased about 3 to 5 times have displayed almost equal ductility. This shows that the specimens of that of reference ones after strengthening. It was noted that CA4, CB4, CC4-2 and CD2 transform to frame behaviour CA4 and CB4 specimens, which had welded connections between as the reference specimens after the infill lost its load-carrying their, showed almost the highest initial stiffness. Therefore, capacity. Specimen CC4 also shows a very similar behaviour welding and shear keys can be reported to prevent relative having the response envelope almost coinciding with that of CA4 displacement between the and make the infill stiffer. and CB4. The only difference is the lower ductility level, and this Specimens CC4 and CD4 had a little less initial stiffness since can be attributed to the higher axial load on columns causing less they do not have welding and shear keys between precast, damage on the infill and frame members. It appears that this but the values are not too different. Presence of lapped-splice in specimen also exhibits frame behaviour after diagonal cracking column longitudinal reinforcement and lower axial load appears and crushing of its infill. CD4 has distinctively higher lateral load to decrease initial stiffness values from the comparison of CR capacity than all other specimens. Moreover, the response and LR specimens. LC4 had provided good increase in initial envelope shows much less ductility, not even reaching half of the stiffness with respect to the specimens with lap-spliced column maximum lateral displacements of other specimens. The infill steel LR, but had less initial stiffness than CC4 and CC4-2, the 328

9 CR LR CA4 CB4 CC4 CD4 LC4 LD4 CC4-2 CD2 Figure 10. Specimens after testing 329

10 Specimen Lapped-splice length Axial load level: N/N 0 * Max. lateral load: kn Drift ratio: ä/h Initial slope: kn/mm Cumulative energy dissipation: J CR LR 20 ö y CR CA CB CC CD CC CD LC4 20 ö y LD4 20 ö y *N 0 ¼ 0:85 f9 c A c þ f y A st y 20ö ¼ 160 mm Table 3. Summary of test results Lateral load: kn Lateral displacement: mm Figure 11. Response envelopes of specimens with continuous column steel Lateral load: kn Lateral displacement: mm CR CA4 CB4 CC4 CD4 CR-2 CC4-2 CD Figure 12. Response envelopes of specimens with lap-spliced column steel 330 LR LC4 LD4 corresponding specimen with continuous column longitudinal reinforcement. The specimens CC4-2, CD2 and LD4 showed initial stiffness values almost the same as specimen CA4 and a great increase with respect to the reference specimens. Precast of specimens CC4-2, CD2 and LD4 had very high concrete strength of almost MPa, and this may be the reason for this high initial stiffness. The amount of dissipated energy was calculated as the area under the hysteretic load displacement curves for each cycle. The area under each half cycle was calculated to find the dissipated energy in that half cycle, then each forward and backward cycle values were added to find the dissipated energy in that full cycle. At the end, cumulative dissipated energy by a specimen was obtained by the addition of dissipated energies in all the full cycles. The total amount of dissipated energy of each specimen is tabulated in Table 3. For all panel types, strengthening increased the total dissipated energy considerably. The increase is about 1. 5to2. 5 times with respect to the reference. The highest energy was dissipated by specimens strengthened with type A and type B. Specimens with continuous column longitudinal reinforcement strengthened with type C and type D have dissipated less energy compared with CA4 and CB4 specimens, since they did not have welding or shear keys. Specimen CC4-2 also dissipated energy as much as specimen CA4. This relatively high energy dissipation capacity can be attributed to the lower axial load on columns causing less damage on the infill and frame members ending up with ductile frame behaviour. The lowest energy dissipation was obtained from CD4, which is even less than that of reference specimen with longitudinal lap-spliced column reinforcement. Low energy dissipation of this specimen was because of its brittle behaviour and insufficiency to develop frame behaviour after crushing of the infill.

11 Observations Seismic performance improvement of strengthened frames is summarised in Table 4 and the following discussions can be developed based on the experimental results. (a) The increase in lateral load-carrying capacity of the strengthened frame varied between 2.27 times and 3.96 times with respect to the appropriate reference frame. (b) The increase in the initial stiffness of the strengthened frames varied between times and times with respect to the appropriate reference frame. (c) The increase in the energy dissipation capacities of the strengthened frames varied between 1.47 times and 2.72 times with respect to the appropriate reference frame. (d) According to the test results, the magnitude of the storey drift index was significantly reduced as a result of the strengthening of the frames. Conclusıons The conclusions presented below are based on the limited data obtained from 11 tests conducted. (a) The occupant-friendly pre-quake seismic rehabilitation technique developed and proposed for seismic strengthening of RC-framed buildings, namely converting the existing hollow brick infill wall into a load-carrying system acting as a cast-in-place concrete shear wall by reinforcing it with high-strength precast concrete, significantly increased the lateral load capacity and rigidity as well as improving the seismic behaviour of the test frames. (b) Frames strengthened by connected only by the use of epoxy mortar proved to be so successful that both shear keys and welded connections came to be redundant. Hence, Ratio Lateral strength Increase Initial slope Cumulative energy dissipation CA4/CR CB4/CR CC4/CR CD4/CR CC4-2/CR CD2/CR LC4/LR LD4/LR Table 4. Behaviour improvement by the proposed method technique can be easily stated to be unbeatable. although making the behaviour less ductile, the application of the method was much simpler and cheaper when type C (nearly square) and type D (full height strip) were used instead of type A and type B which have shear keys and require laborious application. (c) Strengthened infill failed by excessive diagonal cracking on the, and the frame failed by crushing or failure at the column bases or at the beam column joints. After the failure of the infill, the behaviour of the system became similar to typical frame behaviour. Stronger infills of type C and type D with higher axial load on columns provided higher lateral load capacity but hampered frame action, thus, limiting the ductility. (d) Lower axial load with or without presence of lapped-spliced reinforcement created a negative effect on the lateral strength. However, specimens with lower axial load on columns showed more ductile behaviour. Bar slip problems were obviously observed in specimens with lapped-spliced column reinforcement. (e) Seismic strengthening technique with epoxy-bonded precast concrete on brick masonry infills can be an effective strengthening method which is suitable for the existing seismically vulnerable reinforced concrete buildings in Turkey, being faster, easier and more economical than strengthening with cast-in-place shear walls. This method also does not interfere with the function of the building and therefore it is occupant-friendly. ( f ) All of the observations mentioned above are presented in agreement with the results obtained from two-storey frame tests indicating that one-storey frames can suitably be used as test specimens. Result The occupant-friendly seismic rehabilitation technique developed for seismic strengthening of buildings, namely converting the existing hollow brick infill wall into a load-carrying system acting as a cast-in-place concrete shear wall by reinforcing it with highstrength precast concrete, is very effective in improving the seismic behaviour by increasing the lateral strength, initial stiffness, energy dissipation and ductility characteristics while at the same time decelerating strength and stiffness degradation. In addition, this technique would not require evacuation of the building and would be applicable without causing too much disturbance to the occupant. The cost of applying this technique is obviously not more than the cast-in-place reinforced concrete infill wall technique. Considering the mobilisation cost of the occupants out and in, the rent to be paid for the period of construction which is no less than 6 to 8 months and the cost to morale from the trouble caused, the cost-effectiveness of this 331

12 Acknowledgements The financial support given by the Scientific and Technical Research Council of Turkey (TÜBİTAK-IÇTAG I 575) and North Atlantic Treaty Organization (NATO-SFP ) is gratefully acknowledged. REFERENCES Altin S, Ersoy U and Tankut T (1992) Hysteretic response of reinforced concrete ınfilled frames. Journal of Structural Engineering 118(8): Baran M (2005) Precast Concrete Panel Reinforced Infill Walls for Seismic Strengthening of Reinforced Concrete Framed Structures. PhD thesis in Civil Engineering, Middle East Technical University, Ankara. Baran M, Duvarci M, Tankut T, Ersoy U and Ozcebe G (2003) Seismic Assessment and Rehabilitation of Existing Buildings (Wasti ST and Ozcebe G (eds)). Kluwer Academic Publishers, Netherlands, pp Duvarci M (2003) Seismic Strengthening of Reinforced Concrete Frames with Precast Concrete Panels. MSc thesis in Civil Engineering, Middle East Technical University, Ankara, Okuyucu D and Tankut T (2009) Effect of panel concrete strength on seismic performance of RC frames strengthened by precast concrete. Proceedings of WCCE ECCE TCCE Joint Conference: Earthquake and Tsunami. IMO Publication E/09/03, Istanbul. Sonuvar MO, Ozcebe G and Ersoy U (2004) Rehabilitation of reinforced concrete frames with reinforced concrete ınfills. ACI Structural Journal 101(4): Susoy M (2004) Seismic Strengthening of Masonry Infilled R/C Frames with Precast Concrete Panel Infills. MSc thesis in Civil Engineering, Middle East Technical University, Ankara. Tankut T, Ersoy U, Ozcebe G, Baran M and Okuyucu D (2005) Service seismic strengthening of RC framed structures. Seismic Assessment and Rehabilitation of Existing Buildings, International Closing Workshop, NATO Project Sfp , Istanbul. Turk M, Ersoy U and Ozcebe G (2003) Seismic rehabilitation of RC frames with RC infill walls. Proceedings of the Fifth National Conference on Earthquake Engineering, Istanbul, WHAT DO YOU THINK? To discuss this paper, please submit up to 500 words to the editor at by 1 November Your contribution will be forwarded to the author(s) for a reply and, if considered appropriate by the editorial panel, will be published as a discussion in a future issue of the journal.