SEISMIC RETROFIT OF BEAM-COLUMN JOINTS WITH FRP SHEETS

Transcription

1 B-4 ADVANCED COMPOSITE MATERIALS IN BRIDGES AND STRUCTURES MATÉRIAUX COMPOSITES D'AVANT GARDE POUR PONTS ET CHARPENTES Winnipeg, Manitoba, Canada, September 22 24, 28 / 22, 23 et 24 septembre 28 SEISMIC RETROFIT OF BEAM-COLUMN JOINTS WITH FRP SHEETS Alper Ilki Istanbul Technical University, Turkey Idris Bedirhanoglu Istanbul Technical University, Turkey Nahit Kumbasar Istanbul Technical University, Turkey Abstract In this study, four full-scale beam-column joint subassemblages were tested under reversed cyclic loads. The specimens were constructed to represent corner joints of relatively old reinforced concrete structures. Therefore, all specimens were cast using low quality of concrete and plain reinforcing bars with inadequate detailing. Two subassemblages (Group A) included joint, beam, upper stoery and lower storey columns, a transverse beam and a part of slab, while the other two subassemblages (Group B) did not include the transverse beam and slab. One specimen from each group was tested as reference specimens, while the remaining one specimen from each group was tested after the joints were retrofitted using carbon FRP sheets. The aim of the study was both to investigate the effect of FRP retrofit of beam-column joints and the effect of transverse beam and slab on the behavior of reference joints and retrofitted joints. The test results were evaluated in terms of damage development, failure patterns and hysteretic behavior characteristics, such as degradation of strength and stiffness, and energy dissipation. At the end of the study, significantly better performance was obtained for the FRP retrofitted specimens, both in terms of shear strength of the joint and energy dissipation during loading cycles. It should be noted that special attention was paid for the retrofit method to be practically applicable.

2 INTRODUCTION In recent years, utilization of FRP (fiber reinforced polymer) sheets became one of the commonly preferred seismic retrofit approaches for reinforced concrete columns and beams, particularly for enhancing ductility and shear strength. Design and application methods of FRP retrofit of columns and beams are included in various codes and guidelines as well. However, for seismic retrofit of beam-column joints using FRP composites, little knowledge is available. During earthquakes beam-column joints, are among the most critical structural members that may cause total collapse of existing structures. Particularly, external joints, which are not confined by beams from four sides are more vulnerable. This vulnerability increases significantly when quality of concrete is low and reinforcement is composed of plain round bars. Beam-column joint damages observed after 1999 Duzce earthquake are presented in Figure 1. Gergely et al. (2), Ghobarah and Said (21, 22), El-Amoury and Ghobarah (22), Antonopoulos and Triantafillou (23), Mukherjee and Joshi (25), Ghobarah and El-Amoury (25), Al-Salloum and Almusallam (27) and Tsonos (27) investigated the behavior of joints in reinforced concrete frames, which are retrofitted with FRP composites. Engindeniz et al. (25) discussed the different techniques used for repair and strengthening of beam-column joints, including FRP applications. Most of these studies were on T-type joints without transverse beam and slab. Consequently, behavior of both reference and retrofitted specimens are questionable since the effect of transverse beam and slab is not taken into account. Due to neglection of the transverse beam and slab, the performance of reference specimens may have been predicted worse than the case of actual joints with transverse beam and slab. Moreover, the application of FRP retrofitting in the case of joints without transverse beam and slab is much easier and its contribution to the performance may be much higher than in the case of actual joints. In this study, four full-scale beam-column joint subassemblages were tested. The specimens were designed for representing corner joints of relatively old reinforced concrete frames. Therefore, all specimens were cast using low quality of concrete and plain reinforcing bars with inadequate detailing. Two subassemblages (A1 and A2) included joint, beam, upper story and lower story columns, transverse beam and a part of slab, whereas the other two subassemblages (B1 and B2) did not include the transverse beam and slab. One specimen from each group was tested as reference specimens, while the remaining one specimen from each group was tested after the joints were retrofitted using carbon FRP sheets. A1 and B1, and A2 and B2 were identical, with the exception of the difference due to presence of transverse beam and slab in the specimens A1 and A2. Inherently, the application of FRP sheets on the joint core was also different in the cases of specimens A2 and B2. While FRP sheets were applied only on the exterior face of the joint as two plies in case of specimen A2 due to the obstacles such as transverse beam and slab, FRP sheets were fully wrapped around the joint on two faces as one ply in case of specimen B2. The amounts of FRP used in case of specimens A2 and B2 were equal. The test results are evaluated in terms of damage development, failure patterns, energy dissipation and hysteretic behavior. Figure 1: Joints damaged during 1999 Duzce earthquake in Turkey

3 OUTLINE OF EXPERIMENTAL PROGRAM AND SPECIMEN DETAILS Four specimens representing joints of relatively old existing reinforced concrete buildings were tested in this study. Therefore, the concrete strength was intentionally low and all reinforcement was composed of plain round bars. There were no transverse bars in the joint core. Both top and bottom bars of the beams were hooked behind the joint core. The reference specimens were intentionally designed to fail in the joint core. While specimens A1 and A2 included a part of transverse beam and a slab portion, in addition to the joint core, connecting beam and half heights of lower and upper story columns, specimens B1 and B2 were T-type joints composed of a beam and half heights of lower and upper story columns. The characteristics of specimens are outlined in Table 1 and Figure 2. All specimens were cast on the same day ( ) using the same concrete mixture. All specimens were tested around and after 18 days of concrete age. For retrofitting, carbon FRP sheets were bonded on joints in diagonal directions. The characteristics of FRP sheets used for retrofitting are given in Table 2. In case of specimen A2, as seen in Figure 3, two plies of carbon FRP sheets were bonded on the external face and sheets were extended on the sides of upper story and lower story columns to have sufficient anchorage. In case of specimen B2, application was much easier for fully wrapping of the joint through diagonal sheets on exterior and interior faces of the joint, due to non-existence of transverse beam and slab. For obtaining a comparable retrofit in the cases of specimens A2 and B2, only one ply of carbon FRP sheets were bonded on two faces and consequently the FRP amounts used for the retrofit of these two specimens were almost equal. Another intervention in terms of retrofit of specimens A2 and B2 was welding of hooked parts of top and bottom beam longitudinal bars to each other, as well as replacing the low quality concrete behind the hooks with high quality repair mortar as a precaution for slip. Table 1: Specimen details Specimen Explanation Testing date A1 Reference joint - with transverse beam and slab A2 Retrofitted joint - with transverse beam and slab B1 Reference joint - without transverse beam and slab B2 Retrofitted joint - without transverse beam and slab φ8/1/5 8φ16 4φ16 5 2φ8 6 2φ8 φ8/1 8 φ8/1 4φ16 φ8/15/5 8φ16 4φ16 5 φ8/15 4φ16 Figure 2: Reinforcement details (dimensions in mm) Detail of beam top and bottom bars Table 2: Carbon FRP characteristics Tensile strength Elastic Modulus Ultimate Effective area per unit Unit weight elongation width (mm 2 /mm) (g/m 2 )

4 12 mmx2 mm b: column width: 25 mm h: column depth: 5 mm Upper story column Lower story column A2: 2 plies only on exterior face B2: 1 ply on exterior and interior faces Total amount of CFRP for two specimens are equal Figure 3: Retrofitting scheme for specimen A2 (dimensions in mm) CHARACTERISTICS OF CONCRETE AND REINFORCING STEEL Concrete mixture is presented in Table 3. In the mixture CEM I 42.5 type Portland Cement and Epocon SP 515N admixture was used. The unit weight of the concrete mixture was 227 kg/m 3. Since the joint specimens were tested at around the age of 18 days, the compression test results of standard cylinders (15 3 mm mm) obtained at the age of 18 days are given in Table 4. The average tensile strength of concrete obtained at the age of 18 days by splitting-tensile tests was 1.3 MPa. The average slump of fresh concrete was determined as 23.5 cm. Table 3: Concrete mixture (kg/m 3 ) Water Cement Sand Stone powder #1 Aggregate Admixture Table 4: Concrete compressive characteristics at the age of 18 days Specimen Compressive Average Elastic Modulus strength strength Average elastic modulus

5 The mechanical characteristics of plain round longitudinal (16 mm diameter) and transverse bars (8 mm diameter) obtained after tension tests are given in Table 5. Diameter (mm) Yield strength Table 5: Mechanical characteristics of steel reinforcing bars Yield strain Maximum strength Strain corresponding to maximum strength Ultimate strength Ultimate Elongation TESTING SETUP Specimens were tested with the columns in horizontal position as shown in Figure 4. The columns were simply supported at their mid-heights. Simulated seismic loads were acted on the tip of the beam through a MTS actuator, with the capacities of ±25 kn force and ±6 mm displacement. Testing was carried out by controlling the displacements in pushing and pulling directions throughout the test in a quasi-static manner. The actuator was supported by the strong reaction wall and the supports of columns were fixed to the strong floor, which is 12 mm thick. The axial load on column was kept almost constant throughout the test by adjusting a manual hydraulic jack of 6 kn capacity. The axial load on all columns was 12.5% of the axial capacities of the columns (.125 f' c b h). The specimens were instrumented intensely with displacement transducers and strain gages for measuring deformations of joint core, columns, beam and slab, as well as for confirming the appropriateness of the testing setup throughout the loading history. Total amount of data measuring points was around one hundred. The measuring devices on the exterior and interior faces of the joint core can be seen in Figure 5. Reaction wall Actuator Roller support Load cell Hydraulic jack Strong floor Figure 4: Testing setup (dimensions in mm)

6 Figure 5: Measuring system TEST RESULTS AND DISCUSSIONS The hysteresis curves for all specimens are presented in Figure 6 by shear force-drift ratio relationships. Shear force is the applied load to the tip of the beam and drift ratio is the ratio of lateral displacement of beam tip to the beam length from the column surface to the axis of actuator. It should be noted that the drifts caused by rigid rotation of the specimen, as well as the global horizontal displacement of the specimen, are taken into account, while determining the net drift of the beam tip. Shear force (kn) 1 1 A1 8 B1 8 A B Drift ratio Drift ratio Figure 6: Cyclic load-drift ratio relationships of specimens Shear force (kn) As seen in Figure 6, significant increases are obtained in the strength of the joints after FRP retrofitting. It is interesting to observe a quite ductile behaviour for both reference and retrofitted specimens. As seen in Figure 6, specimens did not experience remarkable strength degradation until the drift ratio of 4%. It is also interesting to note that the degradation of strength initiated around the same drift ratios for reference and retrofitted specimens. The ductile-like behaviour of reference specimens is not due to yielding of longitudinal bars, but it is due to slip of plain longitudinal beam reinforcing bars. The strain values recorded on the longitudinal bars were much smaller than the yield strain, when these specimens achieved their load capacities. However, although slip prevented the reference specimens to resist higher loads, this failure mode unintentionally caused a pseudo-ductility, which may be very useful during seismic actions. The failure of reference specimens due to slip can also be confirmed with the damages of the joints, as presented for specimens A1 and B1 in Figure 7 and 8, respectively. It should be noted that together with slip, increasing shear deformations in the joint caused diagonal cracks as well. As expected, these cracks were wider in the case of specimen B1, which did not include a transverse beam and slab. For a comparison of damage patterns of reference and retrofitted specimens, the failures of specimens A2 and B2 are presented in Figure 9 and 1, respectively. As seen in these figures, before the end of the test, the FRP sheets over the joint core were cut due to effect of diagonal tension stresses.

7 This shows that, in the case of specimen A2, the anchorage provided by bonding the extension of FRP sheets on the sides of the upper and lower story columns was sufficient to fully utilize the tensile capacity of FRP sheets in the joint core. In the case of specimen B2, both single ply FRP sheets on exterior and interior joint cores were also cut. Figure 7: Failure of reference specimen A1 (with transverse beam and slab) Figure 8: Failure of reference specimen B1 (without transverse beam and slab) Figure 9: Failure of retrofitted specimen A2 (with transverse beam and slab) The envelopes of the hysteresis curves are presented in Figure 11 for an easier comparison of specimen behaviours. As seen in Figure 11, the rate of degradation of strength is higher in the case of retrofitted specimens. The reason of steeper descending branch may be explained with dominancy of joint shear deformations rather than slip of beam longitudinal bars. In Figure 11 and 12, stiffness and strength degradations of specimens are also presented. Strength and stiffness degradations are determined as the ratio of reduction in strength and secant stiffness at the various target drift ratios, to the maximum resisted load and initial secant stiffness, respectively. The dissipation of energy calculated as the area enclosed by the hysteretic load-displacement relationships for each specimen is presented as a function of achieved drift ratio in Figure 13. As seen in this figure, both retrofitted specimens dissipated significantly higher amount of energy, due to enhancement in their load resistance capacities.

8 Figure 1: Failure of retrofitted specimen B2 (without transverse beam and slab) 1 A1 A2 B1 B Shear force (kn) Drift ratio Figure 11: Envelopes of load-drift ratio relationships COMPARISON WITH ANALYTICAL PREDICTION If joints were adequately designed and detailed, the failure was expected to occur in the beam due to flexure at around the load level of 9 kn. Stiffness degradation (%) A1 A2 4 B1 2 B Drift ratio (mm/mm) Strength degradation (%) 4 A1 3 A2 B1 2 B Drift ratio (mm/mm) Figure 12: Stiffness and strength degradations as a function of drift ratio

9 Cumulative dissipated energy (knmm) 3 A1 A2 25 B1 B Drift ratio (mm/mm) Figure 13: Energy dissipation-drift ratio relationships It is clearly seen that due to inadequate detailing of the joints of reference specimens in terms of transverse reinforcement and insufficient anchorage of the beam longitudinal bars, the theoretical flexural capacity of the beams could not be achieved and joints failed prematurely. However, in the case of retrofitted specimens, the specimens could achieve the theoretically determined flexural capacity of the beams. The shear stresses in the joint core at failure (v exp ) are presented in Table 6 directly and as functions of concrete compressive strength (f' c =8.3 MPa) and square root of concrete compressive strength ( f' c ). Table 6: Comparison of experimentally determined joint stresses at failure Specimen Max. shear force (kn) Max. joint shear stress, v exp v exp. / ' f c v exp. / A A B B ' f c

10 CONCLUSIONS The capacities of both reference specimens were limited by the slip of the beam longitudinal bars. It should be noted that if the beam top and bottom longitudinal bars did not have hooks in the joint, the reference specimens might have had significantly lower strength, as well as ductility. The slip of beam longitudinal bars caused a pseudo-ductile behaviour. Due to this unintentional pseudo-ductility, no remarkable degradation of strength was observed until the drift ratio of 4% for reference specimens. Since the joints of reference specimens were designed weak against shear intentionally, the failure would be more brittle, if slip had not occurred. The existence of transverse beam and slab limited the shear damage in the joint. However, this did not influence the overall behaviour significantly. Joint shear strengths of reference specimens were between.42 and.52 f' c. The one sided diagonal FRP retrofit in case of the joint with transverse beam and slab and two sided FRP retrofit in case of specimen without transverse beam and slab were successful in terms of strength enhancement. The failures of retrofitted specimens were basically due to shear failure of joint after FRP sheets were cut because of diagonal tension stresses in the joint core. However, at around the same load level, longitudinal beam bars also yielded. The descending branches of the load-displacement curves were steeper in the case of retrofitted specimens due to the dominant effect of shear stresses in the joint core. Joint shear strengths of retrofitted specimens were around.7 f' c. It should be noted that in the case of retrofitted specimens, the hooks of beam top and bottom longitudinal bars were welded to each other and the poor concrete around the hooks were replaced with a high strength mortar. As shown by experimental data and observations, retrofitting of inadequately detailed reinforced concrete joints using FRP sheets in an easily applicable manner as explained above can significantly increase the strength of joints. ACKNOWLEDGMENTS This study is a part of the PhD Thesis of I. Bedirhanoglu carried out under the supervision of Dr. A. Ilki. The study was granted by TUBITAK with project number 16M54 and ITU with project number For both projects, the principal investigator was Dr. A. Ilki. The supports provided by ISTON, BETONSA and BASF-Turkey is gratefully acknowledged. Assistance of Orkun Incecik and Kayhan Kolcu is also acknowledged. Master Salih Usta, who recently passed away, is also acknowledged for his valuable contribution in construction of the specimens REFERENCES 1. Gergely J, Pantelides C P, Reaveley L D, Shear Strengthening of RC T-Joints Using Composites, Journal of Composites for Construction, 4(2): 56-64, Ghobarah A, Said A, Seismic Rehabilitation of Beam-Column Joints Using FRP Laminates, Journal of Earthquake Engineering, 5(1): , Ghobarah A, Said A, Shear Strengthening of Beam-Column Joints, Engineering Structures, 24(7): , El-Amoury T, Ghobarah A, Seismic Rehabilitation of Beam-Column Joints Using GFRP Sheets, Engineering Structures, 24(11): , Antonopoulos C P, Triantafillou T C, Experimental Investigation of FRP-Strengthened RC Beam-Column Joints, Journal of Composites for Construction, 7(1): 39-49, Mukherjee A, Joshi M, FRPC Reinforced Concrete Beam-Column Joints under Cyclic Excitation, Composite Structures, 7(2): , Ghobarah A, El-Amoury T, Seismic Rehabilitation of Deficient Exterior Frame Joints, Journal of Composites for Construction, 9(5): , Al-Salloum Y A, Almusallam T H, Seismic Response of Interior RC Beam-Column Joints Upgraded with FRP Sheets. I: Experimental Study, Journal of Composites for Construction, 11(6): , Tsonos A G, Effectiveness of CFRP Jackets in Post-Earthquake and Pre-Earthquake Retrofitting of Beam- Column Subassemblages, Structural Engineering and Mechanics, 27(4): , 27.

11 1. Engindeniz M, Kahn L F, Zureick A H, Repair and Strengthening of Reinforced Concrete Beam-Column Joints: State of the art, ACI Structural Journal, 12(2): , 25.