SEISMIC RETROFIT OF A TYPICAL REINFORCED CONCRETE BUILDING THROUGH FRP JACKETING OF EXTENDED RECTANGULAR COLUMNS

Transcription

1 6 th International Conference on Advanced Composite Materials in Bridges and Structures 6 ième Conférence Internationale sur les matériaux composites d avant-garde pour ponts et charpentes Kingston, Ontario, Canada, 5 May 1 / -5 mai 1 SEISMIC RETROFIT OF A TYPICAL REINFORCED CONCRETE BUILDING THROUGH FRP JACKETING OF EXTENDED RECTANGULAR COLUS M. Comert and A. Ilki Civil Engineering Faculty, Istanbul Technical University Ayazaga Campus Istanbul, Turkey mcomert@itu.edu.tr ailki@itu.edu.tr ABSTRACT: A typical three-storey reinforced concrete building representing a school building is analysed in terms of seismic safety. The structural system of this typical building includes extended rectangular columns with cross-sectional aspect ratio of 3 (5 x 75 mm x mm) and demonstrates typical deficiencies of the time of its construction in Turkey, particularly in terms of low quality workmanship, resulting with low quality concrete and improper reinforcement detailing. A vital deficiency about the reinforcement detailing is the large spacing of transverse reinforcement. The nonlinear analyses of this typical building showed that seismic retrofitting is necessary for the structural system to achieve the required seismic performance level for a school building, which is densely occupied by students. For seismic retrofitting, among several other techniques, jacketing of columns with FRP (fiber reinforced polymer) sheets is considered as one of the feasible approaches, for enhancing the seismic performance of the structure to the required level through ductility improvement. After retrofitting, the nonlinear pushover analysis is performed once more for the assessment of seismic safety. In addition, the comparison of the results for original and retrofitted structure is presented. This comparison clearly demonstrated that the efficiency of the FRP jacketing of extended rectangular columns reduce the structural damage level. 1. INTRODUCTION Earthquakes are major threats for countries, such as Turkey, located in earthquake prone areas. To mitigate the potential losses of earthquakes, a large number of existing buildings in these areas need to be strengthened. The typical deficiency encountered in these buildings is the low quality workmanship, resulting with low quality concrete and improper reinforcement detailing. In this study, a typical existing three-storey reinforced concrete building representing a school building is analysed in terms of seismic safety. The building has the characteristics of typical buildings designed and constructed in 197s. While the building is regular in plan and elevation, its structural system demonstrates typical deficiencies of the time of its construction in Turkey, particularly in terms of low quality concrete and improper reinforcement detailing. A vital deficiency about the reinforcement detailing is the large spacing of transverse reinforcement and inadequate hook details of stirrups. The nonlinear structural analyses of this typical building showed that seismic retrofitting is necessary for the structural system to achieve the required seismic performance level for a school building. Jacketing of columns with FRP (fiber reinforced polymer) sheets is considered as one of the feasible approaches, for enhancing the seismic performance of the structure to the required level through ductility improvement. The structural system of the building includes extended rectangular columns with dimensions of 5x75 (mm x mm). The study compares the deformability performance of the extended rectangular columns and overall structural seismic performance of the building before and after FRP jacketing of columns. During the nonlinear analysis of the retrofitted structure, the results of tests carried out at Istanbul Technical University on the columns with 1

2 cross-sectional aspect ratios of and 3 (Ilki et al. 4) are utilized for determination of the behavior of extended rectangular columns jacketed with FRP sheets.. OUTLINE OF THE STRUCTURE The building, which demonstrates the typical deficiencies in terms of reinforcement detailing and low quality concrete, has three stories and it is regular in plan (Figure 1a) and elevation. The column dimensions are 5 x 75 (mm x mm) and 6 x 6 (mm x mm), the shear wall dimensions are 5 x (mm x mm), and the beam dimensions are 5 x 6 (mm x mm). The reinforcement details of columns, shear walls and beams are presented in Figure 1b. The transverse reinforcement (stirrups) of the columns and beams are 8 mm diameter bars with mm spacing with no intermediate cross-ties. The strength of concrete is assumed to be 15 MPa and the yield strength of the steel (plain round bars) is considered to be MPa. In the analysis, the concrete is modelled through the stress-strain relationship proposed by Mander et al. (1988) and the behaviour of steel is defined through on elasto-plastic steel model with strain hardening. The stress strain curves of concrete and steel are presented in Figure. 16ɸ16 3ɸ18 C4 C8 C1 C16 ɸ18 75 mm 6 mm C3 C7 C11 C15 5 mm 3ɸ18 4ɸ16 5 mm C1, C, C3, C4, C13, C14, C15, C16 Stirrups ɸ8/ ɸ1/3 6 mm C6, C7, C1, C11 4ɸ16 C C6 C1 C14 ɸ8/ mm C5, C8, C9, C1 Lateral Reinf. ɸ1/3 y Interior support sections Exterior support sections C1 C5 C9 x C13 15 mm 3ɸ16 5ɸ16 Beam stirrups ɸ8/ 6 mm 4ɸ16 3ɸ16 5 mm 5 mm (a) (b) Fig 1 - (a) Plan of the building (for all stories) (b) column, shear wall and beam section details Fig - (a) Concrete (b) steel material stress-strain curves

3 3. ANALYSIS OF STRUCTURE 3.1 Nonlinear Modelling and Analysis The single mode pushover analyses are performed in weak (x) and strong (y) directions of the building. During the analyses, Perform 3D (Computers and Structures Inc, 6) nonlinear analysis software is used. The material nonlinearity is considered through the plastic hinge concept. Zero length plastic hinges representing the nonlinear behaviour are assigned to the two ends of the structural members. It should be noted that while converting nonlinear curvatures to rotations the plastic hinge length is assumed to be 5% of the section depth as suggested by Turkish Seismic Design Code (Ministry of Public Works and Settlement Government of Republic of Turkey, TSDC, 7). For determination of characteristics of these plastic hinges, flexural section analyses are carried out for column, shear wall and beam sections given in Figure 1 considering material models presented in Figure. The building is located on ground type of Z3 with characteristic periods of.15 and.6 in the highest seismic risk zone. In Figure 3, the elastic design acceleration spectrum for school buildings defined for Z3 type ground in highest seismic risk zone according to TSDC is presented. The design earthquake has % exceedance probability in 5 years. 1.5 S ae (g) Period (sec) Fig 3 The design acceleration spectrum for school buildings. While determining structural seismic performance levels, TSDC describes three different damage limits for reinforced concrete sections in terms of plastic strains of concrete and steel. The details of these limits are presented in Table 1. In accordance with these strain limits, four different damage ranges are defined for reinforced concrete members (Figure 4). As it can be seen in Figure 4, damage level between zero and minimum damage limit is slight damage, between minimum damage limit and safety limit is moderate damage, between safety limit and failure limit is heavy damage and after failure limit is collapse. Table 1 Section damage limits according to TSDC (Ministry of Public Works and Settlement Government of Republic of Turkey, 7). Damage levels Concrete strain limit* Steel strain limit Minimum damage limit () c.1 s.35 Safety limit ().35.1( s ) cg sm Failure limit ().4.14( s ) cg sm *In equations, s is volumetric ratio of transverse reinforcement and sm is the required volumetric ratio of transverse reinforcement according to TSDC. In addition, c is the maximum compression strain of unconfined concrete, cg is the maximum compression strain of confined concrete and s is the maximum steel strain. s s 3

4 Internal Force Slight Damage Moderate Damage Heavy Damage Collapse Deformability Fig 4 Damage levels of the reinforced concrete members according to TSDC (Ministry of Public Works and Settlement Government of Republic of Turkey, 7). The overall structural performance of the building is decided by the percentage of structural members in different damage levels. In this stage, TSDC defines four different structural performance ranges including immediate occupancy (IO), life safety (LS), collapse prevention (CP) and collapse or partial collapse (C). The expected seismic performance level for a school building under defined earthquake spectrum given in Figure 3 is life safety (LS). For LS performance level, the structure should satisfy the following requirements; i) all member should have sufficient shear strength, ii) the contribution of heavily damaged vertical structural members to shear load carrying should be less than % in each floor, iii) the damage of non of the damage structural members should fall in the range of collapse and iv) the contribution of the structural members whose both ends (critical sections) exceed slight damage level to the shear load carrying capacity should be lower than 3% in each floor. 3. Results The base shear-top displacement curves of the structure in weak (x) and strong (y) directions are given in Figure 5. To evaluate reinforced concrete member performances, the axial load-total curvature interaction curves are used. These curves are obtained for each level of section strain limits. The axial load-total curvature interaction curves for the first storey columns and shear wall in weak and strong directions are presented in Figures 6 and 7, respectively. The damages of other structural members are not shown since the results obtained for the first storey members are sufficient to determine the structural performance. In this building, there is no shear failure in structural members. As can be seen from Figures 6 and 7, almost all extended rectangular columns (C1, C, C3, C4, C13, C14, C15 and C16) exceeded the failure strain limits. According to TSDC, if strains of a vertical structural member of the building exceed the failure strain limits, the overall structural earthquake performance is determined as collapse performance level. Therefore, the performance of this school building falls in the category of collapse and measures are necessary to upgrade the seismic performance level to life safety. Shear (V, kn) Shear (V, kn) Top Displacement, u N1 (m) Top Displacement, u N1 (m) (a) (b) Fig 5 The base shear-top displacement curves in (a) weak (b) strong direction of the structure. 4

5 (a) (b) (c) Fig 6 The damages of the first storey (a) 5 x 75 columns (b) x 5 shear walls (c) 6 x6 columns in weak direction (dimensions in mm) (a) (b) (c) Fig 7 The damages of the first storey (a) 5 x 75 columns (b) x 5 shear walls (c) 6 x6 columns in strong direction (dimensions in mm). 4. RETROFITTING PROCEDURE 4.1 Model for FRP Confined Extended Rectangular Since the compressive strains of the extended rectangular columns (5 x 75 mm x mm) exceeded the failure strain limits, the overall structural performance is assessed as collapse. TSDC does not permit FRP jacketing of columns with cross-sectional aspect ratio greater than. In contrast to the limitation of TSDC, for research purpose, to enhance the seismic performance of the structure, the extended rectangular columns are jacketed with FRP sheets in transverse direction considering the experimental and theoretical studies conducted at Istanbul Technical University before (Ilki et al, 4). In the referenced study, totally 46 specimens with unconfined concrete strengths between 6 1 MPa were axially tested after wrapping different amount of carbon fiber reinforced polymer (CFRP) sheets around the columns in transverse direction. The cross-sectional dimensions of the tested specimens with cross-sectional aspect ratio of 1:3 were 15 x 45 (mm x mm). The obtained test results for the specimens with cross-sectional aspect ratios of 1:3 are presented in Table. In this table, f co and co are the unconfined concrete strength and corresponding axial strain, f cc and cc are the confined concrete strength and ultimate axial strain and r is the corner radius. The specimen names are assigned considering the shape and depth/width ratio of the cross-section, number of transverse CFRP plies and the corner radius. As an example, R-3-3-4a represents the first specimen with rectangular cross-section of aspect ratio 1:3, which is wrapped by 3 plies of transverse CFRP sheets. The corner radius of this specimen is 4 mm. It should be noted that in this test CFRP sheets with elasticity modulus of 3 MPa, effective thickness of.165 mm, tensile strength of 343 MPa and ultimate elongation capacity of.15 were used. As a result of the experimental studiy, an analytical confinement model for concrete members jacketed with CFRP sheets was proposed by Ilki et al. (4). The analytical model is summarised by Equations 1-7. In these equations, f lmax is the maximum effective transverse confinement stress, a is the efficiency factor that is to be determined based on the section geometry as the ratio of effectively confined cross-sectional area to the gross crosssectional area, f is the ratio of wrapping material to the concrete cross-section, h,rup is the rupture strain of the frp ( h,rup =.7 frp ), is the ratio of cross-sectional area of the longitudinal reinforcement to the crosssectional area of wrapped member, is the arching angle(which is assumed as 45 ), t f and n f are the 5

6 effective thickness and the number of plies of wrapping material, and b and h are the width and depth of the rectangular member to be wrapped. 4. Design of FRP Retrofit for Extended Rectangular As seen in Fig. 6, if the curvatures corresponding to damage limits can be increased for extended rectangular columns, the damages of these columns can be reduced and consequently the overall structural performance level can be enhanced. Consequently, the extended rectangular columns are decided to be wrapped with three plies of carbon FRP sheets. The characteristics of the carbon fibers used in the retrofit design are given in Table 3. During design of FRP retrofit, it is assumed that the corners are to be rounded to a radius of 4 mm and the non-structural partition walls were partly demolished from two sides of the columns during the retrofitting works, The confined concrete compressive strength and ultimate axial strain are determined by the use of equations proposed by Ilki et al. (4). During flexural sectional analysis, which is carried out using fiber analysis approach, a bilinear confined concrete stress-strain model and an elasto-plastic steel stress-strain model with strain strain hardening are used. It should be noted that no additional safety coefficients are taken into account while using Equations 1-7 other than those, which are inherently considered by these equations (such as h,rup =.7 frp ). Since the Equations 1-7, are behavior oriented equations, in case of actual retrofit design, additional safety factors should be considered. Consequently, the efficiency of FRP retrofit will be slightly reduced due to additional safety coefficients, which should be used for converting behaviour oriented equations to design equations. Table Experimental results for specimens with 1:3 aspect ratio (Ilki et al., 4). Specimen f co r f cc (MPa) Plies (mm) (MPa) co cc f cc / fco cc / co R R R a R b R a R b R a 4b f lmax f cc fco 1.4 (1) fco f lmax a ρf ε h,rupe frp () A A ρ (3) a 1 1 A 1 (b r) (h r) tanθ (4) 3bh A 4r πr (5) bh nf t f b h ρf (6) bh 6

7 .5 h h f ε ε 1 1 l max (7) cc co b b f co Table 3 Characteristics of the carbon FRP sheets. Tensile Strength (MPa) Elasticity Modulus (MPa) Ultimate elongation Effective thickness (mm) ANALYSIS RESULTS OF RETROFITTTED STRUCTURE Nonlinear pushover analysis is carried out once more for the seismic safety assessment of retrofitted structure. The comparison of the moment-curvature relationships in strong and weak axes for C column (which have dimensions of 5 x 75 mm x mm) before and after retrofitting is presented in Figure 8, and the base shear-top displacement curves of the retrofitted structure with comparison of original structure in weak and strong directions are given in Figure 9. The damages of the first storey reinforced concrete members in weak and strong directions are also shown in Figures 1 and 11, respectively. According to the obtained results, no structural member fail in shear and strains of no member exceed the failure strain limits. The damage levels of extended rectangular columns are reduced from heavy damage or collapse level to moderate damage. On the other hand, the maximum contribution of the heavily damaged members to shear load carrying is approximately 3% in each direction and the maximum contribution of the members that both ends exceed slight damage level to the shear load carrying capacity is lower than 3% in each floor. The heavy damaged columns (which have dimensions of 6 x 6 mm x mm) are not retrofitted. It is clear that the damages of these columns could be reduced through CFRP jacketing as well. This study intends to investigate the seismic safety of the structure. Other risks, such as fire may also affect the retrofitting material components (especially epoxy resin). Although, in this study, it is assumed that the structure is safe under gravity loads and the retrofitting does not contribute to the structural system against gravity forces, it clear that taking precautions such as using of coating materials for other risks may be necessary for actual applications. Moment (knm) After retrofitting Before retrofitting Curvature (1/m) Moment (knm) After retrofitting Before retrofitting Curvature (1/m) (a) (b) Fig 8 Comparison of the moment curvature relationships around (a) strong and (b) weak axes for the extended rectangular columns. Retrofitted structure Retrofitted structure Shear (V, kn) Original structure Shear (V, kn) Original structure Top Displacement, un1 (m) Top Displacement, un1 (m) (a) (b) Fig 9 The first shear-peak displacement curves in (a) weak (b) strong direction of the original and retrofitted structure. 7

8 (a) (b) (c) Fig 1 The damages of the first storey (a) 5 x 75 columns (b) x 5 shear walls (c) 6 x 6 columns in weak direction (dimensions in mm) (a) (b) (c) Fig 11 The damages of the first storey (a) 5 x 75 columns (b) x 5 shear walls (c) 6 x6 columns in strong direction (dimensions in mm). 6. CONCLUSIONS The efficiency of the external confinement with FRP sheets for extended rectangular columns for enhanced deformability was shown by the authors in a previous study carried out in Istanbul Technical University. In this study, a typical three-storey building is analysed with and without retrofitting through nonlinear pushover analysis. The following findings are derived from the analyses; i) the original structure with improper reinforcement details and low quality of concrete did not satisfy the seismic performance requirements according to TSDC, ii) retrofitting of the extended rectangular columns (5 x 75 mm x mm) by CFRP sheets enhanced the deformability capacity of the columns significantly and leading a remarkably better overall seismic performance. 7. REFERENCES Computer and Structures Inc. (CSI)., 6. Perform 3D Nonlinear Analysis and Performance Assessment for 3D Structures User Guide. Berkeley, California, USA. Ilki A., Kumbasar N. and Koc V., 4. Low Strength Concrete Members Externally Confined with FRP Sheets. Structural Engineering and Mechanics, Vol.18, pp.1-8. Mander, J.B., Priestley, M.J.N., Park, R Theoretical Stress-Strain Model For Confined Concrete. Journal of Structural Division (ASCE), Vol. 114, pp Ministry of Public Works and Settlement Government of Republic of Turkey, 7. Turkish Seismic Design Code. Ankara, Turkey. 8