Seismic Behavior of Composite Shear Wall Systems and Application of Smart Structures Technology

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1 Steel Structures 7 (2007) Seismic Behavior of Composite Shear Wall Systems and Application of Smart Structures Technology Qiuhong Zhao 1 and Abolhassan Astaneh-Asl 2, * 1 Assistant Professor, University of Tennessee, Knoxville, Civil and Environmental Engineering Department, 109A Perkins Hall, Knoxville, TN , USA 2 Professor, University of California, Berkeley, Civil and Environmental Engineering Department, 781 Davis Hall, Berkeley, CA 94720, USA Abstract Shear wall systems are one of the most commonly used lateral load resisting systems in high-rise buildings. This paper concentrates on the experimental and analytical studies of two composite shear wall systems and presents a summary and discussion of research results. In addition, the paper discusses application of smart structures technology into the design of these systems. The composite shear wall system studied herein consists of a steel boundary frame and a steel plate shear wall with a reinforced concrete wall attached to one side. The steel plate shear wall is welded to the boundary frame and connected to the reinforced concrete wall by bolts. In the system called traditional the reinforced concrete wall is in direct contact with the boundary steel frame, while in the system called innovative there is a gap in between. Keywords: Smart Structures Technology, Composite Shear Wall, Seismic Engineering, Cyclic Test 1. Introduction Reinforced concrete shear walls have been widely used as lateral load resisting system in the past in high-rise buildings, but there were always concerns on the local strength, ductility and construction efficiency of these systems in steel high-rise buildings, especially in high seismic zones. In recent years, more and more steel plate shear walls have been used with satisfactory results on construction efficiency and economy. Yet there were still concerns on overall buckling of the steel plates that will result in reduction of the overall shear strength, stiffness and energy dissipation capacity (Zhao, 2004), as well as large inelastic deformation of the steel plates that will result in large cyclic rotations of the moment connections and large inter-story drifts (Allen, 1980). On the other hand, composite shear walls might compensate for the disadvantages of reinforced concrete shear walls and steel shear walls and combine the advantages together. The composite shear walls have been used recently in a few modern buildings including a major hospital in San Francisco (Dean, 1977), but not as common as the other lateral load resisting systems. Therefore, seismic behavior of these systems and corresponding design guidelines are of high interest to design engineers. As a result, a project was conducted at the University of California, Berkeley *Corresponding author Tel: astaneh@ce.berkeley.edu to investigate the seismic behavior of two composite shear wall systems through large scale cyclic tests and advanced finite element analyses. 2. Project Background The composite shear wall project described in this paper concentrated on the seismic behavior of two composite shear wall systems denoted as traditional and innovative (designed by the second author), as shown in Fig. 1. Both systems are dual lateral load resisting system as defined in current codes (ICBO, 1997), and consist of a composite shear wall (primary system) welded inside a moment frame (secondary system) in a single-bay. The composite shear wall is made of a steel wall and a reinforced concrete (RC) wall connected together by bolts. In the traditional system, the four edge surfaces of the RC wall are in direct contact with the steel boundary frame, while in the innovative system there is a gap in-between. It is anticipated that by introducing the gap, the performance of the RC wall under severer seismic events could be improved, and the RC wall could be pre-cast and bolted to the steel wall on site to further increase construction efficiency. 3. Experimental Studies 3.1. Cyclic test on composite shear wall system Two half-scale specimens were constructed representing sub-assemblies of a generic building over three floors with the innovative composite shear wall system (Specimen

2 70 Qiuhong Zhao and Abolhassan Astaneh-Asl Table 1. Components of composite shear wall test specimens Steel wall plate Pre-cast RC wall Beam Column Wall bolts dia. thickness Thickness Rebar dia. Rebar spacing Reinf. ratio section* section* 4.8 mm 76 mm 10 mm 102 mm 0.92% 13 mm W12 26 W *Cross section properties refer to the AISC Manual (AISC, 1994). Figure 1. Main components of composite shear wall system. Figure 2. A composite shear wall Specimen with details of RC wall. One) and the traditional composite shear wall system (Specimen Two) as the lateral load resisting system. Each specimen included two full stories in the middle and two half-stories at the top and bottom. Structural components of the specimens are shown in Table 1. As illustrated before, both specimens had exactly the same components, except that in Specimen One there was a gap of 32 mm between the RC wall edges and the steel boundary frame in the middle two stories. The wide flange (WF) columns and beams were made of A572 Grade 50 steel with yield stress of 345 MPa, and the steel wall plate was made of A36 steel with yield stress of 248

3 Seismic Behavior of Composite Shear Wall Systems and Application of Smart Structures Technology 71 Figure 3. Composite shear wall specimen and test set-up. MPa. The concrete had a minimum f c of 28 MPa. The test specimen and details of the reinforced concrete walls are shown in Fig. 1 and 2. Test set-up for the composite shear wall tests is shown in Fig. 3. During the test, cyclic shear displacements were applied by the actuator to the top of the specimen through the top loading beam, and the shear force was transferred to the lab floor by the bottom reaction beam and reaction blocks. As shown in Fig. 4, the same cyclic displacements were applied to both specimens, which were established according to the specifications for Qualifying Cyclic Tests of Beam-to-Column and Link-to-Column Connections in Seismic Provisions for Structural Steel Buildings (AISC 1997). A set of linear variable displacement transducers (LVDT) and strain gauges were installed on the test specimens and test set-up in order to measure the displacement and strain at critical locations of the specimen and monitor slippage of the test set-up Cyclic behavior of composite shear wall system Specimen One, with a 32 mm gap around the RC wall, behaved in a very ductile and desirable manner. Up to overall drifts of about 0.006, the specimen was almost elastic. At this drift level some yield lines appeared on the beams as well as column base. At overall drifts of about 0.012, the compression diagonal in the steel wall panels was buckling and diagonal tension field was forming. The specimen could tolerate 33 cycles, out of which 27 cycles were inelastic, before reaching an overall drift of and maximum shear strength of about 2790 kn. At this level of drift, fractures were widespread in the walls and frame members due to low-cycle fatigue, and the bolts connecting the steel wall and RC wall were almost gone. Shear strength of the specimen dropped to about 80% of the maximum capacity, and the specimen was considered failed. Specimen Two also behaved in a ductile manner. Up to overall drifts of about 0.006, the specimen was almost elastic. At this drift level some yield lines appeared on the bottom and middle beam webs as well as column base plate. The specimen was able to reach cyclic overall drift of after undergoing 23 cycles, 17 of the cycles being inelastic. The maximum shear force reached was about 3020 kn during the 19th cycle. Throughout the test, the gravity load carrying column remained essentially stable while non-gravity carrying lateral load resisting elements underwent well-distributed and desirable yielding. During the 23rd cycle, the upper steel shear wall plate fractured totally along the north and bottom edges due to low-cycle fatigue, and the bolts connecting the steel wall and concrete wall were almost gone. Shear strength of the specimen dropped to about 80% of the maximum capacity, and the specimen was considered failed. Figure 5 shows both specimens after the test, as well as the hysteresis curve for the third story of both specimens. Based on the test observations and post processing of Figure 4. Loading history of composite shear wall tests.

4 72 Qiuhong Zhao and Abolhassan Astaneh-Asl Figure 5. Composite shear wall specimens after the test and hysteresis curve for the third story. test data, it is clear that in the innovative composite shear wall system, damage to the concrete wall was much less than in the traditional composite shear wall system. The steel wall didn t have excessive global buckling compared to some other steel shear wall tests conducted at Berkeley (Zhao 2004); instead the buckling happened locally between the bolts. The sequence of yielding of components was very desirable with yielding showing in WF beams and steel walls first. At the end of the test, the WF columns showed yielding at the base but didn t buckle. The composite shear walls and WF beams did yield extensively and dissipated energy, which made the composite shear wall system very ductile with inter-story drift over 4.4%. Therefore an R-factor of 8.0, in the codes today, was confirmed. An R of 9-10 is more appropriate. 4. Analytical Studies Finite element analyses were conducted on the composite shear wall specimens, along with parametric studies. Two models were constructed with model one representing Specimen One (innovative system) and model two representing Specimen Two (traditional system), as shown in Fig. 6. Accordingly, there was a 32 mm gap between the four edge surfaces of the RC wall and the boundary frame in model one, while there was no gap in

5 Seismic Behavior of Composite Shear Wall Systems and Application of Smart Structures Technology 73 Figure 6. Finite element models of composite shear wall specimens. model two. All the other geometric, material and element properties as well as boundary conditions of these two models were the same. In order to facilitate the calculation process, a negligible gap was introduced between the RC wall edge surfaces and the boundary frame in model two, as well as between the steel wall and RC wall in both models. Most of the structural components were modeled as nonlinear shell elements, except for the bolts which were modeled as 1-D beam elements. The steel material properties were simplified to a bilinear model considering strain-hardening, and the reinforced concrete material property was simplified to an elasto-plastic model considering the contribution from rebars. In addition, the elastic modulus E of the steel material for plates was reduced by 30% to take into account for initial warping, geometric imperfections, residual stresses, etc. MSC Nastran was used to conduct the nonlinear pushover analysis on the structural system. An implicit nonlinear solver was used to consider geometric and material nonlinearities during the push-over analyses, as well as contact phenomena (MSC Corp., 2005). In order to simulate the contact between the RC wall and the surrounding steel surfaces, the RC wall and the surrounding steel parts were defined as separate contact bodies. By applying contact methodology, motion of the RC wall and the surrounding steel parts on the boundary gap would be monitored, so that transferring of forces and stresses on the boundary would be conducted once the steel and concrete surfaces get into contact. The lateral force vs. overall displacement curve from the push-over analysis matched with the test results to a reasonable extent, for both specimens as shown in Fig. 7. Parametric studies were developed on this basis. Three cases were run for the composite shear wall systems to Figure 7. Comparison of experimental and analytical push-over curves for composite shear wall specimens. identify the key design parameters. In each case, only one parameter in the structural model was modified. Parametric studies showed that for the composite shear wall system studied in this paper, the steel wall is the major component and its stiffness and strength contribute the most to the overall system stiffness and strength. Increasing the steel wall thickness would be a very effective way to strengthen the whole system; however, premature failure of the system might occur if the WF columns were not strong enough. In the mean time, using higher strength steel for the steel wall would also be an effective way to strengthen the composite shear wall system, while using high strength concrete for the RC wall wouldn t affect the system behavior as much. 5. Application of Smart Structures Technology Smart structures technology involves development of intelligent material or structures that can monitor their own condition, detect impending failure, control damage, and adapt to changing environments (Chong, 2003). The idea of smart structures technology was shown in the design of the innovative composite shear wall system

6 74 Qiuhong Zhao and Abolhassan Astaneh-Asl 6. Conclusions Figure 8. Function of gap in the innovative composite shear wall system. which helps control structural damage and adapt structural behavior to external seismic events. By introducing the gap in the innovative composite shear wall system, it is anticipated that the system lateral stiffness would be reduced and the RC wall behavior would be improved under severer seismic events. In the traditional composite shear wall system, both the steel and RC walls will be active in resisting lateral loads as soon as a lateral displacement is applied. As a result, larger base shear will be present in the structure due to relatively larger stiffness of the combined system, and the RC wall could be damaged under relatively small lateral displacement. In the innovative composite shear wall system, however, due to the existence of the gap, the RC wall will not get involved in resisting lateral loads until the inter-story drift has reached a certain value, as shown in Fig. 8. When the drift is under the specified value, only steel shear wall and the boundary moment frame provide strength, stiffness and ductility, and the role of RC wall is to provide out-of-plane bracing for the steel plate. When the drift is over the specified value, the gap is closed at corners and both steel and RC walls become active and provide strength, stiffness and ductility. Then the participation of RC wall brings in the much needed extra stiffness to help reduce the drift and P- effects, compensates for loss of stiffness of steel shear wall due to yielding, and helps in preventing lateral creep and collapse failure of the structure due to P- effects. An additional possible application of the smart structures technology to the composite shear wall systems would be the use of visco-elastic material as a filler in the gap around the RC walls in the innovative system, such that more damping could be introduced to the system and the energy dissipation capacity of the whole system would be increased under seismic effects. The introduction of smart materials such as replacing the concrete with a lighter material that could provide enough bracing to the steel wall would also be a potential application. The projects described in this paper addressed the issues of cyclic behavior of two composite shear wall systems, and proposed seismic design recommendations. Through the experimental studies, it is clear that both systems were very ductile under large cyclic displacements with maximum inter-story drifts over 4.2%. Therefore an R factor of 8.0 or even 9.0 could be used in the seismic design of these systems. The experimental studies also showed the importance of keeping the gravity load carrying members in these systems intact under seismic effects, while the non-gravity carrying members could yield extensively and dissipate energy. The project also verified the idea of innovative composite shear wall system and compared its performance with the traditional composite shear wall system. Experimental results showed that by bolting a RC wall to a steel shear wall on one side, the excessive global buckling of the steel wall was prevented. In the mean time, the gap in the innovative composite shear wall system introduced more stable behavior and reduced the damage to the concrete wall. The analytical studies on the composite shear wall system showed some of the major factors that control the overall shear strength of the system. Further refined analytical studies on more parameters would help in identifying the key parameters for seismic design. Smart structures technology could be applied to the design and construction of the composite shear wall systems, and further improvement of system behavior could be achieved by introducing new materials. Acknowledgments This project was funded by the National Science Foundation, Directorate of Engineering, Civil and Mechanical Systems. The technical assistance and input from Program Directors Dr. S. C. Liu and Dr. P. Chang at NSF were much valuable and sincerely appreciated. The research was part of the U.S. Japan Cooperative Research on Composite and Hybrid Structures of the National Science Foundation. The guidance and technical input of all involved in the program, in particular Professors Stephen Mahin and Subhash Goel, directors and organizers of the program are sincerely appreciated. The Structural Steel Educational Council, American Institute of Steel Construction (AISC) and the Herrick Corporation also provided valuable input and support. Judy Liu, formerly graduate student at the University of California, Berkeley provided valuable help in developing, analyzing and designing the test set-up. Her work is very much appreciated. Ricky Hwa, undergraduate research assistant participated in preparing specimens, instrumentation, and

7 Seismic Behavior of Composite Shear Wall Systems and Application of Smart Structures Technology 75 conducting tests. His dedicated and valuable work was very helpful to the success of the project. Finally, this experimental program could not have been completed without the resources of the laboratory and staff of the Department of Civil and Environmental Engineering at the University of California at Berkeley. References AISC (1999). Load and Resistance Factor Design Specification. American Institute of Steel Construction Inc., Chicago AISC (1997). Seismic Provisions for Structural Steel Building. American Institute of Steel Construction Inc., Chicago AISC (1994). Manual of Steel Construction- Load and Resistance Factor Design, 2nd Edition. 2 Volumes, American Institute of Steel Construction Inc., Chicago Allen H.G., Bulson P.S. (1980). Background to Buckling. McGraw Hill Book Company, U.K. Chong, K.P. (2003). Health Monitoring of Civil Infrastructures. Smart Materials and Structures 12: Dean R.G., Canon T.J., Poland C.D. (1977). Unusual Structural Aspects of H.C. Moffit Hospital. Proceedings of the 46th Annual Convention, SEAOC, Coronado, CA. ICBO. (1997). The Uniform Building Code. Vol 2. The International Conference of Building Officials, Whittier, CA. MSC Corp. (2005). MSC.Nastran Implicit Nonlinear (SOL 600) User s Guide. MSC Software Corporation, USA. Zhao Q., Astaneh-Asl A, (2004). Cyclic Behavior of an Innovative Steel Shear Wall System. Proceedings of the 13th World Conference on Earthquake Engineering, Vancouver, Canada.