SEISMIC STUDY OF HYBRID SHEAR WALL

Transcription

1 10NCEE Tenth U.S. National Conference on Earthquake Engineering Frontiers of Earthquake Engineering July 21-25, 2014 Anchorage, Alaska SEISMIC STUDY OF HYBRID SHEAR WALL Xu, L. 1, Shen, X. 2 and Shen, J. 3 ABSTRACT The paper presents an experimental and associated numerical study on a hybrid shear wall system as a seismic force resisting element for taller buildings. Different from traditional sandwich shear wall, the hybrid shear wall in the study consists of two exterior precast wall layers and one interior cast-in-place concrete layer. This type of wall system has been increasingly used in moderate- to high-rise buildings in low-seismicity areas, and its seismic behavior is little known. Revealing its corresponding seismic behavior will help develop a rational seismic design details for ductile seismic force resisting system as a formidable alternative to conventional cast-in-place shear walls by combining inherent the benefits of speedy construction and sustainability of precast technology with improved structural integrity and ductility. In this research study, both experimental observation and FEM simulation validation were used to analyze the hybrid shear wall system. Three sets of full-scale shear walls including two hybrid shear walls and one cast-in-place shear wall were tested subjected to cyclic horizontal. A series of 3D non-linear finite element models which included all significant details and specifications were created in ABAQUS to simulate the experiments. The pushover analysis method was employed to reproduce the test procedures. The response of the structure was computed both at macro and micro levels in order to validate the accuracy of the analytical model. After a good agreement between the experimental observations and simulation results, a comprehensive parametric study was conducted to determine seismic design parameters, and to explore damage mechanism of the hybrid shear wall system. 1 Dept. of CAEE, Illinois Institute of Technology, IL Dept. of Civil Engineering, Anhui University of Architecture, China Dept. of CCEE, Iowa State University, IA Lifeng Xu, Xiaopu Shen and Jay Shen. Seismic Study of Hybrid Shear Wall. Proceedings of the 10 th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014.

2 Tenth U.S. National Conference on Earthquake Engineering Frontiers of Earthquake Engineering July 21-25, 2014 Anchorage, Alaska Seismic Study of Hybrid Shear Wall Xu, L. 1, Shen, X. 2 and Shen, J. 3 ABSTRACT The paper presents an experimental and associated numerical study on a hybrid shear wall system as seismic force resisting structures for taller buildings. Different from traditional sandwich shear wall, the hybrid shear wall in the study consists of two exterior precast wall layers and one interior cast-in-place concrete layer. This type of wall system has been increasingly used in moderate- to high-rise buildings in low-seismicity areas, and its seismic behavior is little known. Revealing its corresponding seismic behavior will not only help develop theoretical background for design, but also will help to take advantage of it: less labor, less contamination, good integrity as cast-in-place shear wall, etc. In this research study, both experimental observation and FEM simulation validation were used to analyze the hybrid shear wall system. Three sets of full-scale shear walls including two hybrid shear walls and one cast-in-place shear wall were tested subjected to vertical pressure and cyclic horizontal load in Anhui University of Architecture, China (2010). A series of 3D non-linear finite element models which included all significant details and specifications were created in ABAQUS to simulate the experiments. The pushover analysis method was employed to reproduce the test procedures. The response of the structure was computed both at macro and micro levels in order to validate the accuracy of the analytical model. After good agreements between the experimental observations and simulation results were consistently achieved, a comprehensive parametric study was conducted to determine seismic design parameters, and to explore damage mechanisms and design optimization of the hybrid shear wall system. 1. Introduction The shear wall system could be categorized into precast shear wall and cast-in-place shear wall according to distinct construction technology. Engineers are searching for a new shear wall system that could balance two wall systems advantage together, which is to construct an ideal shear wall under a neat environment and less labor but finally gets a better global seismic resisting capacity. This is the orient motivation for this research about the sandwich shaped hybrid shear wall system. According to the PCI committee s agreement [1], the production history of precast sandwich shear wall panels in North America was traced to 50 years ago. A lot of theoretical 1 Dept. of Civil, Architectural, and Environmental Engineering, Illinois Institute of Technology, IL Dept. of Civil Engineering, Anhui University of Architecture, China Dept. of CCEE, Iowa State University, IA Lifeng Xu, Xiaopu Shen and Jay Shen. Seismic Study of Hybrid Shear Wall. Proceedings of the 10 th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014.

3 analyses and experiments have been done towards the traditional sandwich shear wall [2-5]. Different from traditional walls, the sandwich shaped hybrid shear wall includes a cast-inplace concrete layer to replace the inner insulation layer in order to get better system integrity and ductility. This type of shear walls has been used for non-seismic design of buildings, its seismic behavior remains unknown. Both experimental and numerical simulation studies were conduct to investigate the hybrid shear wall to reveal its cyclic behavior and explore design details leading to a ductile seismic design of such system. 2. Experimental Study Three full scale sets of confirmatory tests with different forms of wall panels are presented: Wall 1 (W1) is a cast-in-place shear wall panel as a comparison specimen; Wall 2 (W2) is a hybrid shear wall with a rigid panel connection (reinforcement enhanced area), Wall 3 (W3) is also a hybrid shear wall but with less reinforcement in the connection. W1 is a continuous panel with the dimensions of 2800(h) 2000(w) 200(t)mm which is cast in field, while the hybrid wall panel W2 and W3 are consists of two precast panels with the dimension of 2800(h) 1000(w) 200(t)mm. In the hybrid shear wall, the thickness of exterior precast wall panel at two ends is 50mm, with a reserved 100mm in between as the cast-in-place concrete with reinforced enhanced core. The H shaped supporting is 550mm height which is fixed to the ground by the anchor bolts; the top beam connecting to the electrolyte servo systems in the vertical and horizontal direction. All the shear walls are connected to the supporting and the beam by reinforced enhanced area, which consists of the Ф6@200 rebar symmetrically extended from one component to another among the cast-in-place concrete. The difference between the W-2 and W-3 is the reinforced enhanced zone in the middle portion of the cast-in-place layer, the specifications as shown in Fig. 1.

4 W2 Plan Form W2 Side View W3 Side View W3 Plan Form Figure 1. Sandwich Hybrid Shear Wall Panel Experiment Settings 2.1 Accessory Test Field-cast shear wall sample W-1 with concrete intensity rating 30. The bearing steel inside it is HRB 335. The precast wall panels in sample W-2,W-3, intensity rating C35/45 came from German Silwade Company. The bearing steel inside the precast wall panels is BSt500. After shipped in cargo containers to home labs from abroad concrete-filled cores are re-casted on the spot to be superimposed cores. The concrete intensity rating is C30 and steels are HPB235 and HRB335. The mechanical properties of the major materials as follows,

5 Table 1. Steel Mechanical Properties Diameter (mm) Grade Yield Strength (MPA) Failure Strength (MPA) Elongation (%) CHN 6 HRB CHN 8 HRB CHN10 HRB CHN12 HRB GRM 6 BSt GRM 8 BSt GMN10 BSt GRM 12 BSt GRM14 BSt Table 2. Cubic Compressive Strength of Concrete Cast-in-Place Wall Specimen Hybrid Wall Internal Core Specimen 1(MPA) Specimen 2(MPA) Specimen 3(MPA) Average(MPA) Loading History Loadings in two direction are applied on the test members: The vertical pressure force is about 600KN according to the pull down ratio 0.1 Vertical loading employs counterforce rack device. When loading, first apply 40% of the full loading pressure force to the top of the wall and repeat this 2-3 times in order to remove the malconformation in the internal structure of samples. Then the axial pressure force is added to the full load and remains unchanged in the trials. The horizontal load is low-cycle repeated loading, which is divided into two phases: Phase 1 employ load control with the loading pace 50KN.First to have monotonic step loading to crack; then add the range, 25KN of the limit load, to the building blocks yielding and all-level

6 loading recycles once at the same time. In Phase 2, to employ displacement control, with step loading according to the times of apex displacing while yielding and recycling twice. 2.3 Experimental Result and Observations Several observations and conclusions are summarized according to the above experiments, 1. Based on the envelope area of hysteresis curves below, Cast-in-place Shear Wall performances an excellent energy dissipate capacity, while the hybrid shear wall with rigid connections has a better capacity than that with moderate connections, which indicates the importance of the rigid connection in seismic resisting capacity of hybrid shear wall. W1 W2 W3 Figure 2. Hysteresis Curve of Different Shear Walls 2. The force state and breakage form of hybrid shear wall panels are in accordance with the cast-in-place shear wall on the whole, which all belong to bending -shear failure. 3. Comparing the testing processes of wall panel samples W-1.W-2 and W-3, it shows that the locations for initial cracks to appear are basically the same; the cracks trends and macroscopic distributions are similar; the locations of the maximum cracks are basically the same, all within the range of 500mm-700mm ( ratio of the total height) above the bottom of the wall panel. 4. No relative slide between the exterior precast concrete layers and interior cast-in-place layer is visible by naked eyes. 5. Unlike the cast-in-place shear wall, the compression crush zone in hybrid shear wall is partially cracking instead of whole section cracking. 6. The maximum drift ratio is approximately 1.6% for hybrid shear wall under the current experiment. 3. FEM Simulation As we know, experimentation is more accurate, however it s time consuming and definitely very costly. Nowadays, computer simulation is really capable of capturing the major failure modes and showing comparable results with actual test specimens. Thus, in order to explore single hybrid wall s seismic behavior under the distinct parameters, convincing numerical models should be created and validated by the comparison between the experimental observations and simulation results. 3-D models were then set up in ABAQUS following experimental specifications accordingly.

7 Top Girder W-1 Section Top-view Shear Wall A Supporting A W-2 Section Top-view W-3 Section Top-view A-A Section 3.1 3D Nonlinear Finite Element Model Material Model Figure 3. Model Setting in ABAQUS As a main component in shear wall, concrete mechanical behavior plays an important role through the whole simulation procedure, and the model s mechanical capacity and fracture mode is very sensitive to the concrete input data, Improper concrete definition will not only lead to un-precise simulation results but also result in a negative Eigen-value which finally cause FEM calculation error. The chosen of the concrete material model is the most critical step among whole simulation, since concrete is a typical quasi-brittle material and it is difficult to represent stiffness degradation caused by micro-cracks growth. Also, the concrete s compression strength will be dramatically enhanced due to the confinement effect which is already proved by the existing experiments nowadays[10]. Accordingly, in current simulation, concrete model is based on the balance of plastic damage model[8,9] and Mander s confinement model[10]. What s more, noticing that the steel data from accessory test could not directly be applied into the simulation, in order to take into consideration of rebar s pinch effect, true stress are adopted instead of nominal stress in the ABAUQS according to the following equations, σ true = σ nominal (1+ε nominal ) (1) ε true = ln(1+ε nominal ) (2) where σ nominal and ε nominal is the nominal stress and strain from test, σ true and ε true is the corresponding true stress in Eq.1 and Element and Model Specifications All the components dimensions, locations, cross sections are precisely according to the existing experiments: shear walls connected to the beam and the support by the inter rebar; this wall mesh is refined at lower portion since the concrete cracking mainly occur at that position; the friction coefficient between the precast wall panel is assumed to be 0.3; the rebar is

8 embedded to the concrete and the cohesive behavior is considered. Solid elements(c3d8r) are adopted for concrete simulation and truss element(t3d2) for rebar, 32 surface contact pairs are adopted to simulate the contact or friction behavior among the cast-in-place layer, two precast panels, beam and the support, reference point and the rigid body is applied on the top beam so as to make sure that the beam failure is excluded in the simulation. Dynamic explicit method is used in simulation, there are totally two steps: step one is to apply vertical load, step two is to apply horizontal load(using displacement control), both of the above steps are adopting smooth steps to guarantee the accuracy and efficiency quasi-static analyses. 3.2 Comparison Between the Experiment and Simulation Load-displacement Curve. In the analysis model, different friction coefficients between the cast-in-place layer and the precast concrete layer were adopted, varying from 0.3 to 0.5, also limit to the fact that the Mander s concrete model theory is based on his experiment test whose confinement is stronger than current experiment, the behavior of confined concrete should be somehow between the Mander s concrete model and the plain concrete model, thus several level of concrete models were used to represent its corresponding behavior, especially for the compression decay. Figure 4. Force-Displacement Comparison The above curves (Fig. 4) indicate that the horizontal resisting capacity of shear wall is very close between the experiments records and the FEM simulation results, the W2 has a larger lateral resisting capacity since it has a more rigid reinforced enhanced area embedding in the panel connection. The initial stiffness in W-1 and W-2 matches good, while in W-3 simulation results is larger than the experimental results, this phenomenon might due to the factor of experiment setting,most probably the foundation is not as rigid as assumed. The over strength factor in W-2 (Ωo 1.6)is larger than W-1and W-3(Ωo 1.1),indicating that the hybrid shear wall with a more reinforced enhanced in connection has a larger potential lateral resisting capacity beyond system yielding than W-1 and W Cracking process(take W-2 test as an example). The cracking process for hybrid shear wall in ABAQUS validate the experimental observation records for W-2 member:" When peak value of the horizontal load added to +225kN,

9 initial horizontal cracks appear at the side of the wall of 680mm above the bottom of the pulled sidewall with the length of around 450mm. When loaded to +300kN, the second horizontal crack appears at 450mm above the bottom of the wall" Stage I(Monotonic) Stage II(Monotonic) Experiment Observations (Cyclic) Figure 5. Concrete Cracking Developing Process (South Side View) A remarkable simulation result is that according to the experiment, the largest plastic range mainly concentrates in the vicinity of the section in connection between the supporting and the interior concrete layers, which is also proved by the compression damage according to Von Mises as in Fig. 6, In the contour plot, the largest compression damage occur around two end rebar which connect the middle portion concrete and the support, two exterior precast wall panels suffer less damaged than the interior cast-in-place layer. Actually there should be some micro compression damage at the zone 1 and zone 2 which may not be captured by experimental observations. Zone 2 Zone 1 Figure 6. Largest Plastic Area Comparison Maximum equivalent plastic strain. (Take W-1 as an example) Since Wall 1 is cast-in-place concrete shear wall panel, accordingly, there is no connection or exterior precast panels involved in the simulation. Different from the hybrid shear wall (Seen from Fig. 7), the destroy pattern in the crushing zone is whole cross section cracking instead of partial cracking.

10 . Side View Max Point Tension zone cracking Crushing zone U-1 Displacement Figure 7. PEEQMAX for Shear Walls 3.4.Parametric Study Many parameters may influence the shear wall s mechanism, but among them, emphasis should mainly be placed on the height/width ratio, axial compression level, longitudinal rebar spacing, longitudinal rebar ratio and wall thickness, based on the experience gained from the traditional shear wall system research. The good agreements between the experimental observations and simulation results indicate the validity of the previous prototype, based on this prototype, dozens of different trail models with different levels were then created and analyzed in ABAQUS to look insight into the parameter influence. The results are concluded in the conclusion which may help for the design optimism. 4.Conclusions The results of an experimental campaign on sandwich hybrid shear wall panels with different connection, used as seismic resisting wall panels, have been presented. Subsequently numerical simulation results with linear and non-linear finite element models were also carried out. Through the above study, several useful observations and simulation results are summarized to help us to better understand the hybrid shear wall s seismic mechanism. (1) Comparing to cast-in-place shear wall, the hybrid shear wall is less effective in energy dissipation, and the larger longitudinal rebar ratio in hybrid shear wall panel connection will benefit its cyclic behavior. (2) Under the monotonic load, there is no much difference between the cast-in-place wall s strength level and hybrid wall s. However, the fracture pattern in compression zone is different. For cast-in-place wall, its whole section crushed at the compression corner, while for hybrid wall, only the middle portion of concrete cross section bulb out.

11 (3) The fracture drift ratio of the hybrid shear wall is approximately 1.6% in current experiments, based on the assumption from FEMA 695 that states the ultimate displacement as the displacement at a level 80% of the capping strength in the descending branch of the pushover curve. (4) Shear wall s cracking diagonal is mainly determined by the height/width (H/W) ratio, high diagonal cracking trends are expected in seismic design since high H/W ratio usually lead to ductile fracture. (5) Axial force and the longitudinal rebar ratio are critical to the shear wall s strength level. The longitudinal rebar spacing has few to do with the lateral resisting capacity. (6) Longitudinal rebar ratio is sensitive to the system s over-strength ratio(ω 0 ), while different compression level and wall thickness result in the same Ω 0. Acknowledgement The financial support provided by Silwade Precast L.T.D in German, is gratefully acknowledged. The technical support of the laboratory staff at the Anhui University of Architecture and the research group in Dept. of CAEE of Illinois Institute of Technology, is greatly appreciated. References 1. PCI Committee on Precast Sandwich Wall Panels. (1997). State-of-the-Art of Precast/ Prestressed Sandwich Wall Panels. PCI Journal, Vol. 42, No.2, pp Benayoune, A., Samad, A. A. A., Trikha, D. N., Ali, A. A. A. & Ellinna, S.H.M. (2008). Flexural Behaviors of Precast Concrete Sandwich Composite Panel Experimental and Theoretical Investigations. Construction and Building Materials Journal, Vol. 22, pp Mahendran, M. & Subaaharan, S. (2002). Shear Strength of Sandwich Panel Systems. Australian Journal of Structural Engineering, 3(3), pp Gara, F., Ragni, L., Roia, D. & Dezi, L. (2012). Experimental tests and numerical modeling of wall sandwich panels, Engineering Structures Journal, Vol. 37, pp Hamid, A. & Fudzee, M. (2013). Seismic Performance Of Insulated Sandwich Wall Panel(ISWP) Under Inplane Lateral Cyclic Loading, International Journal of Emerging Technology and Advanced Engineering, Volume 3, Issue 3,pp Okamura, H. & Ouchi, M. (2003). Self-compacting concrete. Journal of Advanced Concrete Technology, Vol. 1, pp Shishido, T., Shiraiwa, S., Ishii, J., Yamamoto, S. & Kume, H. (1999). Pouring works of high fluidity concrete for immersed tunnel by steel-concrete composite structure. Proceedings of the International Work on Self- Compacting Concrete, Vol. 30, pp Lee, J. & Fenves, G. (1998). A plastic-damage concrete model for earthquake analysis of dams, Earthquake Engineering & Structural Dynamics, Volume27,pp Lubliner, J., Oliver, J. Oller, J. & Onate, E. (1989). A plastic-damage model for concrete, International Journal of Solids and Structures - INT J SOLIDS STRUCT, Vol. 25, pp Mander, J. B., Priestley, M. J. N. & Park, R. (1988). Theoretical stress-strain model for confined concrete, Journal of Structural Engineering ASCE, 114(8),pp