Study on the effect of the infill walls on the seismic performance of a reinforced concrete frame

Transcription

1 Vol.1, No.4 EARTHQUAKE ENGINEERING AND ENGINEERING VIBRATION December, 211 Earthq Eng & Eng Vib (211) 1: DOI: 1.17/s x Study on the effect of the infill walls on the seismic performance of a reinforced concrete frame Zhang Cuiqiang 1, Zhou Ying 2, Zhou Deyuan 1 and Lu Xilin 2 1. Research Institute of Structural Engineering and Disaster Reduction, Tongji University, Shanghai 292, China 2. State Key Laboratory of Disaster Reduction in Civil Engineering, Tongji University, Shanghai 292, China Abstract: Motivated by the seismic damage observed to reinforced concrete (RC) frame structures during the Wenchuan earthquake, the effect of infill walls on the seismic performance of a RC frame is studied in this paper. Infill walls, especially those made of masonry, offer some amount of stiffness and strength. Therefore, the effect of infill walls should be considered during the design of RC frames. In this study, an analysis of the recorded ground motion in the Wenchuan earthquake is performed. Then, a numerical model is developed to simulate the infill walls. Finally, nonlinear dynamic analysis is carried out on a RC frame with and without infill walls, respectively, by using CANNY software. Through a comparative analysis, the following conclusions can be drawn. The failure mode of the frame with infill walls is in accordance with the seismic damage failure pattern, which is strong beam and weak column mode. This indicates that the infill walls change the failure pattern of the frame, and it is necessary to consider them in the seismic design of the RC frame. The numerical model presented in this paper can effectively simulate the effect of infill walls on the RC frame. Keywords: infill walls; RC frame structure; strong column and weak beam; strong beam and weak column; nonlinear time history analysis 1 Introduction In the field of engineering, it is generally considered that infill walls in a concrete frame serve as a nonstructural element. In actual practice, the influence of infill walls on the stiffness of frame structures is considered by a reduction factor to the period of the reinforced concrete (RC) frame without infill walls. The disadvantage of this method, however, is that it disregards the constraint of the infill walls on the RC frame. In the Wenchuan earthquake, many frames suffered plastic hinge damage at columns instead of beams, which is different from the failure mode expected in the seismic code. The constraint effect of the infill walls changes the failure mode of the frame. Some researchers (Qian and Zhao, 28; Li et al., 29; Yang et al., 29; Guo et al., 28) pointed out that infill walls in RC frames change the frame failure mode by changing the stiffness of the RC frame. Others (Chaker and Cherifati, 1999; Shi et al., 1996) who conducted experimental studies concluded that the Correspondence to: Zhou Ying, State Key Laboratory of Disaster Reduction in Civil Engineering, Tongji University, Shanghai 292, China Tel: ; Fax: yingzhou@tongji.edu.cn PhD candidate; Associate Professor; Professor Received June 8, 211; Accepted October 27, 211 location of the infill walls plays different roles in the mechanical properties of the frame and the infill walls can sometimes increase the frame stiffness as much as seven times over the bare frame. Therefore, the effect of the infill walls must be taken into account in the design of the RC frame if the required failure mode in the Chinese seismic code is to be realized. Generally, there are three mechanical models of infill walls in the RC frame as follows: (1) the effective stiffness model (Code for Seismic Design Buildings, 21), in which the infill walls are considered by decreasing the period of the bare frame whereas the weight of the infill walls is converted to load applied to the frame; (2) frame-infill-wall parallel model (Cao et al., 1997), in which the infill walls are considered as the shear wall to work together with the frame, the frame and infill walls have the same displacement. The stiffness of the infill walls adopts.2.3 times the initial stiffness after cracking; and (3) diagonal strut model (Mónica Puglisi et al., 29a,b), in which the infill walls are considered as the diagonal strut and pin connection used at the ends. The first model can consider the stiffness of the infill walls, but the location of the infill walls in the RC frames cannot be considered. The second mechanical model does not take the constraint between the RC frame and infill walls into account. The third mechanical model can take the stiffness of the infill walls into consideration,

2 58 EARTHQUAKE ENGINEERING AND ENGINEERING VIBRATION Vol.1 but it does not consider the co-work behavior of the beam and the filled wall. In this paper, the diagonal strut model is used to simulate the stiffness of the infill walls; meanwhile a modified section of beam is used to simulate the cowork behavior of the infill walls. The results show good agreement with the actual damage. 2 Damage observation During the Wenchuan earthquake, many buildings suffered severe damage. Many RC frame structures, designed according to the code, developed a column hinge mechanism which is different from the desired mechanism specified in the China code of seismic design of buildings (CCSDB, GB511-21). In the CCSDB, a beam hinge mechanism is preferred to dissipate the earthquake energy and prevent collapse when subjected to a rare earthquake, with a 2% to 3% probability of exceedance in 5 years. However, the seismic behavior of RC frames in the Wenchuan earthquake was beyond the expectations of researchers. Column hinges developed on the RC frames during the earthquake. Typical damage such as observed in a teaching school building located in Dujiangyan is presented. The elevation of the teaching building is shown in Fig. 1. Different extents of damage are seen on the top end of the columns in the first floor. The columns at the axis E (Fig. 2) were only slightly damaged, the columns at the C (Fig. 2) and D (Fig. 2) axes sustained moderate damage, while the columns at the A (Fig. 2) and B (Fig. 2) axes suffered the severe damage. In addition, much cracking was observed in the first floor infill walls and the veneer fell to the ground. There is no obvious damage on the second floor except for some cracking in the infill walls and falling of veneer in the staircase room. No damage was observed in the floors above. Typical damage is presented in Fig Target building 3.1 Building information The target building in this paper is located in the Dujiangyan City, Sichuan Province. The structural plan is shown in Fig. 3 and Fig. 4. The specific cross section information is presented in Table 1. Because of the requirement of the architectural function, there are no Fig. 1 Elevation of the building (a) Crushing on the column top end (b) Infill wall damage at axis E (c) Column damage at axis B Fig. 2 First floor damage

3 No.4 Zhang Cuiqiang et al.: Study on the effect of the infill walls on the seismic performance of a reinforced concrete frame 59 E L2 L2 L2 E L2 D C B A Z3 L2 L2 L2 Z3 L2 L2 L2 L2 L2 L2 L2 L2 B Z2 Z2 Z2 L2 Z2 A L2 L4 D C L2 L2 Fig. 3 Plan layout of the first floor Fig. 4 Plan layout of the fifth floor Table 1 Section of the beam and column (mm) L2 (mm) (mm) L4 (mm) (mm) Z2 (mm) Height of story (m) 1F F F F F Note: -: there is no element infill walls in the first floor except for three areas (Fig. 3, shadowed areas). The thickness of the infill walls is 24 mm. 3.2 Material information The specific information of material used in this building is presented in Table 2. 4 Analytical model Table 2 Material of target building Material Grade Design value of strength Compression(N/mm 2 ) Tension(N/mm 2 ) Concrete C Steel bar HRB Steel bar HPB Masonry mortar MU 15+ M Note: -: the value in China Code for design of masonry structures (GB 53-21) 4.1 Assumption The column ends in this paper use fiber elements to simulate the coupling behavior between the biaxial bending and axial deformation. In the middle of the column, a bi-directional shear spring element is used to simulate the bi-directional shear behavior. Beam ends employ a tri-linear spring element to simulate the bending behavior. In the middle of the beam, a bi-linear spring element is used to simulate the shear behavior. The concrete constitutive model in this paper is illustrated in Fig. 5 (Hoshikuma et al., 1997), and the steel bar constitutive model is shown in Fig. 6 (Li, 24). The infill walls are simulated by the compression only element. The width of the section is.25 times the δ δ c τε t 7 ε t σ t 8 Φε t λσ c ε c ε ε u Fig. 5 Concrete constitutive model and hysteretic relationship

4 51 EARTHQUAKE ENGINEERING AND ENGINEERING VIBRATION Vol.1 length of the diagonal line (Paulay and Priestley, 1991; Hu, 21). The normalized constitutive relationship of the masonry infill wall (Zhu, 199) is given in Eq. (5). The constitutive relationship of the diagonal strut is a tri-linear spring model (Kwon and Kim, 29) shown in Fig. 7. The tri-linear displacement-force relationship consists of cracking force (V c ), maximum compression force (V m ), yield displacement (U c ) and maximum displacement at peak force (U m ). The masonry maximum compression strain corresponding to the peak compression stress is defined as.5, and the ultimate strain is defined as.2. The cracking shear force is defined as.6 times the maximum compression force. The compression force is calculated by the following equations. U m = tmdm (1) σ 3 M U C =.6 tmd m (2) 7 σ y K = Et d m m (3) ε ' m θσ y ε y εy ε m θσ ' y ε K C = K (4) / = / f m / > (5) M ' -σ y 4 Fig. 6 Steel bar constitutive model and hysteretic relationship V (Force) V m V c Compression U c U m U (Displacement) Fig. 7 Tri-linear constitutive relationship of the diagonal strut y x Fig. 8 Section of the beam considering the infill walls where: is the masonry peak compression stress; t m is the thickness of the masonry infill wall, set as 24 mm; and d m is the width of the diagonal strut, set as,25 times of the diagonal length. E is the elastic modulus of masonry, set as 24 N/mm 2 ; and α is set as.2. f m is the masonry peak stress. In order to consider the effect of the infill walls on the beam, the beam section is modified by adding one-third of the inter-story height infill walls onto the original RC cross section to form an enlarged cross section (Fig. 8). The width of the flange of the cross section is 12 times the thickness of the slab. It is necessary to use the masonry wall peak stress and ultimate strain in the concrete constitutive relationship to simulate the property of the infill walls (Zhu, 199). 4.2 Recorded ground motions Figure 9 shows the time history of the ground motion recorded at the Wolong station in the N-S direction. The ground motion recorded at the Wolong station during the Wenchuan earthquake and the Fourier amplitude spectrums are shown in Fig. 9. The peak ground acceleration (PGA) was Gal in the N-S direction, gal in the E-W direction, and Gal in the U-D direction. The earthquake lasted longer than 12 s, about 18 s. Figure 9 also shows the Fourier amplitude spectrums of the ground motion. According to the Fourier amplitude spectrums analysis, the main frequency range in the horizontal directions is 5 15 Hz, and the main frequency range in the vertical direction is 5 6 Hz. The elastic response spectra with 5% damping are presented in Fig. 1. The ground motion has a spectral acceleration of.5.7g between.1 s to.6 s of the period range.

5 No.4 Zhang Cuiqiang et al.: Study on the effect of the infill walls on the seismic performance of a reinforced concrete frame Analytical model of the target building In order to compare the effect of the infill walls, three models are analyzed in this paper. One model is the bare RC frame (without infill walls), and the other two are RC frames with infill walls. The difference between the latter two RC frames is that one considers the co-work of the infill walls and beam and the other does not. The models are shown in Figs. 11 to Definition of yield If the element develops any one of following conditions, the element will be considered to be yielding ( Li, 24). Definition of yield: (1) axial compression force ratio.6 (over 6 % of central compression capacity); (2) steel bars yield-ratio.5 (half or more of rebar tension/compression yielded); (3) rebar maximum strain in tension 2. ε y (steel bar yielding strain); (4) steel/rebar maximum strain in compression 5. ε y (steel bar yielding strain); (5) concrete compression strain ε (concrete strain corresponding to the concrete peak stress). 5 Analytical results 5.1 Comparison of dynamic properties Acceleration (1 3 Gal) Three analytical models are presented in this paper Time (s) N-S Direction Acceleration (Gal) Acceleration (Gal) Acceleration (Gal) Period (s) (a) N-S response spectra 8 RS of recorded ground motion in EW direction 7 RS under frequently occured earthquake RS under rarely occured earthquake Period (s) (b) E-W response spectra 6 RS of recorded ground motion in UD direction 5 RS under rarely occured earthquake RS under frequently occured earthquake RS of recorded ground motion in NS direction RS under rarely occured earthquake RS under frequently occured earthquake Period (s) (c) U-D response spectra Fig. 1 Response spectrum of ground motion Amplitude (1 3 Gal/Hz) Frequency (Hz) Fig. 9 Ground motion recorded in Wolong site and the Fourier amplitude spectrum of ground motion (N-S direction) Fig. 11 Analytical model without infill walls

6 512 EARTHQUAKE ENGINEERING AND ENGINEERING VIBRATION Vol.1 Fig. 12 Analytical model with infill walls to the study the effect of the infill walls. They are as follows: (1) the first model is a bare RC frame that does not consider the infill walls; (2) the second model is a RC frame that considers the infill walls by the diagonal strut and does not consider the co-work of the beam and infill walls; and (3) the third model is a RC frame that considers the infill walls by the diagonal strut and considers the interaction of the beam and infill walls. A comparison of the periods of the three models is presented in Table 3. In this table, the period with infill walls refers to the model with infill walls that considers the interaction of the infill walls and beam. Table 3 Comparison of the periods Mode shape No. 1st 2nd 3rd Translation in X Normalized Translation in Y Normalized Torsion Normalized Period of the first model Period of the second model Period of the third model Interstory drift (1-3 ) (a) 1st floor Interstory drift (1-3 ) (b) 2nd floor Interstory drift (1-3 ) Interstory drift (1-3 ) Interstory drift (1-3 ) Interstory drift (1-3 ) (c) 3rd floor Fig. 13 Hysteresis loop of inter-story in X and Y direction (the first model)

7 No.4 Zhang Cuiqiang et al.: Study on the effect of the infill walls on the seismic performance of a reinforced concrete frame Interstory drift (1-3 ) (d) 4th floor Fig. 13 Continued Interstory drift (1-3 ) Interstory drift (1-3 ) (a) 1st floor Interstory drift (1-3 ) (b) 2nd floor Interstory drift (1-3 ) (c) 3rd floor Interstory drift (1-3 ) Interstory drift (1-3 ) Interstory drift (1-3 ) Interstory drift (1-3 ) Interstory drift (1-3 ) (d) 4th floor Fig. 14 Hysteresis loop of inter-story in X and Y direction (the second model )

8 514 EARTHQUAKE ENGINEERING AND ENGINEERING VIBRATION Vol Interstory drift (1-3 ) (a) 1st floor Interstory drift (1-3 ).3 (b) 2nd floor Interstory drift (1-3 ) (c) 3rd floor Interstory drift (1-3 ) Interstory drift (1-3 ) Interstory drift (1-3 ) Interstory drift (1-3 ) Interstory drift (1-3 ) (d) 4th floor Fig. 15 Hysteresis loop of inter-story in X and Y direction (the third model) From the comparison, the order of the mode shape is changed by the infill walls. The first mode shape changes from X direction translation to Y direction translation. The infill walls change the mode shape stiffness of the structure. The mode stiffness in the X direction is enhanced to 2.92 times the original mode shape stiffness, and 1.63 times in the Y direction. The torsion mode shape is 1.79 times the original one. 5.2 Hysteresis loop The input time history in the analytical model is the ground motion recorded at the Wolong station. From the three-dimensional nonlinear dynamic analysis, the hysteretic relationship between the ratio of shear force to weight and the inter-story drift are presented in Figs. 13, 14 and 15. Figure 13 is the bare RC frame. Figure 15 is the RC frame with infill walls that consider the co-work of the beam and infill walls. From the comparison of the hysteresis loop, the infill walls not only change the stiffness of the RC frame, which increases the shear force of floor, but also change the strength distribution of the RC frame, causing the first floor to be the weak floor.

9 No.4 Zhang Cuiqiang et al.: Study on the effect of the infill walls on the seismic performance of a reinforced concrete frame 515 C D B C D E A A B C D E C D Fig. 16 Distribution of the beam hinges and column hinges (the first model) C D A A B C D E C D Fig. 17 Distribution of the beam hinges and column hinges (the second model) C D A Fig. 18 Distribution of the beam hinges and column hinges (the third model )

10 516 EARTHQUAKE ENGINEERING AND ENGINEERING VIBRATION Vol.1 A B C D E C D Fig. 18 Continued 5.3 Plastic hinge distribution The plastic hinge distribution regardless of the infill walls is presented in Fig. 16. The plastic hinge distribution regardless of the co-work of the beam and infill walls is shown in Fig. 17. The plastic hinge distribution considering the infill walls and the co-work of the infill walls and beam is presented in Fig Conclusions This paper presents an analysis of the advantages and disadvantages of three mechanical models used to simulate infill walls in a RC frame. According to typical damage observed in the Wenchuan earthquake, the failure mode of the RC frame is a column plastic hinge mechanism rather than the beam hinge mechanism. Recorded ground motion is analyzed, the main frequency range in the horizontal directions is 5 15 Hz, and the main frequency range in vertical direction is 5 6 Hz. The elastic response spectra with 5% damping are also presented. The ground motion has a spectral acceleration of.5.7g between.1 s to.6 s of the period range. Three analytical numerical models are developed to study the effect of the infill walls. By comparing the three models, the following conclusions are obtained. (1) For the RC frame with infill walls, the dynamic properties are apparently changed. And, the period of the RC frame is also changed. Because of the increase in the stiffness to the RC frame, the RC frame will carry more inertia force. (2) From the hysteresis loops of the inter-story, for RC frames with discontinuous infill walls in elevation, the infill walls not only change the stiffness distribution of the entire structure, but also change the distribution of the strength. Therefore, a weak story can easily be formed in the RC frame without an infilled wall. For the RC frame in this paper, the weak story is developed on the first floor. (3)The diagonal strut model considering the co-work of the beam and infill walls can simulate the damage of the RC frame with infilled walls. Regardless of the cowork of the beam and infill walls, there are discrepancies between the simulation and the actual damage. (4) From the point of view of the simulation and observed seismic damage, both the stiffness contribution of the infill walls to the beam and the strength contribution should be considered. Acknowledgement The authors are grateful for the partial financial support from Kwang-Hua Fund for College of Civil Engineering, Tongji University, and the National Natural Science Foundation of China (Grant No , ). References Cao Wanlin and Wang Guangyuan(1997), Calculation of Earthquake Action on Frame with Special-shaped Columns and Light-weight Filled Walls at Elastic Stage, Journal of Earthquake Engineering and Engineering Vibration, 17(3): (in Chinese) Chaker AA and Cherifati Arslan (1999), Influence of Masonry Infill Panels on the Vibration and Stiffness Characteristics of R/C Frame Buildings, Earthquake Engineering and Structural Dynamics, 28(9): Code for Seismic Design Buildings (21), Beijing: China Architecture & Building Press. (in Chinese) Guo Zixiong, Wu Yibin and Huang Qunxian (28), Research and Development in seismic Behavior of Inf illed Frame Structures, Journal of Earthquake Engineering and Engineering Vibration, 28(6): (in Chinese) Hoshikuma J, Kawashima K, Nagaya K and Taylor A W(1997), Stress-strain Model for Confined Reinforced Concrete in Bridge Piers, Journal of Structural Engineering, ASCE, 123(5): Hu Yuxian (21), Earthquake Engineering, Beijing: Earthquake Publish Company. (in Chinese) Kwon Oh-Sung and Kim Eungsoo (29), Case Study: Analytical Investigation on the Failure of a Two-story RC Building Damaged During the 27 Pisco-Chincha

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