SEISMIC PERFORMANCE EVALUATION OF RETROFITTED WOODEN-HOUSE BY COLLAPSING PROCESS ANALYSIS

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1 Research, Development, and Practice in Structural Engineering and Construction Vimonsatit, V., Singh, A., Yazdani, S. (eds.) ASEA-SEC-1, Perth, November 28 December 2, 2012 SEISMIC PERFORMANCE EVALUATION OF RETROFITTED WOODEN-HOUSE BY COLLAPSING PROCESS ANALYSIS TOMIYA TAKATANI 1 and HAYATO NISHIKAWA 2 1 Department of Civil Engineering & Architecture, Maizuru National College of Technology, 234 Shiraya, Maizuru, Kyoto, Japan. takatani@maizuru-ct.ac.jp 2 Education and Research Supporting Centre, Maizuru National College of Technology. nisikawa@maizuru-ct.ac.jp Major earthquakes in Japan have caused serious damage to a great number of existing wooden houses built by some old seismic design codes before ACM bracing method developed by one of the authors has already applied to two existing wooden houses for seismic retrofit, and the seismic performance of ACM bracing method was experimentally confirmed by vibration measurement procedure using an oscillator-measurement system. On the other hand, it is very important to accurately evaluate the seismic performance of a retrofitted wooden house against a large earthquake ground motion. In this paper, the seismic performance of the wooden house retrofitted by ACM brace was numerically confirmed by a collapsing process analysis based on the Distinct Element Method, and also was compared with the results of vibration measurements. Keywords: Wooden house, collapsing process analysis, seismic retrofit, transmission function, ACM bracing. 1 Introduction A great number of wooden houses in Japan have suffered severe damage such as collapsing against several earthquakes with Magnitude 7 to 9, because most of the collapsed wooden houses were built by some old earthquake design codes before Seismic retrofitting policy for a lot of old wooden houses has become an urgent and important problem to solve as quickly as possible in Japan. One of the authors has already developed an advanced seismic retrofitting method for existing RC building structures using ACM bracing method which consists of CFRP (Carbon Fiber Reinforced Plastic) plate and steel plates (Takatani et al. 2011, Takatani 2012), and then has developed a new ACM bracing method using an e-plate for old wooden house by an improvement of RC structural ACM bracing technique. E-plate is a thin plate of CFRP with 1.2mm thickness and 25mm width. This seismic retrofitting method has applied to two wooden houses. On the other hand, vibration experiments using an oscillatormeasurement system were carried out in two wooden houses retrofitted by ACM bracing method, and the effect of seismic retrofitting work was confirmed by some transmission 1

2 Instructions for Preparing Manuscripts Tomiya Takatani and Hayato Nishikawa functions obtained in vibration experiments. However, the seismic performance effect of ACM bracing method has not been confirmed against a strong earthquake ground motion by which is caused a collapsing phenomenon of wooden house, because these vibration experiments were conducted to evaluate natural frequency characteristics of the retrofitted wooden house by means of a sweep vibration technique with small amplitude. In this paper, a seismic response analysis of wooden house is carried out by a structural analysis software Wallstat (Nakagawa et al. 2010, Nakagawa 2011) based on the Distinct Element Method proposed by Cundall et al. (1979) in order to investigate the collapsing behaviour of wooden house and the seismic performance effect of ACM bracing method. Earthquake ground motion wave data with JMA seismic intensity of 6 upper level is used as an input motion in the collapsing analysis. Some transmission functions are evaluated by seismic responses at arbitrary measuring points, and also transmission functions obtained in the collapsing analysis of Wallstat are compared with those obtained in vibration experiments using an oscillator-measurement system. 2 Outline of Collapsing Analysis In this paper, structural analysis software of Wallstat is employed in order to investigate seismic response behaviour and collapsing process of wooden house during a large earthquake ground motion. This software has an original analysis technique (Nakagawa et al. 2010) using the basic theory of the Distinct Element Method, and can be taken into consideration the extremely nonlinear properties of timber members breaking or being dispersed. Figure 1 illustrates a typical framing plan of Japanese two-story wooden house built by a traditional woodenbased building method. In the collapsing process analytical calculation, a wooden house shown in Figure 1 can be modelled by a lot of timber elements such as beam and post connected with non-linear spring, and also can be modelled by lumped mass and the weight of each floor in wooden house model can be obtained from each structural element as illustrated in Figure 2. Timber characteristics of the compression and tensile elasto-plastic springs consist of an elastic part and a slip-type one indicated in Figure 3(b), and also timber characteristics of the rotational spring are assumed to be a slip-type relationship between the bending moment M and the angle of rotation θ shown in Figure 3(c). Vertical shear wall indicated in Figure 4(a) can be modelled by the replacement of truss component with a load-displacement nonlinear relationship shown in Figure 5. Also, bracing shear wall illustrated in Figure 4(b) can be modelled by the replacement of the compression and tensile truss components defined by a set of bi-linear and slip skeleton curve shown in Figure 5, too Due to the limited space, there are no tables indicated the detailed data concerning the non-linear spring and the load-displacement relationship shown in Figures 3 and 5, which are used in the collapsing process analysis. Figure 1 Framing plan of wooden house 2

3 Research, Development, and Practice in Structural Engineering and Construction Vimonsatit, V., Singh, A., Yazdani, S. (eds.) ASEA-SEC-1, Perth, November 28 December 2, Numerical Results In this paper, transmission functions evaluated from floor responses by Wallstat are compared with the results obtained from an oscillator-measurement system. These functions can be defined by the spectral ratio of Fourier spectra at two arbitrary points of Figure 2 Weight of floor in the analytical model of wooden house (Nakagawa 2011) (a) Schematic diagram of joint (b) Characteristic of elasto- (c) Characteristic of rotational element plastic spring spring Figure 3 Outline of joint modelling (Nakagawa 2011) (a) Shear wall spring (b) bracing shear wall Figure 4 Shear wall spring and bracing shear wall (Nakagawa 2011) Figure 5 Hysteretic characteristics of shear wall and bracing (Nakagawa 2011) 3

4 Instructions for Preparing Manuscripts Tomiya Takatani and Hayato Nishikawa wooden house. For the oscillator-measurement system, vibration response wave data obtained from two sensors and an oscillator located on the second floor of wooden house were used in the accurate evaluation of transmission functions. Transmission functions evaluated from both the oscillatormeasurement system and the simulation by a collapsing process analysis are shown in Figures 6 and 7. Peak frequencies in transmission function before seismic retrofit obtained from an oscillator-measurement system are 4.2Hz in ridge direction and 5.5Hz in span direction, respectively. Transmission function after seismic retrofit shifts to high frequency range in comparison with that before seismic retrofit and also peak frequencies after seismic retrofit in their transmission functions in both ridge and span directions are higher than those before seismic retrofit. As can be seen from the transmission function obtained by Wallstat in Figure 7, peak frequencies are in almost agreement with those obtained from an oscillator-measurement system. Figure 8 shows collapsing process behaviour of wooden house without seismic retrofit work under Japan Railway (JR) Takatori wave record used as input earthquake ground motion. In this collapsing process analysis, NS and EW components in JR Takatori wave record are applied to the ridge direction and the span direction of wooden house s floor plan, respectively. If a wall with gray color has a damage, the wall color changes to other color. When the degree of its damage in- (a) Ridge direction (b) Span direction Figure 6 Transmission functions obtained from an oscillator-measurement system (a) Ridge direction (b) Span direction Figure 7 Transmission functions obtained by Wallstat 4

5 Research, Development, and Practice in Structural Engineering and Construction Vimonsatit, V., Singh, A., Yazdani, S. (eds.) ASEA-SEC-1, Perth, November 28 December 2, 2012 creases during earthquake motion, the wall color changes from gray to yellow, orange, and red. It can be seen from Figure 8 that some walls have severe damages after 4 seconds and also both walls and roof on the first floor begin to collapse after 6 seconds. After 8 seconds, large deformation occurs at the center of the first floor, and then the wooden house leads to the collapse after 10 seconds. Due to the limited space in the present paper, seismic behaviour of the retrofitted wooden house is not included here, but it should be pointed out that seismic behaviour of wooden house during earthquake motion can be numerically simulated by a collapsing process analysis. Span direction Ridge direction (a) 0 sec (b) 2 sec (c) 4 sec (d) 6 sec (e) 8 sec (f) 10 sec Figure 8 Seismic response of no-retrofitted wooden house during JR Takatori wave record 5

6 Instructions for Preparing Manuscripts Tomiya Takatani and Hayato Nishikawa 4 Conclusions In this paper, a structural analysis based on the Distinct Element Method was conducted in order to investigate the collapsing behaviour of wooden house and the seismic performance effect of ACM bracing method using e-plate. Transmission functions obtained in the collapsing analysis were compared with those obtained in vibration experiments using an oscillator-measurement system. In summary, the following conclusions can be made based on the results presented in this paper. (1)Seismic performance of an old wooden house retrofitted by ACM bracing method can be numerically confirmed by a collapsing process analysis. Seismic response of the wooden house greatly depends on the spectral characteristics of input earthquake ground motion used in the collapsing analysis. (2)Transmission function of the retrofitted wooden house almost makes a good agreement with vibration measurement result obtained by an oscillator-measurement system. It was found that transmission function after seismic retrofit shifts to high frequency range in comparison with that before seismic retrofit. Although the seismic retrofitting work of wooden house using ACM bracing method is not presented in detail in this paper, it is necessary for an intensive study on the effect of ACM bracing method on the collapsing behaviour during a strong earthquake ground motion. The collapsing process phenomenon of wooden house with or without seismic retrofit may be needed to make some concrete conclusions. Moreover, as the seismic behaviour of wooden house during earthquake motion can be considered to be more sensitive to the peak frequency range in Fourier spectrum of the earthquake motion used in the collapsing analysis, further investigation may be needed to simulate the collapsing process phenomenon of wooden house against several earthquake motions with different peak frequency range. Acknowledgements The authors would like to thank the Japan Meteorological Agency and the National Research Institute for Earth Science and Disaster Prevention in Japan for their earthquake ground motion wave data in K- NET and KiK-net system. References Nakagawa, T., Development of analysis method for collapsing behaviour of wooden post-andbeam house during earthquake, Building Research Data, Building Research Institute (in Japanese), Nakagawa, T., Ohta, M., Collapsing process simulations of timber structures under dynamic loading III: Numerical simulations of the real size wooden houses, Journal of Wood Science, Vol.56, No.4, , Sakai, Y., Koketsu, K. and Kanno, T., Proposal of the destructive power index of strong ground motion for prediction of building damage ratio, Journal of Structural and Construction Engineering, Architectural Institute of Japan, No.555, 85-91, 2002 (in Japanese). Tajima, M., Murakami, M., Gotou, M., Inayama., M and Fukuda, M., Development of structural design method on conventional post and beam structures : Part 35 Result of Shearthed Diaphram with Angle Brace Test, Summaries of Technical Papers of Annual Meeting, Architectural Institute of Japan, 25-26, 2000 (in Japanese). Takatani, T., Relationship between seismic index and response of RC structure retrofitted by ACM braces, Proceedings of the 12th East Asia-Pacific Conference on Structural Engineering & Construction (EASEC-12), Hong Kong, China, Takatani, T., Application of ACM brace retrofitting countermeasure to steel structure, Proceedings of the 2nd International Conference on Rehabilitation and Maintenance in Civil Engineering (ICRMCE 2012), Solo, Indonesia,