Seismic Collapsing Response Analysis of Wooden House Retrofitted by ACM Braces

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1 Seismic Collapsing Response Analysis of Wooden House Retrofitted by ACM Braces T. Takatani Department of Civil Engineering & Architecture, Maizuru National College of Technology H. Nishikawa Education and Research Supporting Centre, Maizuru National College of Technology ABSTRACT: 3-D non-linear collapsing process analysis of two-story wooden house was conducted against several strong earthquake ground motions with the seismic intensity of 6+ to 7 level, in order to investigate the seismic behaviour of the wooden house retrofitted by ACM braces. A non-linear behaviour of timber elements in wooden house during earthquake motion can be simulated by this collapsing analysis. As a result, seismic response of the wooden house depends on the seismic intensity of input earthquake ground motion in the collapsing analysis. Also, there seems to be a possibility that the wooden house with the upper structural seismic index value of less than 1. may be collapsed by a strong earthquake motion with 7 level. 1 INTRODUCTION In recent years, major earthquakes in Japan caused serious damage to a great number of existing wooden houses built by some old seismic design codes before One of the authors has already developed an advanced seismic retrofitting method for existing wooden house using ACM bracing method which consists of Carbon Fiber Reinforced Plastic (CFRP) plate and steel plates (Takatani 212). It is very important for a structural engineer to take account of a seismic response of wooden house in the design process of seismic retrofit and also to make an accurate evaluation of the seismic collapsing behaviour of the retrofitted wooden house during a strong earthquake ground motion. In this paper, 3-D seismic collapsing process analysis of wooden house based on the theory of the Distinct Element Method (Cundall et al. 1979) is conducted in order to accurately evaluate the seismic behaviour of wooden house with or without ACM bracing seismic retrofit. The effect of the seismic retrofit using ACM bracing method on the collapsing behaviour is numerically investigated. Five earthquake ground motions with the Japan Meteorological Agency (JMA) seismic intensity, I JMA =6+~7, are used in this 3-D collapsing process analysis of a typical traditional two-story wooden house in Japan. 2 COLLAPSING ANALYSIS OF WOODEN HOUSE Target of the collapsing process analysis in this paper is a Japanese traditional two-story wooden house built in The upper structural seismic index of this wooden house is.45 in the span direction. ACM bracing seismic retrofit can be designed so as to satisfy the upper structural seismic index of 1.. Figure 1 shows the wooden house floor plans before and after seismic retrofit. This wooden house was retrofitted by outside, inside and corner ACM bracing techniques. Some red lines of X sign in Figure 1(b) mean ACM brace locations. Some types of ACM bracing seismic retrofit for wooden house have already developed by Takatani (212). A collapsing process analysis software of Wallstat is conducted in order to investigate the seismic response behaviour and the collapsing process of wooden house during a strong earthquake ground motion. This software has an original analysis tech- (a)before seismic retrofit (b)after seismic retrofit Figure 1. Floor plan of wooden house.

2 nique (Nakagawa et al. 21) using the basic theory of the Distinct Element Method. Figure 2 illustrates a typical framing plan of Japanese two-story wooden house built by a traditional wooden-based building method. In the collapsing process analytical calculation, a wooden house can be modelled by a lot of timber elements such as beam and column connected with non-linear spring as shown in Figure 2, 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 3., and then the beam component can be judged to have been broken. 2.2 Modelling of joint spring Joint spring can be modelled by both an elastoplastic spring and a rotational spring as shown in Figure 5(a). Timber characteristic of the compression and tensile elasto-plastic spring consist of an elastic part and slip-type part indicated in Figure 5(b), and also timber characteristics of the rotational spring is assumed to be a slip-type relationship between ending moment M and angle of rotation shown in Figure 5 (c). When the elasto-plastic spring or the rotational spring of the joint exceeds the maximum structural strength or moment and their strength values becomes, the joint will be adjudged to have been broken and then the spring will be annihilated. Figure 2. Framing plan of wooden house. 2.1 Modelling of frame Timber frame is modelled by two elasto-plastic rotational springs (plastic hinge) and an elastic beam component as shown in the left-hand side of Figure 4. The spring can be defined by a relationship between the bending moment M and the angle of rotation with the skeleton curve indicated in the right-hand side of Figure 4. The bending moment starts to fall once if it is over the maximum bending moment, and the rotating spring changes to a pinned joint state at the point if the bending moment reaches 2.3 Modelling of shear wall and bracing Vertical shear wall indicated in Figure 6 can be modelled by the replacement of truss component with a load-displacement non-linear relationship shown in Figure 8. Also, bracing shear wall illustrated in Figure 7 can be modelled by the replacement of compression and tensile truss components defined by a set of bi-linear and slip skeleton curve shown in Figure 8, too. 2.4 Parameters of structural element In this paper, Young s modulus and the maximum bending moment of timber component are assumed to be 2,MN/m 2 and 1kNm, respectively. External and internal walls in wooden house can be assumed to be lath mortal wall and clay wall, respectively. For seismic retrofit work, plywood is used as Figure 3. Weight of floor in the analytical model of wooden house (Nakagawa 21). Figure 4. Schematic diagram and skeleton curve of frame spring (Nakagawa 21).

(a) Schematic diagram of joint element (b) Characteristic righting moment of elasto (c) Characteristic righting moment of -plastic spring rotational spring Figure 5.

3 (a) Schematic diagram of joint element (b) Characteristic righting moment of elasto (c) Characteristic righting moment of -plastic spring rotational spring Figure 5. Outline of joint modelling (Nakagawa 21). Figure 6. Shear wall spring. Figure 7. Outline of bracing shear wall. Figure 8. Hysteretic characteristics of shear wall and bracing (Nakagawa 21). Table 1. Parameters for hysteretic characteristics of vertical shear wall (Nakagawa 21). Wall P 1 P 2 P 3 P 4 D 1 D 2 D 3 D 4 h (kn) (m) (%) Clay Wall Lath Mortal Wall Structural Plywood h : viscous damping factor Table 2. Parameters for hysteretic characteristics of elasto-plastic spring (Tajima et al. 2, Nakagawa 21). Ks 1 Ks 2 Ks 3 D 1 D 2 (kn/m) (m) Stub Tenon Corner Bracing 5, Table 3. Parameters for hysteretic characteristics of bracing spring. Brace P 1 P 2 P 3 P 4 D 1 D 2 D 3 D 4 h (kn) (m) (%) Timber Brace ACM Brace (e-plate) Table 4. Earthquake ground motion records. Earthquake Number of Records K-NET KiK-net The Western Tottori prefecture Earthquake in The Mid Niigata prefecture Earthquake in The Noto Peninsula Earthquake in 27 4 The Niigataken Chuetsu-oki Earthquake in 27 2 The Iwate-Miyagi Nairiku Earthquake in

4 Displacement(m) Amplitude(cm/s) Displacement(m) Amplitude(cm/s) Displacement(m) Amplitude(cm/s) internal wall. Parameters of hysteretic characteristics of their walls are shown in Table 1. Parameters of stub tenon jointed at the interface between beam, column and corner bracing used for seismic retrofit work are described in Table 2. Table 3 shows the parameters of hysteretic characteristics of timber brace and ACM brace element. This software has an original analysis technique (Nakagawa et al. 21) 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 disperse. Nishikawa et al. (212) conducted collapsing analysis of a two-story wooden house with and without ACM bracing seismic retrofit against some earthquake motions to investigate the effect of seismic retrofit of the wooden house. 3 EVALUATION OF COLLAPSING PROCESS OF WOODEN HOUSE 3.1 Input earthquake motion Table 4 shows five earthquake records in K-NET and KiK-net systems used in this paper, which were obtained in the earth s crust between 2 and 28 in Japan and were the moment magnitude M W = 6.6~6.9 with the seismic intensity of I JMA =5+~7. Three earthquake acceleration waves in Table 4 and their Fourier spectra are indicated in Figures 9 and 1, respectively. Three seismic intensities I JMA are 6.3~6.7 and their predominant frequencies are about 1Hz. 3.2 Collapsing Process and Seismic Response Figure 11 illustrates collapsing behaviours before and after seismic retrofit of a wooden house against K-NET Ojiya in the Mid Niigata prefecture Earthquake in 24. Wall elements in wooden house during earthquake are indicated in yellow, orange and red colours with the increase of damage degree of wooden house. In addition, as can be seen from the upper structural seismic index values in Table 5, the difference between the upper structure seismic index values before and after seismic retrofit have a signif Time(s) (a) K-NET Ojiya Frequency(Hz) (a) K-NET Ojiya Tiime(s) (b) K-NET Anamizu Time(s) (c) KiK-NET Hino Figure 9. Displacement waves of input motion Frequency(Hz) (b) K-NET Anamizu Frequency(Hz) (c) KiK-NET Hino Figure 1. Fourier spectra of acceleration.

icant effect on the seismic response against a strong earthquake motion, because the upper structural seismic index of the first floor in the ridge direction increases more than.

5 icant effect on the seismic response against a strong earthquake motion, because the upper structural seismic index of the first floor in the ridge direction increases more than.2 by a seismic retrofit. Upper structure seismic index of wooden house means a ra- 2 F 1 F s s Table 5. Upper structural seismic index value. No Seismic Retrofit Seismic Retrofit Ridge Direction Span Direction Ridge Direction Span Direction (a) No-seismic retrofit (b) Seismic retrofit Figure 11. Collapsing process behaviour of wooden house under K-NET Ojiya (I JMA =6.7). s 2s 2s tio of the holding strength to the necessary one of wooden house, and also indicates the resistant strength against a strong earthquake motion with I JMA =6+ level. Therefore, if the upper structural seismic index value is over 1., the retrofitted wooden house has an enough seismic resistant strength against a strong earthquake with the seismic intensity of I JMA =6+ level. It is found from Table 5 that the wooden house after seismic retrofit doesn t have an enough seismic resistant strength. As can be found from the collapsing behaviour during K-NET Ojiya in Figure 11, the first floor of wooden house before seismic retrofit begins to destroy after 8 seconds and then collapses after 12 seconds. On the other hand, the damage degree of wooden house after seismic retrofit is slightly smaller than before seismic retrofit and then collapses after 12 seconds, too. This is because the displacement wave of K-NET Ojiya in Figure 9(a) was measured under a strong earthquake motion with the seismic intensity of I JMA =7 level and the amplitude of displacement was over.3m. Figure 12 illustrates collapsing behaviours before and after seismic retrofit of wooden house during K- NET Anamizu in the Noto Peninsula Earthquake in 27. Although wooden house before seismic retrofit collapses after 16 seconds, wooden house after seismic retrofit has no large damage after 5 seconds. This implies that the seismic retrofit has a significant effective effect on the collapsing behaviour against an earthquake with the seismic intensity of I JMA =6+ level. Figure 13 indicates collapsing behaviours before and after seismic retrofit of wooden house during KiK- NET Hino in the Western Tottori prefecture Earthquake in 2. The first floor of wooden house be- s 2s 2s s (a) No-seismic retrofit 1s 2s s (a) No-seismic retrofit 3s 5s 2s (b) Seismic retrofit Figure12. Collapsing process behaviour of wooden house under K-NET Anamizu (I JMA =6.3). (b) Seismic retrofit Figure 13. Collapsing process behaviour of wooden house under KiK-NET Hino (I JMA =6.6).

6 fore seismic retrofit in Figure 13(a) begins to destroy after 8 seconds and then collapses after 12 seconds. On the other hands, the damage degree of wooden house after seismic retrofit in Figure 13(b) is almost the same collapsing behaviour of wooden house before seismic retrofit. This is because the displacement wave of KiK-NET Hino in Figure 9(c) was measured under a strong earthquake motion with the seismic intensity of I JMA =7 level. In this collapsing analysis, there are three wooden houses collapsed before seismic retrofit and two wooden houses collapsed after seismic retrofit against earthquake motions in Table 5. Because two earthquake motions, which was collapsed the wooden house after seismic retrofit, have seismic intensity of I JMA =7, any other effective seismic retrofit countermeasure seems to be needed for a strong earthquake motion with the seismic intensity of I JMA =7. Consequently, the upper structural seismic index value of wooden house must be over CONCLUSIO In this paper, a structural analysis based on the Distinct Element Method was conducted in order to investigate the collapsing behaviour of wooden house during a strong earthquake ground motion and the seismic performance effect of ACM bracing method using e-plate. A traditional two-story wooden house was modelled before and after ACM bracing seismic retrofit, and the collapsing behaviour of the wooden house against some earthquake motions with the seismic intensity of I JMA =6+~7 were investigated by 3-D seismic collapsing process analysis. 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 seismic intensity of input earthquake ground motion used in the collapsing analysis. (2)There seems to be a possibility that the wooden house with the upper structural seismic index value of less than 1. is easily collapsed by a strong earthquake motion with the seismic intensity of I JMA =7 level even if the wooden house is retrofitted by ACM bracing method. (3)As the wooden house retrofitted by ACM bracing method does not suffer severe damage against any earthquake ground motion with the seismic intensity of I JMA =6+ level, ACM bracing method for old wooden house may be one of the effective seismic retrofitting countermeasures. Although the effect of earthquake ground motion spectrum on the seismic response of wooden house is not presented in detail in this paper, it is necessary for an intensive study on the effect of earthquake ground motion spectrum on the seismic response of the retrofitted wooden house during some earthquake ground motion wave data, which have the same level intensity with a different peak frequency in Fourier spectra. In addition, further investigation may be needed to simulate the seismic collapsing process phenomenon of an existing wooden house with or without seismic retrofit and make some concrete conclusions. ACKNOWLEDGMENTS 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 Cundall, P. A. and Strack, O. D. L A discrete numerical model for granular assemblies, Géotechnique, Vol.29, No.1, Nakagawa, T., Ohta, M. 21. 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, Nishikawa, H., Takatani, T Collapsing simulation of wooden house retrofitted by ACM braces during seismic ground motion, Proceedings of the 212 World Congress on Advances in Civil, Environmental and Materials Research (ACEM-2), Seoul, South Korea, Tajima, M., Murakami, M., Gotou, M., Inayama., M and Fukuda, M. 2. 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, (in Japanese). Takatani, T Load-displacement relationship of ACM wooden-house retrofitting brace, Proceedings of World Conference on Timber Engineering (WCTE 212), Auckland, New Zealand.

Collapsing Simulation of Wooden House Retrofitted by ACM Braces During Seismic Ground Motion

The 2012 World Congress on Advances in Civil, Environmental, and Materials Research (ACEM 12) Seoul, Korea, August 26-30, 2012 Collapsing Simulation of Wooden House Retrofitted by ACM Braces During Seismic