STRENGTH AND ENERGY DISSIPATION OF TIMBER SHEAR WALLS SUBJECTED TO ARBITRARY DEFORMATION HISTORY

Study on fatigue behavior of timber structures subjected to repeated earthquake motions: Part1

Abstract

 Conventional test program for timber shear walls in Japan uses a loading protocol consisting of monotonic loading following reverse cyclic loading up to 1/50rad. While repeated cyclic loading is thought to decrease force and deformation capacity due to the fatigue behavior, specimens based on the conventional test program might not sustain enough cycles at large deformation range. Although timber shear walls had better have large deformation capacity, it is not always empirically confirmed. Recently, buildings are required to protect not only the safety but the property after a major earthquake, and passive control techniques for timber structures have been developed in order to enhance the seismic performance. Therefore, evaluation of timber shear walls' fatigue behavior have become an important issue.

 Several experiment-based studies on timber shear walls' fatigue behavior can be found, and some of them have proposed hysteresis model for time history analysis. However, time history analysis are rarely used for seismic design of timber detached houses owing to time and effort. Response spectrum method, which is approximated calculation method for maximum earthquake response, is alternate seismic design rule in order to specify the performance. In response spectrum method, characteristic of building is determined by two parameters: equivalent stiffness and equivalent damping ratio. Fatigue behavior should be taken into account as deterioration of the two parameters.

 In this study, the final goal is to develop a response spectrum method considering fatigue behavior of timber shear walls. Part 1 deals with evaluation of strength and energy dissipation of two typical timber shear walls subjected to various deformation history. Chapter 2 introduces static loading tests. Various loading protocol were applied. Chapter 3 introduces shaking table tests. Two types of earthquake motions having same spectrum shape and different duration time were applied. Chapter 4 introduces evaluation formulae for strength and equivalent damping ratio. The followings are findings of this research.

 1) Envelope curve of force-deformation angle relation has dependency on loading protocol. The envelope curve in monotonic loading is the upper limit, and the others go down as the number of cycles becomes large.

 2) Plywood type shear walls are vulnerable to repeated loading rather than brace type shear walls, which is likely to be related to the failure mode. As for plywood type shear walls, when the number of cycles was small like monotonic loading, failure mode of nail joint was punching out. However, when the number of cycles was large, nail joint was fractured by bending.

 3) When two earthquakes having the same response spectrum shape and different duration time were repeatedly applied, the one having longer duration time rapidly brought deformation increase. It is likely to be related to fatigue behavior owing to repeated loading similar to the above finding 1).

 4) Both brace type and plywood type specimens kept seismic performance against repeated earthquake motions if the maximum deformation response was less than one third of the ultimate deformation.

 5) Evaluation formulae for strength and equivalent damping ratio considering fatigue behavior were proposed. The formula for strength has a unique parameter controlling damage effect when the deformation amplitude is expanded. The formula for equivalent damping ratio is empirically derived based on the fact that equivalent damping ratio has strong dependency on the already experienced maximum deformation rather than the number of cycles. They showed acceptable agreement with results of static random loading tests and shaking table tests.