SIMPLIFIED EVALUATION OF MEMBER FORCE FOR WOODEN HORIZONTAL HYBRID STRUCTURE CONSIDERING NONLINEARITY OF WOODEN ELEMENTS

Study on dynamic behavior of wooden horizontal hybrid structure : Part3

Abstract

 Techniques for large wooden constructions are actively developed for the purpose of reduction of environmental load. One of the solutions for fire and seismic resistance is composite structure, which utilizes different structures like steel or concrete. Wooden structure laterally connected to different structure is discussed, which is called "wooden horizontal hybrid structure". Since it involves stiff cores, most of seismic force acting on wooden parts is sustained by the cores, and wide open space with less shear walls is realized. However, such force transmission complicates the displacement mode as well as the stress distribution in the structure. The authors have proposed dynamics continuous model of horizontal hybrid structure assuming that core part is infinitely-rigid. Although the model can simulate displacement mode of horizontal hybrid structure having various arrangement of shear walls, the structure is assumed to be linearly elastic. While allowable strength design is conducted using elastic model, wooden structures generally have non-linearity even in small deformation. The equivalent stiffness is required to be determined depending on the target deformation. Horizontal hybrid structure, however, shows complicated displacement mode, and it makes it difficult to define deformation and the corresponding equivalent stiffness of each element. Therefore applicability of equivalent linear model is discussed by comparison of analysis result with test result, and simplified evaluation of member force is addressed.

 In this research, prediction by the proposed method is compared to test results conducted by the authors⁹⁾. The referred shake table test on one third scaled model of wooden horizontal hybrid structure is introduced. One side of the specimen is laterally connected to steel jig which represents stiff core. The specimen is 3-story 3-span wooden frame with shear walls. Uni-directional input motions with various intensity are applied to the specimen. Three specimens were tested and the parameters were wall arrangement and nail interval of floor diaphragm which characterizes the stiffness and the strength. The followings are findings of this research.

 1) The prediction method of displacement mode which the authors had proposed was applied to the specimens. Since non-uniform stiffness balance of each element was observed, the balance could be idealized to fit the assumption of the dynamics model. It showed close agreement with test result.

 2) Although stiffness balance of each element was changed according to earthquake intensity, displacement mode and the corresponding equivalent mass ratio were relatively stable. It insists that the existence of core is likely to prevent concentration of deformation on particular element.

 3) 30 to 50% of seismic force acting on wooden part was transmitted to core part through floor diaphragm. No. 2 having stiffer floor diaphragm showed the higher ratio compared to the other specimens. They corresponded to predicted values with acceptable accuracy.

 4) Connection moment between wooden part and core part could be conservatively evaluated using tensile force of bolt and assumed stress condition.

 5) The ratios of connection moment to connection shear force, which represents equivalent floor length, could be conservatively evaluated by assuming triangular distribution.

 6) Based on findings 3), 4) and 5), connection moment between wooden part and core part which is obtained as connection shear force multiplied by equivalent floor length gives tensile force of bolt with some safety margin.