A STUDY ON SHEAR PERFORMANCE OF GABLE ROOF FRAME IN TRADITIONAL WOODEN BUILDING

Abstract

 Wooden roof frame is required to have angle braces and intertruss bracings based on the current specification code. In the past natural disasters except for strong wind by typhoons, wooden roof frames have rarely suffered severe damages. The current code has been thought to work well. However, traditional-type wooden roof frame does not satisfy the current code. In addition, the roof frame is boarding diaphragm consisting of narrow boards fastened by nails, which is thought to have low shear stiffness and strength unlike plywood sheathing floor diaphragm.

 A roof frame works as horizontal diaphragm transmitting lateral force to shear walls. In the current allowable stress design, stress distribution of a floor diaphragm is calculated using continuous shear beam. The allowable strength is determined by referring to test results of elements such as roof frame covered by wooden boards and beam frame with angle braces. Since the design method is originally applied to modern wooden roof frame, the applicability to traditional-type wooden roof frame should be investigated.

 In this paper, shear performance of gable roof, which is one of the typical roof systems, subjected to lateral force in the longitudinal direction is discussed. Although previous studies often consider pure shear condition, equally distributed forces with two specific boundary conditions are applied to roof frame in order to simulate earthquake force or wind force. Following the full-scale experiment, the behavior is simulated by two-dimensional framing analysis model whose characteristics is determined by referring to results of element tests. Parameter study with various specifications of each element is also conducted, and the effect of the stiffness balance on the shear behavior of roof frame is investigated. Finally, the framing model is converted to simplified model which is capable of expressing mathematical relation of each parameter.

 The followings are findings of this research.

 1) In cantilevered roof frame, shear force-deformation angle relations of roof frame and beam frame could be simply added.

 2) In roof frame specimen, shear force was sustained by roof frame while moment was sustained by beam frame. It is likely to be assured by rotational resistance of transverse walls.

 3) Average shear stress along the span controls the behavior of roof frame, and it is reasonable to suppose that the average shear stress should be checked in each half span divided by ridge beam. Actually, flexural resistance of beam frame had an effect of making shear deformation in the roof frame more equal. However, it is difficult to be assured when connections exist in the tie beams.

 4) Two-dimensional framing analysis model was constructed based on results of element tests, and it gave close agreement with test results.

 5) Parameter studies of framing analyses showed that replacement of nuki with brace in vertical frame had little effect on performance of roof frame due to relatively large contribution of moment resistance by tenon connections.

 6) Simplified model derived from framing model could give mathematical relation of each parameter. It can provide simple formulae of whole behavior with less effort.

 Although the effect of in-plane shear stiffness on dynamic behavior of a building is not mentioned in this paper, the above 3) insists that 2-span dynamic model, which has been proposed by the authors^14)-16), can be utilized. It will be discussed in the future.