Evaluation of Splitting Capacity of Bottom Rails in Partially Anchored Timber Frame Shear Walls

Abstract: The horizontal stabilization of timber frame buildings is often provided by shear walls. Plastic design methods can be used to determine the load-carrying capacity of fully and partially anchored shear walls. In order to use these methods, a ductile behaviour of the sheathing-to-framing joint must be ensured. If hold-downs are not provided, the vertical uplifting forces are transferred to the substrate by the fasteners of the sheathing-to-framing joints. Since the forces in the anchor bolts and the sheathing-to-framing joints do not act in the same vertical plane, the bottom rail will be subjected to bending in the crosswise direction, and splitting of the bottom rail may occur. If the bottom rail splits the applicability of the plastic design method for partially anchored shear walls is questionable. This doctoral thesis addresses the problem of brittle failure of the bottom rail in partially anchored timber frame shear walls. The first part of the study comprised of two basic experimental programs, for single-sided and double-sided sheathed shear walls. The aim was to evaluate the different failure modes and the corresponding splitting capacity of the bottom rail. Two brittle failure modes were observed: (1) a crack opening from the bottom surface of the bottom rail; and (2) a crack opening from the side surface of the bottom rail along the line of the fasteners of the sheathing-to-framing joints. It was found that the distance between the washer edge and the loaded edge of the bottom rail has a decisive influence on the type of failure mode and the maximum failure load of the bottom rail. Two theoretical models for the load-carrying capacity for each type of failure mode based on a fracture mechanics approach are studied and validated. The two analytical closed-form solutions are in good agreement with the test results. The fracture mechanics models seem to capture the essential behaviour and to include the decisive parameters of the bottom rail. These parameters can easily be determined and the fracture mechanics models can be used in design equations for bottom rails in partially anchored shear walls. Also, an extended fracture mechanics model for the load-carrying capacity for each type of failure mode is presented and evaluated. The present study discusses the splitting behaviour of the bottom rail and provides methods to determine the splitting capacity for two brittle failure modes, splitting of the bottom surface (mode 1) and of the side surface of the rail (mode 2). By these means brittle failure of the bottom rail can be avoided and the full plastic load-carrying capacity of the sheathing-to-framing joints can be utilized.