The Effect of the Geometry and Grain Direction of the Knee Brace on the Strength and Aesthetics of the Norwegian Stave Churches

Abstract

The ancient Norwegian stave churches are testaments to the longevity of timber construction with proper design and preservation. These eight century old structures have received little structural analysis for centuries, so this thesis works to attain a better understanding of one important structural component: the Norwegian knee brace. This knee brace is found vertically bracing columns and beams, horizontally bracing beam connections, and stiffening the aisle walls and rafters, laterally bracing the churches. This thesis analyzes two geometries that the knee brace is found in, one nearly symmetrical and the other exceptionally asymmetrical, to determine the different mechanical behaviors and thus the importance of the geometry of the knee brace. The knee braces of the stave churches are constructed from the root flare, the part of the tree where the grain naturally curves as the trunk transitions to the roots. Wood is anisotropic, best resisting load parallel to the grain, making this material selection critical. To understand the importance of this defining characteristic of these braces, samples of the same geometry are loaded in compression and tension with three different grain directions: curving grain, grain that is parallel to one leg of the brace and perpendicular to the other, and grain that is at a 45° angle to both legs. The findings of this thesis indicate that the curving-grained samples perform stronger and stiffer than the straight-grained samples, underscoring the importance of material selection and geometry for the timber knee brace.