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The Effect of Eccentric Overturning Restraint in Complete Shear Wall Assemblies

The following is an excerpt from the article The Effect of Eccentric Overturning Restraint in Complete Shear Wall Assemblies, by Steve Pryor, P.E., S.E., Building Systems Research & Development Manager, Simpson Strong-Tie.

Abstract

In this paper various aspects of the effect of eccentricity in overturning restraint devices used in light-framed wood shear walls is discussed. To accurately assess the demand on the tension end post, nonlinear finite element modeling is used to investigate the effect that sheathing/nailing can have on the boundary posts in shear walls. Results are presented at both the allowable stress design level and at the strength limit state, and these results are compared against a bare post analysis. Additionally, both one and two story conditions are analyzed. The results show that the sheathing/nailing can significantly reduce the internal post stresses, and this reduction is proportional to post stiffness and nail spacing. Finally, various aspects of the In-Grade testing program used to determine wood resistance values in the United States are discussed because of the impact they have on the actual distribution of strength in a given grade of lumber.

Introduction

There has been much discussion recently regarding the effects of eccentricity in the shear wall overturning restraint systems traditionally offered by various manufacturers [4;6;8]. Extensive full scale testing has been performed that suggests that these effects are minor or don’t control for the configurations tested [8:27]. Others have countered that this conclusion is incorrect because a: the worst strength-reducing characteristic of a particular grade of wood may not have been present at the point of critical stress; and b: the tests were conducted in a single story configuration and would, therefore, not be applicable to multi-story configurations [6:37-38]. Additionally, a claim has been made that sheathing/nailing cannot offer any resistance to the boundary posts to reduce the effects of an eccentric overturning connection [6:39].

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Wood Shear Wall Installation