MODELING KNEE-BRACE FASTENER SLIPPAGE IN POSTFRAME BUILDINGS WITH DIAPHRAGM-ACTION DESIGN

Knee braces should not be an add on after the post-frame building is designed because they do affect the behavior of the structure. It is recommended that the building designer first check the need for knee bracing when diaphragm action is included in the design. Knee braces may benefit buildings with certain geometrical configurations and loading conditions where diaphragm action alone is not enough. Modeling knee bracing within a post-frame building is complex. The complexity extends to the roof-truss design because the truss design must include the knee brace reaction forces. The building designer is responsible for reviewing truss drawings to verify that knee brace loads are applied correctly, and that correct governing load combinations are applied. In this study, a structural analog that includes knee braces, with and without slippage, was developed to model the effects of knee braces on designs of post-frame buildings with diaphragm action. In the five buildings analyzed in this study, use of knee braces produced inconsistent results. Knee braces may increase or decrease horizontal eave deflection, load demand on the diaphragm and endwalls, load demand on the foundation, and stress unity in the posts. In all buildings with knee braces, posts were subjected to additional bending stresses under gravity loads. This behavior was more pronounced in buildings with long truss spans. Knee braces should not be specified in the design documents unless their effects on the building are considered by structural analysis. Keywords. Diaphragm action, Knee braces, Modeling connection slippage, Post-frame buildings.