Discrete Bracing Analysis for Light-Frame Wood-Truss Compression Webs

Abstract

Light-frame wood roof trusses often require lateral bracing to reduce the effective length of compression web members for buckling purposes. Three simple analysis methods were compared in a physical test program to determine if any could adequately predict required brace strength and stiffness. A test program measured brace force and lateral deflection at midheight of 774 nominal 2 × 4 (38 × 89 mm) lumber columns of two grades and four lengths loaded in axial compression and braced with a nonlinear support. Lumber length or grade had little practical effect on the relative accuracy of the three analysis methods. Brace instability, observed in 1% of the tests, suggests the need to limit brace loads. Using 2% of the axial load to predict brace force was generally conservative but may not be appropriate, because it does not ensure sufficient brace stiffness. Two other evaluated methods, modified to achieve design conservatism, can provide a rational basis for bracing design that considers both required strength and stiffness.