Abstract:
As part of a roof framing system, light-frame wood trusses often require lateral bracing to reduce the effective length for flexural buckling of compression web members. This observational research investigates the design requirements for discrete compression web bracing intended to provide such lateral support. Four simple brace analysis methods based on rules-of-thumb and theoretical means were compared with the results from a physical test program to determine if any could be used to predict the brace strength and stiffness requirements. These methods include those developed by Plaut, Winter, and Tsien. In addition, the popular "2%" rule-of-thumb was investigated. For the test program, the lateral brace force and deflection were measured at mid-height for 774 2x4 Douglas-fir columns of four different lengths (4, 6, 8, and 10 foot) and two different lumber grades ("Select Structural" and "Standard). The testing incorporated a practical range of brace stiffnesses estimated by finite element analysis to characterize the support offered by a traditional lateral/diagonal bracing system. The brace force and deflection were measured for each column at an axial load equal to the estimated 5% exclusion strength limit for an
effectively braced column of a given length and grade. We found little practical effect of column length and grade on the relative
accuracy of the four brace analysis methods. However, a statistically significant length
effect was observed for all but the 2% rule-of-thumb. Since brace instability was
observed when the lateral load extended into the non-linear range of brace support in
about 1% of the tests, we recommend limiting the brace load to a level below the proportional limit of the brace assembly. The 2% rule-of-thumb was found to be the most conservative method of predicting the brace force in this test program. However, it may not be appropriate for brace design because it does not ensure sufficient brace stiffness to achieve the desired column strength. Given its complexity and comparative inaccuracy as a predictor of the brace support requirements when compared with Winter's method, use of Tsien's equation is not recommended. The results from this study suggest that, with some further modification to achieve design conservatism, the methods developed by Plaut and Winter provide a rational basis for discrete compression web bracing design.
