Practical Approach to Designing Wood Roof Truss Assemblies

The objective of this research was to use a three-dimensional (3D) analysis method to evaluate “system effects” in light-frame roof truss assemblies. The goal of this study was to develop an improved and practical design method for 3D roof truss assemblies used in residential construction. A truss plate manufacturer (TPM) design software was used to lay out assemblies and to design individual trusses. The TPM software used a conventional design procedure (CDP) by analyzing one truss at a time in two dimensions. A commercially available structural analysis program was utilized to model and analyze 3D truss assemblies as a system. This system design procedure (SDP) is being proposed as a tool to analyze and design 3D roof truss assemblies. Three truss assemblies, L shape, T shape, and a complex assembly, were analyzed. The structural responses including combined stress index (CSI), truss deflections, and reactions from both CDP and SDP were compared and the system effects were evaluated. From this investigation, it is concluded that there are three system effects observed by the SDP, but not accounted for by CDP. These are: reduced applied load effect, truss-to-truss support effect, and stiff truss effect. Based on this investigation, the maximum CSI for most trusses in all three assemblies reduced by 6–60% because of system effects. SDP can help to improve the design of truss assemblies by directly including system effects that are not accounted for by the CDP.