System Factors for Light-Frame Wood Truss Assemblies

Light-frame wood roof assemblies may be stronger than what the current single member design approach suggests because of repetitive member load sharing and partial composite action. These strengthening effects are partially addressed in the National Design Specification for Wood Construction through a repetitive use factor of 1.15, applied to the allowable bending stress for repetitive wood members. This paper examines the effect of load sharing and composite action on wood pitched-roof assemblies through a new analysis model. Individual truss and roof assembly strength distributions were computed using the new model. The results show that slope, span, and other truss characteristics cause significant changes in the system factor for a given roof structure configuration. Computed system factors are presented in terms suitable for working stress and reliability-based design methodologies. Computed working stress design system factors vary from 1.15 to over 1.50 for the roof structures studied, suggesting t...