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 l oad 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 a llowable bending stress for repetitive wood members. This paper examin es the effect of load sharing and composite action on wood pitched-roo f assemblies through a new analysis model. Individual truss and roof a ssembly strength distributions were computed using the new model. The results show that slope, span, and other truss characteristics cause s ignificant changes in the system factor for a given roof structure con figuration. Computed system factors are presented in terms suitable fo r working stress and reliability-based design methodologies. Computed working stress design system factors vary from 1.15 to over 1.50 for t he roof structures studied, suggesting that the current 1.15 factor ma y be conservative for many roof structures. This conservatism must be retained until more sophisticated analysis models, such as the one pro posed herein, are fully verified and accepted for design.