SAFETY FACTORS FOR METAL-PLATE-CONNECTED WOOD TRUSSES - THEORETICAL DESIGN VERSUS TEST SPECIFICATIONS

Metal-plate-connected (MPC) wood truss joints can fail in any of four modes. The failure modes are: steel shear, steel tension, tooth withdr awal, and net section of the wood at the joint. The purposes of safety factors in structural design and the relationships between joint safe ty factors versus truss safety factors were summarized. A detailed dis cussion of the theoretical safety factors for MPC wood truss joints wa s presented and statistical distributions were derived for minimum joi nt properties based on research literature. Theoretical safety factors for the joints range from 1.3 to 3.0 depending on the mode of failure . Three model building code requirements for truss tests were summariz ed. Truss test safety factors specified by a model building code can b e as high as 3.0 times the total load plus the dead load. There appear s to be an inconsistency between the safety factors used in the design of the individual components and safety factors specified by the mode l building codes for truss tests. This study found a need for a logica l link between design safety factors of truss components and the model building code-specified truss test safety factors. Research is needed to define the expected performance of MPC trusses in a test environme nt versus the expected performance of the truss lumber and joints.