Comparison of ASD and LRFD Codes for Wood Members. I: Axial Loading

Abstract

A comparison of the economies of the structural design of wood members subjected to axial loads using the allowable stress design (ASD) and reliability-based [load and resistance factor design (LRFD) format] codes is presented. Analyses considered the combinations of dead load only and dead plus live loads due to floor occupancy, snow, wind, and seismic activity. Ratios of live-to-dead loads ranged from zero to 10. The relative economies of the ASD and LRFD codes show the combined influences of the time-related factors assigned to the various load types, load and resistance factor values, and live-to-dead load ratios. The behavioral equations in both codes are now virtually the same. Since the allowable material stresses of the LRFD code are equal to the ASD value multiplied by a constant, the resulting economies are not species- or grade-dependent. For tension and zero-length compression loading, the LRFD format provided the more optimal design for all loading combinations except snow and seismic loadings when the live loads exceed three and four times the dead load, respectively. For columns with the potential for buckling, the LRFD format yielded larger buckling adjustment factors for wind and seismic loadings than did the ASD code. For all other loading arrangements, the ASD format produced larger factors. For floor loading, a 20% average over the LRFD code was observed for very slender columns.