Comparison of ASD and LRFD Codes for Wood Members. II: Flexural Loading

Abstract

As in the companion paper for axial loading, a comparison of the economies of the structural design of wood members subjected to flexural loading 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, as well as dead plus live loads resulting from floor occupancy, snow, wind, and seismic activity. Ratios of live-to-dead loads ranged from zero to 10. Other factors that were found to have some influence on the efficiencies of beams susceptible to lateral torsional buckling were the ratio of modulus of elasticity to bending strength, unsupported beam length divided by beam depth, beam aspect ratio, and coefficient of variation of the beam modulus of elasticity. For laterally stable bending and flexural shear, the LRFD format provided the more efficient design for all loading combinations except snow and seismic loadings with live load-to-dead load ratios greater than three and four, respectively. For nominal 2 × 4 members, there was generally little difference between codes when the bending members were laterally unbraced. For the deeper sections, the ASD code proved to be more optimal. This was especially true when the variation in modulus of elasticity was low (i.e., for machine-stress-rated lumber).