Differences in LRFD and ASD outcomes for hardwood glued-laminated bridges

This study compares and calibrates the design outcomes of load and resistance factor design (LRFD) to allowable stress design (ASD) procedures for hardwood glued-laminated (glulam) timber bridges. Three hardwood species were considered: northern red oak, red maple and yellow poplar. The percent change between the LRFD and ASD outcomes was found for bending moment, shear stress and bearing stress, then a calibration was performed which placed the LRFD outcomes to within ±5% of the ASD outcomes. The absolute magnitude of the difference in design outcome by method, when based on 10-year load duration, increased with span length, with the LRFD outcomes being less conservative than the ASD outcomes. A less conservative design results in a smaller beam section. The LRFD outcomes were calibrated using a format conversion factor that convened 10-year, dry-use ASD allowable strength values to 10-year, wet-use LRFD base resistance values. Changing to a 2-month load duration from a 10-year load duration reduced cross sectional beam and panel section requirements in the LRFD designs.