DIFFERENCES IN LRFD AND ASD OUTCOMES FOR HARDWOOD GLUED-LAMINATED BRIDGES

This study compares and calibrates the design outcomes of load and res istance factor design (LRFD) to allowable stress design (ASD) procedur es for hardwood glued-laminated (glulam) timber bridges. Three hardwoo d species were considered: northern red oak, red maple and yellow pol. ,lar: The percent change between the LRFD and ASD outcomes was found f or bending moment, shear stress and bearing stress, then a calibration was performed which placed the LRFD outcomes to within +/-5% of the A SD outcomes. The absolute magnitude of the difference in design outcom e by method, when based on 10-year load duration, increased with span length, with the LRFD outcomes being less conservative than the ASD ou tcomes. A less conservative design results in a smaller beam section. The LRFD outcomes were calibrated using a format conversion factor tha t converted 10-year dry-use ASD allowable strength values to 10-year; wet-use LRFD base resistance values. Changing to a 2-month load durati on front a 10-year load duration reduced cross sectional beam and pane l section requirements in the LRFD designs.