Bridge Load Rating Calculator

This calculator applies the AASHTO Load and Resistance Factor Rating (LRFR) equation, from Section 6 of AASHTO's Manual for Bridge Evaluation, to determine whether an existing bridge member can safely carry a given load: Rating Factor = (C − γD·D) / (γL·L·(1+IM)), where C is the member's nominal capacity, D and L are the unfactored dead and live load effects, γD and γL are the dead and live load factors, and (1+IM) is the dynamic load allowance accounting for the bounce and vibration of a moving vehicle (1.33 for the standard HL-93 design truck, versus a smaller 1.15 for fatigue checks). A rating factor at or above 1.0 means the member has enough reserve capacity to carry the rating vehicle at that limit state; below 1.0 triggers a load posting restricting heavier vehicles. This calculator computes the more conservative Inventory rating (using γD=1.25, γL=1.75) as the primary result, then separately computes the Operating rating with reduced factors (γD=1.25, γL=1.35) representing the maximum load a bridge can occasionally carry rather than sustain routinely — operating rating factors are always higher, or equal, since they're less conservative.

The reported "Rating (Tons)" multiplies the inventory rating factor by the 36-ton HL-93 design truck weight to express capacity in familiar tonnage terms. This is a single-member, single-limit-state check: real bridge rating requires evaluating every controlling member (moment, shear, multiple girders) across HL-93 and permit-vehicle loadings, and factors must reflect the specific analysis method, condition, and system factors from the current AASHTO Manual for Bridge Evaluation.