Bridge Deck Load Calculator

HL-93 is not a single truck weight. It is a notional live load model introduced with the AASHTO shift to Load and Resistance Factor Design, defined as the greater demand produced by a design truck of 72 kips in axles of 8, 32, and 32 kips or by a design tandem of two 25 kip axles spaced 4 feet apart, each superimposed on a uniform design lane load of 0.64 kip per linear foot representing the statistical effect of general traffic. The dynamic load allowance of 33 percent is applied only to the truck or tandem and never to the lane load, because a moving vehicle striking deck irregularities, expansion joints, and approach slab discontinuities imparts an impact effect beyond its static weight, whereas the lane load is already a smoothed statistical aggregate. Dead load is assembled from components rather than assumed. Normal-weight concrete at 150 pounds per cubic foot makes an 8 inch deck 100 pounds per square foot, a 2 inch asphalt wearing surface adds roughly 25 pounds per square foot, and concrete barriers contribute on the order of 350 to 450 pounds per linear foot on each side. AASHTO separates structural components (DC) from wearing surface and utilities (DW) because future overlay thickness is far less certain than the deck itself and carries a higher load factor, 1.50 against 1.25. Multiple presence factors from 1.20 down to 0.65 account for the falling probability that every lane carries a maximum vehicle at once. The result is a preliminary demand suitable for early sizing only; a real design still requires live load distribution factors, deflection and fatigue checks, and analysis of the girders that actually carry the deck.

American bridge loading standards begin with the organization founded in 1914 as the American Association of State Highway Officials, which published the first national Standard Specifications for Highway Bridges in 1931 to end the patchwork of state-specific loadings. Those early editions used H-series loadings named for gross truck weight in tons, such as H15 and H20, representing a two-axle truck. As tractor-semitrailer traffic grew, the HS series was added in 1944, with HS20-44 becoming the workhorse design vehicle for American highway bridges for the next half century. The association added Transportation to its name in 1973, becoming AASHTO. The move to probability-based design followed. Ontario issued its own Highway Bridge Design Code in 1979, the first major limit-states bridge code in North America, calibrated against measured truck weight surveys rather than a nominal legal vehicle. AASHTO commissioned a comparable effort through National Cooperative Highway Research Program work in the late 1980s, with the specification development led by John Kulicki of Modjeski and Masters and the reliability calibration carried out by Andrzej Nowak and colleagues, who fitted load and resistance factors to a target reliability index of about 3.5. That work produced the HL-93 notional load, which combines truck and lane effects instead of relying on a single vehicle, and the first edition of the AASHTO LRFD Bridge Design Specifications appeared in 1994. Federal Highway Administration policy subsequently required LRFD design for new bridges on the National Highway System, and the specification has been revised on a regular cycle since.