Unique Truck Loads on Interstates in Wyoming and Similar States and the Impact on Bridge Safety Design

Interstate 80 (I-80) through Wyoming carries a large volume of cross-continental trucks and large energy-industry trucks compared to many other states. Moreover, frequent weather closures position trucks side-by-side and end-to-end for miles so that they travel as a convoy once the road opens. The unique truck traffic and traffic patterns of Wyoming potentially create larger demands on bridges than were considered in the development of the national design specifications, and these differences may hold true for other states with unique traffic situations. This study was performed to assess bridge safety (in terms of national design expectations) along the I-80 corridor. Rational load cases for situations of single vehicles and multipresence were developed to model the traffic pattern characteristics thought to exist on I-80 across Wyoming. The 75-year design-life live-load model was applied for reliability studies. Reliability indexes were produced through Monte Carlo simulation. The results indicated that truck traffic and traffic patterns in Wyoming create larger demands than considered in the national specifications. Short multispan bridges were found especially critical. To maintain target-bridge safety, two recommendations were made: (1) specifications should incorporate the published commentary about low-boy tandem load as part of the loading specifications and (2) the state DOT (and that of other states with similar truck-traffic conditions) should increase the live-load factor, γ_L, for interstate bridges. Recommendations for the live-load factor are presented.