Multiple Presence Factor for Truck Load on Highway Bridges

Trucks may simultaneously appear on a highway bridge in the same lane or different lanes, which represent the governing load for short- and medium-span bridges. In practice specifications, the multiple presence factor (MPF) to cover these simultaneous loads was developed using intuition or models without support of measured weight data of trucks in motion. This paper presents a research effort of deriving a MPF based on weigh-in-motion (WIM) truck data, for both the strength and fatigue limit states, including about 68 million trucks gathered for 436 months and from 43 sites in California, Oregon, Michigan, and New York. The resulting MPFs are proposed in this paper as functions of bridge-span length, truck-traffic volume, and number of lanes available. They show that the code-specified MPF values are conservative and sometimes overconservative by 400% or more, which may have caused too high requirements for load-rating existing bridges, especially for shorter spans and low truck traffic. The recommended MPF may be considered to be adopted in specifications for highway bridge design and evaluation. This paper also presents a new truck-by-truck analysis approach for understanding the load effect of trucks in motion, taking advantage of increasingly available WIM data of large quantity to avoid unsupported assumptions in live-load modeling.