Abstract
Traffic loads on bridges exhibit significant variations regionally, from state to state, and from site to site. Accounting for actual live loads in the bridge design process is important to improving the overall reliability and safety of bridges. In some cases, the code-specified live loads may underestimate traffic loading on a bridge. The current load and resistance factor design (LRFD) live load calibration is based on a biased sample of truck weights collected as part of an Ontario, Canada, truck weight survey conducted in 1975. In the past 35 years, truck traffic has significantly increased in volume and weight, which may necessitate adjusting the LRFD live load factors in certain cases on the basis of current truck traffic conditions. Although the quality and quantity of traffic data being collected by highway agencies has improved since 1975, it has not been used to update the bridge design loads. NCHRP Project 12–76 was initiated in 2006 to develop a set of protocols and methodologies using recent truck traffic data to update live loads for LRFD bridge design. Various levels of complexity are available using the site-specific truck weight and traffic data to calibrate live load models. One simplified calibration approach focuses on the lifetime maximum live load for updating the live load model or the load factor for current traffic conditions. Another, more robust, reliability-based approach for calibration is proposed in the protocols. The models are applicable for the design of bridge members, for both ultimate capacity and cyclic fatigue, and are implementable for both main structural members and the design of bridge decks.
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