Multi-presence factors in the AASHTO code are designed to account for the occurrence of multiple lanes experiencing maximum standard AASHTO loads. Permit vehicles represent a source of loading that exceeds standard vehicle weights. The presence of a single permit vehicle in addition to the loads from standard weight vehicles is arguably accounted for implicitly in the multi-presence factors. However, such an assumption needs to be verified based on the specifications for permit vehicle weights and traffic patterns within the state of Florida. In addition, the presence of multiple permit vehicles in configurations that result in loads in critical locations may conceivably exceed the capacity of the bridge.

This study examined the loads (additional trucks or otherwise) that should be considered concurrent with the different permit loads for the purpose of calculating appropriate operating ratings.  This study relied heavily on the processing and analysis of weigh-in-motion records from the State’s network of telemetered data collection stations.  From this, a probabilistic model of the relative likelihood of various concurrent vehicle combinations was developed to describe realistic worst case loading configurations.