Multiple Presence Statistics for Bridge Live Load Based on Weigh-in-Motion Data

A study was done to determine truck load spectra for bridges in New Jersey with emphasis on multiple truck presence statistics. Truck weight data were collected by the New Jersey Department of Transportation from 25 weigh-in-motion sites throughout the state between 1993 and 2003 (with some gaps). The sites encompass a variety of site-specific conditions, including truck volume, road and area type, and number of lanes. For each truck, the recorded parameters include the time of passage, speed, travel lane, number of axles, and axle loads and spacings. Of particular interest are the frequency and correlation among trucks simultaneously occurring on a bridge either as following, side by side, or staggered. This multiple presence directly affects simulation, future extrapolation of live load effects, and code calibration. In fact, the case of two trucks occurring side by side has been considered the governing case for the design of bridges during the development and calibration of the AASHTO LRFD Bridge Design Specifications. However, until recently, multiple presence statistics have been based on few observations of truck traffic at a small number of sites. Statistics are presented for various truck loading cases, and the effects of truck volume, area type, road type, and bridge span length on such statistics are examined. It is observed that truck volume and bridge span length have a significant effect on the frequency of multiple truck presence, whereas area and road type have only slight effect. It is also observed that the rate of increase in the percent occurrence of following loading events is lower for bridge span lengths of up to 100 ft (30 m) as compared with longer spans, whereas for staggered loading patterns the opposite is true. The frequency of side-by-side trucks was found to remain relatively constant with respect to span length.