Dynamic Load Allowance for Through-Truss Bridges

The objective of the present study was to experimentally evaluate the statistics of dynamically induced stress levels in steel through-truss bridges as a function of bridge component type, component peak static stress, vehicle type, and vehicle speed. Better understanding of critical bridge rating parameters will enable more accurate bridge evaluations of this type of structure. Three 60-year-old, steel through-truss bridges with similar characteristics were investigated in the present study. Several bridge components on each of the three bridges were instrumented (truss members, stringers, and floor beams), and dynamic strain data were collected under controlled and normal traffic conditions. The dynamic strain histories were processed to obtain bridge component peak static response and peak dynamic response, resulting in the determination of the dynamic load allowance (DLA) for each of the instrumented bridge components for each of several truck crossings. The calculated DLA value are plotted as a function of member peak static stress for each bridge member instrumented. The DLA data are examined as a function of component type, component location, truck type, number of axles, truck speed, and truck direction. This study has demonstrated that the DLA is dependent on truck location, component location, component type, and component peak static stress but appears to be nearly independent of vehicle speed.