Measurement-Based Evaluation of Noncomposite Steel Girder Bridges

The results from an experimental study into the behavior of noncomposite slab-on-steel girder bridges are presented. Two bridge units were tested under increasing static loads beyond first yield of the girders with a flatbed trailer loaded with concrete barriers. The load tests were unique in that dump trucks were applied between trailer load cycles to monitor the effect of overloads on the bridge response. Measured deformations included strain, vertical deflection, and girder-slab slip.

Lateral load distribution was evaluated experimentally by expressing the bending moment in each girder section (derived from the measured strains) as a percentage of the total bridge moment due in part to bearing restraint and deck stiffness. Measured distribution factors compared well with finite element analysis but were below empirical factors from AASHTO Standard (1996) and LRFD (1998) Specifications. There was better agreement with the LRFD-based values. The heavy trailer loads had no significant impact on the load distribution and partial composite action of the girders. More significant changes occured between different transverse load positions of the dump truck.

A proposed experimental bridge rating process was developed that consists of a hierarchy of steps of increasing complexity and effort, some of which do not require load testing. The process terminates once a satisfactory rating is achieved. Procedures for using measured distribution factors and section moduli for the girders to improve the AASHTO Allowable Stress and Load Factor ratings are given.