Live-Load Behavior of a Twin I-Girder Bridge

Very little data exists on the live-load behavior of twin-girder bridges despite their unique configuration and minimal redundancy. To investigate the live-load behavior of twin girder bridges, the State Highway 52 Bridge over the Snake River was instrumented and studied. The live-load response was monitored by attaching 62 strain gauges on girders, stringers, and intermediate diaphragms at nine cross-sectional locations. The live-load was applied with two trucks that were driven along three predetermined load paths. A finite-element model of the bridge was calibrated with the measured response from the live-load tests. The calibrated finite-element model was used to quantify moment distribution factors and load ratings for the bridge. The finite-element distribution factors were compared with those calculated according to methods provided in the AASHTO standard and LRFD specifications. The distribution factors from both specifications were found to be unconservative for the girders and overly conservative for the stringers. The model also was used to quantify the effect of the transverse diaphragm members on the live-load distribution. Distribution factors were calculated with and without the diaphragm members. The diaphragms were found to increase the distribution of moments by over 20% for both positive and negative moments.