Analysis, Testing, and Load Rating of Historic Steel Truss Bridge Decks

This report documents the results of a study of on-system historic metal truss bridges in Texas. On-system bridges are those located on the state highway system, and the surviving on-system historic trusses in Texas were typically constructed in the 1920s and 1930s. The primary objective of this study was to investigate methods that can be used to develop an accurate and realistic load rating for an older on-system truss bridge.

In order to examine issues involved in the structural evaluation of older on-system truss bridges, two case study bridges were investigated in detail. In addition, a full-scale laboratory experimental investigation was conducted on a single bay of a typical truss bridge floor system.

Evaluation of the case study bridges indicated that the primary structural deficiency in these bridges was inadequate capacity of the steel floor beams and stringers, based on conventional load rating techniques. The analysis and field load testing conducted on the case study bridges and the laboratory investigation therefore focused primarily on the bridge floor systems.

The results of this study indicate that the use of standard AASHTO load rating techniques can substantially underestimate the strength of the floor beams and stringers. A significantly more accurate prediction of the structural response of the floor members to truck live loads can be achieved by conducting an elastic finite element analysis of the bridge floor system. Comparison with extensive field load test results and with laboratory test results shows that finite element analysis provides a more realistic but still somewhat conservative prediction of floor member response. Analysis of the floor system using a finite element model can be used to support a significantly improved load rating for historic on-system truss bridges.