Load Testing Application for Truss Bridge Design Verification: Live Load Testing

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This is the second of two companion papers discussing application of load testing techniques for verification of bridge design. The bridge was a six-spans continuous steel truss structure, about 338-m long and 14.45-m wide, in the State of New York. Design of the bridge was completed using an unconventional approach where the top chords of the main trusses, floor beams, and stringers were designed to act composite with the concrete deck. In addition to axial forces, such a design also gives rise to secondary moments in the truss main members under design loads. This aroused interest in verification of the design through an instrumentation, monitoring, and load testing program. Comparing the members’ actual service load axial forces and moments with those used in the design, it was concluded that axial forces were overestimated in the design by about 20 percent for service dead load and by about 25 percent for service live load. Similar comparison for moments, indicated that service dead load moments were within 20 percent of those used in the design and service live load moments were underestimated by about 55 percent. The above differences for service dead load can be attributed to the way the deck pours were accounted for in the design and the possibility of construction loads being on the structure during the deck pour monitoring. For service live load, these differences can be explained by possible discrepancies in estimating service live load from the test results and the fact that the analysis for service live load in the design was performed ignoring the contribution of the composite concrete deck. Adequacy of the structural design under actual axial forces and moments was confirmed by checking the AASHTO interaction equations for steel members under combined axial and bending loading conditions. The major contribution of this and the companion paper is that, they introduced a new approach for estimating total dead load effects by only monitoring strains in a limited number of truss members during staged deck construction and load testing of a very large truss bridge structure, it validated the unconventional design under both dead load and live load as a viable method for design of truss bridge structures, and it supplemented the very limited literature on dead load monitoring of bridges.