Load Testing Application for Truss Bridge Design Verification: Dead Load Monitoring

This is the first of two papers discussing application of load testing techniques for verification of bridge design. The bridge was a truss structure in the State of New York, which atypical to conventional design, the top chords of the main trusses, floor beams, and stringers were designed to act composite with the concrete deck. Under load, in addition to axial forces, such a design creates secondary moments in the truss main members and complicates the analysis. This inspired the need for verification of the bridge design through an instrumentation, monitoring, and load testing program. The objective of this paper is to verify dead load design. Achieving this objective required instrumentation of main members of the erected structure and monitoring of strains in those members during concrete deck construction. For total dead load, a pseudo analytical approach was used to incorporate load effects due to self-weight of the structure. Five members of the downstream truss were instrumented with vibrating wire gages to record strains in those members during the first three deck pours. Finite element (FE) and deck monitoring results were utilized in the pseudo analytical approach to investigate actual axial forces and moments due to service total dead loads. The investigation indicated that those forces and moments were within 20 percent of those estimated based on FE analysis during the bridge design. The way the deck pours were accounted for in the design and presence of construction loads on the deck during the monitoring might have contributed to this difference. The paper’s major contribution is that it 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, it validated the unconventional design as a viable method for design of truss bridge structures, and it supplemented the very limited literature on dead load monitoring of bridges.