Monitoring and load distribution study for the land bridge

Abstract

A monitoring program and a live load distribution study were conducted for the Land Bridge, located on State Highway 131 between Ontario and LaFarge in southwest Wisconsin. The bridge is a 275-ft long curved double trapezoidal steel box girder construction. Hybrid HPS70W and A588 weathering steels were used for the construction of the bridge. The monitoring program included measurements of live load and thermal strains as well as displacements for the girders over a four-year period. The effects of the in-service live load, in terms of both the applied stresses and the number of load cycles, were found to be insignificant. The thermal stress levels were found to be more significant but with only a limited number of load cycles. It was also found that there was no significant change in the load pattern, for both the stress level and number of load cycles, over the four years of the monitoring program for the bridge. The observed stress levels in the bridge were found to be below the fatigue stress threshold prescribed by AASHTO. This indicated that an infinite life could be expected for the bridge when fatigue is a consideration for the steel box girders. The live load distribution study for the Land Bridge included a field testing, a 3-D numerical simulation, and a comparative study of the results with those determined by the provisions of the AASHTO standard and LRFD specifications. Good agreement was achieved between the load distribution factor values that were obtained from the field testing and the numerical simulation. The comparison of the results with the values obtained from the AASHTO specifications indicated that over-conservative results yielded from the standard specifications while the results from the LRFD specifications were under-conservative. It is recommended that an additional study be performed to overcome this shortcoming of the current design specifications.