Finite-Element Analysis and Load Rating of Flat Slab Concrete Bridges

Abstract

A significant portion of the nation’s aging bridge inventory consists of flat slab concrete bridges. Many of these bridges were constructed in the middle third of the 20th century, and although they are in generally good condition, they were not designed to carry modern highway loads. This research builds on prior studies that indicate that the equivalent strip width method—prescribed by AASHTO and widely used for the analysis of flat slab bridges—may be overly conservative and lead to underprediction of bridge structural capacity. The development of finite-element (FE) analysis software designed specifically for the load rating of flat slab bridges is presented. The FE software formulation and convergence were verified by comparison with predictions from commercial FE software under realistic loading scenarios. Results of live load tests of an instrumented, in-service flat slab bridge are reported. The FE model-predicted slab moments were shown to be conservative relative to the moments inferred from the load test data for a range of truck positions. Fourteen in-service flat slab bridges were load rated with both FE analysis and the equivalent strip method to assess the degree of conservatism inherent in the AASHTO approximate analysis. The results show an average increase in rating factor of approximately 26% when using FE analysis and that 58% of the bridges predicted to be under capacity using AASTHO approximate methods are sufficient based on FE analysis.