Experimental Verification of Shakedown Approaches for Bridge Design

Recently, it has been suggested that the lives of many deficient short-span composite bridges could be extended significantly if these structures were allowed to enter into the inelastic range for a low number of cycles. This limit state is known as the shakedown or incremental collapse limit. The results of one test with a half-scale, two-beam, two-span composite bridge aimed at clarifying some important aspects of composite behavior related to shakedown are reported. The test is unique in that various amounts of composite action (50 and 80 percent) were used in the two spans and in that actual moving loads were used to load the structure. The experimental and theoretical load calculations for the test compared favorably after the actual material properties were used in the section capacity calculations. The results indicate that for spans with 80 percent or greater interaction, the members are capable of sustaining a large number of cycles without significant strength deterioration. Although the slip at the beam-slab interface increased with cycling, strain hardening and redistribution of forces were sufficient to allow the system to reach its theoretical elasto-plastic capacity. In the span with 50 percent interaction, however, the maximum load achieved during the test was 30 percent lower than the theoretical prediction. This implies that if 50 percent or less of the required shear connection is provided, one should not expect the structure to carry cyclic loads into the inelastic range.