Experimental determination of the ultimate capacity of a composite prestressed bridge unit (FHWA-OK-89-05) 2162

Abstract

A 55 ft. long prestressed, composite steel girder-concrete slab bridge unit was tested to ultimate load. The unit had previously undergone a series of tests including sustained loading, fatigue loading, and static loading to first yield. The 54 ft.' simply supported span was subjected to static loads applied at points 20: ft. from the supports. Vertical deflection as measured at midspan, and horizontal slippage at the slab-girder interface was measured at several locations along the span. Strain gage readings at midspan throughout the depth of the unit were recorded. The bridge unit behaved in a ductile manner, deflecting 18. 6 inches before failure occurred. Failure was by crushing of the slab at the load application points. The unit supported an applied load 13% greater than the ultimate load predicted. The excess capacity is attributed to strain hardening of the bottom flanges of the girders. Slippage at the slab-girder interface was observed to be small. Existing analysis procedures were found to be adequate for predicting the ultimate capacity of the unit. Load at first yield in a previous test was observed to be lower than predicted. It was determined that the reduction in yield load can be explained by the presence of residual tensile rolling stresses in the bottom flanges of the girders.