Ultimate strength of three reinforced concrete highway bridges

The destructive testing of three reinforced concrete highway bridges, recently made redundant by road realignment, is summarized. The procedure used to test the bridges to ultimate conditions is described and load capacities of about 20 times class 1 axle loads are reported for all structures. Analyses based on conventional ultimate strength theory can account for only two-thirds of these ultimate loads and then only if second order effects are included. A nonlinear finite element computer program has been developed and used to analyze one of these structures. Excellent prediction of the ultimate load is made by the program. It is therefore suggested that compressive membrane action, which is automatically modelled in the finite element solution, plays a significant role in the enhancement of load capacity.The paper concludes that a more sophisticated approach to the assessment of bridge load capacity is necessary if realistic estimates of actual strength are to be made. Limited experience with a nonlinear finite element program suggests one such approach. If used with care, some relief to the bridge replacement program can be expected. Key words: highway bridges, ultimate load capacity, finite element analysis, reinforced concrete, field testing, compressive membrane action.