Reliability-based optimum design of reinforced concrete girders

A practical optimization approach to the design of reinforced concrete girders for highway bridges is presented. The approach is entirely based on the American Association of State Highway and Transportation Officials (AASHTO) standard specifications for highway bridges. All behavior and side constraints specified in the AASHTO bridge code are considered in the optimum design process. Two optimization formulations are presented. The first uses load and resistance design factors (LRFD) and the second one is entirely based on a reliability approach. In these formulations, the independent and dependent variables are identified, and the constraints are formulated in both the AASHTO-LRFD format and the AASHTO-reliability-based format. The solutions for both formulations are obtained by using a nonlinear optimization software. Numerical examples illustrate the application of the proposed approach to the design of reinforced concrete T-girders. Sensitivity analyses are also performed for finding the effects of various parameters including steel to concrete cost ratio and allowable reliability level on the optimum solution.