Evaluation of AASHTO-LRFD design methods for thermal loads in fixed-flexible twin-walled R/C bridge piers

The design of reinforced concrete fixed-flexible twin-walled bridge piers for lateral loads is an ambiguously defined task for bridge engineers. As the bridge experiences lateral loads, primarily from temperature fluctuations and time-dependent effects, the walls undergo cracking, requiring designers to consider sectional stiffness reductions. Two types of finite-element models were generated of the recently constructed Wakota Bridge in South St. Paul, Minnesota, one using a design-level program (SAP2000) and the other using a research-level program (ABAQUS). For an arbitrary temperature load, a commonly used refined design method, implemented in the design-level program, was evaluated for accuracy of reduced section properties relative to a more descriptive progressive cracking solution provided by the research model. The refined design method with four stiffness update segments was found to provide a balance between accuracy and analysis effort. A staged construction model of the Wakota Bridge, defined in SAP2000 to incorporate time-dependent effects of the construction sequence, indicated that pier forces for the design options in the AASHTO-LRFD Specifications for simulating reduced section properties (i.e., refined analysis with stiffness updates versus gross sections with reduced load factors) correlated to within approximately 10%. Additionally, of the two temperature change procedures in the AASHTO-LRFD Specifications, Procedure B produced moments for the Wakota Bridge that were as much as 25% larger than those from Procedure A.