Evaluation of strut-and-tie modeling for bridge pier applications

Strut-and-tie modeling is an analysis and design tool for reinforced concrete elements which assumes that internal stresses are transferred through a truss type mechanism. The tensile ties and compressive struts serve as truss members connected by nodal zones. The internal truss idealized by the strut-and-tie model implicitly accounts for the distribution of both flexure and shear.;In 1998, the AASHTO LRFD Bridge Specifications (1998) incorporated the strut-and-tie modeling procedure for the analysis and design of deep reinforced concrete members where sectional design approaches are not valid. In most instances, hammerhead piers can be defined as deep reinforced concrete members and therefore, should be designed using the strut-and-tie modeling approach. However, little has been done to develop a consistent approach to the design of hammerhead pier caps employing the strut-and-tie modeling method.;The present study is focused on developing a uniform design procedure for applying the strut-and-tie modeling method to hammerhead piers. In addition to developing a design procedure, a survey of all fifty State Transportation Departments was conducted to ascertain the degree of implementation of the AASHTO LRFD Bridge Specifications for substructure design.;A design study was conducted using four hammerhead piers that were previously designed using the strength design method specified by the AASHTO Standard Specifications in order to evaluate strut-and-tie modeling procedures. The four pier caps were designed using the strut-and-tie modeling procedure and the results compared to the results of the sectional design method. For each hammerhead pier cap, the strut-and-tie method required more flexural steel than the sectional method. Based on the design studies, a well-defined procedure for designing a hammerhead pier utilizing the strut-and-tie model was established that may be used by bridge engineers