Seismic Behavior of Poorly Detailed RC Bridge Piers

Existing bridges may have been designed and constructed according to codes without any provisions for seismic design and detailing. Particularly in countries with moderate seismicity, such as Switzerland, modern seismic design standards might have been introduced only in recent years, as this hazard may have been underestimated previously. Existing bridge piers may thus have small deformation capacities due to their layout and detailing. Critical detailing includes lap-splices in the potential plastic hinge region above the foundation, low transverse reinforcement ratios and a lack of confining reinforcement. Displacement-based methods, which compare the deformation capacity with the imposed demand, may be used for the assessment of these bridges. As the assessment is performed by practicing engineers and the bridge stock to be assessed is large, models to predict the deformation capacity need to be easily applicable while yielding good results. Therefore, the purpose of this study is to contribute to the development of modeling approaches fulfilling these criteria. A series of tests that are representative of existing piers with the above mentioned detailing deficiencies was used as experimental database to check and validate the models. Two modeling approaches were investigated in detail: the plastic hinge modeling approach and a kinematic approach for shear critical wall-type piers. The first part of this report deals with the plastic hinge modeling approach. Equations to determine the plastic hinge length, the flexural deformation and the shear deformation as well as strain limits for the deformation capacity are reviewed. Based on the experimental data a procedure to predict the force-deformation response of wall-type piers is identified. The influence of lap-splices on the response and shear deformations are accounted for in a simple manner. The second part of this report deals with shear strength degradation and a kinematic approach for shear critical rectangular wall-type piers with which this degradation can be predicted. The model is based on the kinematics that develop if extensive shear cracking occurs in a pier and has been developed elsewhere. It is validated here against an extended experimental database and used to illustrate the influence of some key characteristics, such as the reinforcement contents and the aspect ratio, on the force-deformation response, particularly with regard to the deformation capacity. Comparison of the predictions with the experimental data showed that the plastic hinge modeling approach is, despite its simplicity, well suited to predicted the response of the only partially flexure controlled walls investigated here. With this approach, deformation capacities approximately corresponding to the peak load are obtained. To also take into account the post-peak response, the kinematic approach should be used, which is shown to capture very well the onset of shear degradation and axial load failure.