Modeling Axial-Shear-Flexure Interaction of RC Columns for Seismic Response Assessment of Bridges

Reinforced concrete bridge columns exhibit complex hysteretic behavior owing to combined actions of axial force, shear force and bending moment during earthquakes. To efficiently simulate their nonlinear behavior for seismic assessment of bridges, a coupled hysteretic model is first developed to account for the nonlinear shear-flexure interaction (SFI) behavior under constant axial load. The proposed SFI model consists of coupled flexure and shear springs, whose behavior are governed by the primary curves and a set of loading/unloading rules to capture the pinching, stiffness softening and strength deterioration of columns due to combined loads. The model is implemented in a displacement-based finite element framework and calibrated against a large number of column specimens from static cyclic to dynamic shake table tests.;Recognizing that the SFI may increase the inelastic displacement of bridge columns, a simple demand model for estimating the inelastic displacement and ductility is developed. Guided by the rigorous dimensional analysis, the inelastic displacement responses of bridge columns considering SFI are presented in dimensionless form showing strong correlation with the dimensionless parameters (e.g. structure-to-pulse frequency, nonlinearity index, and aspect ratio). The proposed demand model is validated to predict accurately the displacement directly from structural and ground motion characteristics.;Subsequently, the proposed SFI model is applied to evaluate the system level seismic responses of three prototype bridges where the soil-structural interaction effects are also considered. Response quantities of bridges (e.g. drift, acceleration, section force and section moment etc.) are derived and compared to evaluate the effects of structural characteristics and the SFI of columns.;Lastly, in order to account for the axial load fluctuation during earthquakes, a coupled axial-shear-flexure interacting (ASFI) hysteretic model is developed. The ASFI model utilizes a novel concept of normalization to parameterize the primary curves at different axial load levels. An axial load independent stress level index is also developed to enable the transition between different reloading and unloading branches at variable axial load levels. The model is validated against the experimental data and allows for efficient seismic response assessment of bridges under multi-directional ground motions.