Seismic behavior and modeling of reinforced concrete building columns

Post-earthquake reconnaissance and experimental research indicate that columns with insufficient transverse reinforcement and poor seismic details are vulnerable to earthquake damage in existing buildings. One of the major objectives of this research was to identify main factors contributing to shear failure and gravity load collapse of lightly reinforced concrete columns. Four full-scale column specimens were constructed and tested statically as part of the experimental investigation. The columns were tested in double bending under uni-directional lateral load.;Test results showed that the response of older columns with nominally identical properties depended largely on the magnitude and history of axial and lateral loads. Specimens with low axial load lost their lateral strength substantially at low displacement ductility, but sustained axial load at large displacements. Under the same flexural demand and very high axial load, lateral stiffness and strength increased at low displacements, however, the specimen had a sudden shear and axial load failure. Under monotonic lateral load, and under very low compressive or tensile axial loads, the lateral strength degradation was less severe.;The analytical phase of this investigation included development of a model to predict the shear strength of columns, and comparison with the experimental results from a database of more than fifty columns with shear failure. Based on a statistical evaluation, a strength reduction factor is proposed for design and assessment purposes. The proposed shear strength model is also compared with other existing models.;The analytical investigation also included examination of the behavior of columns with significant stiffness and strength degradation due to shear failure after the flexural strength is attained. Based on test results, simplified analytical procedures are developed to model the flexure, longitudinal bar slip, and shear behavior under monotonic lateral load. The flexure and longitudinal bar slip models are based on fiber section moment-curvature analysis with uniaxial material properties. The shear model is developed using the data from experiments and previous theoretical investigations. A model with three springs in series is proposed to combine individual monotonic deformation models. The measured cyclic response of test columns compared well with the response calculated from the proposed spring model.