Seismic assessment of existing reinforced concrete columns
Pham, Thanh Phuong
Date of Issue2014
School of Civil and Environmental Engineering
Existing Reinforced Concrete (RC) columns, commonly designed and built with light transverse reinforcement, were prevalently used in buildings worldwide before significant advancements in building code provisions were instituted in the mid-1970s. Post-earthquake investigations have shown that these columns are vulnerable to earthquake-induced collapse, posing a threat to public safety in future earthquakes. In this context, seismic assessment of existing RC columns plays a key role and needs further development. In this research, two series of experimental program consisting of sixteen column specimens were investigated to study the seismic behavior and failure mechanism of existing RC columns. The first test series was conducted on nine RC columns having light transverse reinforcement and plain longitudinal reinforcement. The seismic performance of the test specimens was assessed in terms of crack patterns, hysteretic response, initial stiffness, shear strength, drift capacity and energy dissipation capacity. The test results were discussed and compared with available experimental data of similar columns reinforced with deformed rebars. The comparisons highlighted that apart from lower shear capacities; columns with plain rebars had relatively large initial stiffness and drift capacities, even higher than columns with deformed longitudinal rebars. Further comparisons with the existing model for evaluation of existing structures revealed that the ASCE 41 provision tends to over-estimate initial stiffness, shear strength but substantially underestimates drift capacities of columns with plain rebars. A reduction factor therefore was proposed to improve the accuracy of ASCE 41 when employed for shear strength evaluation of RC columns with plain longitudinal reinforcement. An analytical model was developed to study the unique failure mechanism observed from the first test series. A new load-transferring path was introduced which is not only capable of explaining the failure mechanism but also allows for the development of a shear strength model for the column specimens. To enhance the applicability of the model, it was further corroborated with 89 short columns screened from existing literature. Good agreement between analytical and experimental results was achieved. To further understand the seismic response of existing RC columns, a second test series was conducted in this study with the focus on the effects of seismic loading directions. Seven RC columns with light transverse reinforcement and having deformed rebars were tested under non-principal loading directions. The obtained test results were analyzed, discussed and compared with similar columns previously tested under principal directions. The direction of seismic loads was found to have a significant effect on the drift capacities and failure modes of both rectangular and square columns. The extent of change in shear strength resulting from the different angles of seismic loading directions could be predicted. A Finite Element Model was developed to supplement the results of the second test series. Based on both experimental and numerical results, a simple upper limit for shear strength contributed by concrete was proposed.