Seismic performance of limited-ductile RC columns in moderate seismicity regions

Many existing buildings in Australia and other places of the world with low to moderate seismicity, particularly old buildings, are not designed for credible seismic actions. Consequently there are considerable uncertainties over the safety of such buildings and the suitable risk management strategies that should be adopted. This study focuses on gravity collapse drift modelling and seismic collapse assessment of limited ductile RC columns which are commonly found in soft-storey buildings in low to moderate seismicity regions. The RC columns of interest are typically lightly reinforced & poorly confined with vertical spacing of transverse reinforcements as large as the column depth. Such columns are categorized as shear critical and automatically deemed unsafe by the measures originally developed for the conditions of high seismic regions. Importantly, by such measures, collapse is conventionally defined at the point of nominal shear failure; that is the point of 20% degradation in peak lateral strength. This criterion however, does not take into account the properties of the projected ground motions in the low to moderate intra-plate seismicity region of Australia. In Australia, the seismically induced displacement demand does not increase indefinitely with the effective period of vibration of the structure. Instead, the displacement demand is constrained to a limit once the period of the structure reaches the corner period of excitations ( second). In conditions of displacement controlled behaviour, the actual displacement capacity of a structure prior to overturning/ gravity collapse is a more direct and realistic measure of the acceptance limit than the limit defined at the point of nominal shear failure. Considering the outlined condition and objectives, four lightly-reinforced and poorly-confined concrete columns were tested up to gravity collapse. The columns were subjected to constant axial load and reversed cyclic displacements of increasing amplitude. An iterative equilibrium/compatibility-based shear-slip model was developed to simulate the commonly observed rotational deformation (in relatively slender columns) caused by the opening of the critical shear crack prior to gravity collapse. The developed model was simplified and contributed to the development of Model (I) which was proposed for a deterministic estimation of total drift capacity at gravity collapse. Model I is limited to the limited-ductile RC columns with the flexural-shear mode of failure. Based on the analysis of the experimental results corresponding to a dataset of 56 limited ductile RC columns, two additional probabilistic drift capacity models have also been developed (at nominal shear failure and the limit of gravity collapse). The developed models are mainly functions of axial load ratio which address the need for minimizing input requirements while providing realistic predictions. Realistic backbone curves were constructed and incremental dynamic analyses were conducted for probabilistic seismic collapse assessment of a practical range of limited ductile RC columns. Axial load ratio, longitudinal and transverse reinforcement ratios, column size and aspect ratio and soil type were parameterized. A simplified probabilistic solution was proposed for collapse assessment based on the fragility curves of the columns analysed.