Seismic axial collapse of short shear span reinforced concrete shear walls

Abstract

This thesis addresses the seismic behaviour investigation of reinforced concrete (RC) shear walls in a low-to-moderate seismicity region which faces its own class of challenges inherited from non-seismic construction practice. In the current global norm of shear wall research in high seismicity region, discussions on slender walls in tall buildings and squat walls in low-rise structure with low axial load, the effectiveness of boundary confinement detailing and more recently on the out-of-plane deformation are among the common topics. In low-to-moderate seismicity region, different problem arises due to the existence of lightly reinforced slender shear walls which induced single large crack scenario. However, this study focuses on a special class of RC shear wall problem which is out of the global norms listed above. This special class of structural walls is situated in a low-to-moderate seismicity region but also experiencing high typhoon wind actions (such as Hong Kong). The lack of strength hierarchy consideration has resulted in the construction of over-strength coupling beams. The high coupling degree is known to decrease the shear span of shear walls and increase the axial compression load in the lower wall panel. In Hong Kong, the closely packed architectural layout of small residential units in tall building construction has intensified the structure gravity density which transforms into axial load increment in the shear walls. The coexistence of transfer structure supporting shear walls above where shear stress concentration effects are present has further “shorten” the shear span. This special class of short shear span RC walls which are not found in global literature deserves in-depth study. Hence, this thesis is documenting the research on heavily reinforced short shear span rectangular RC shear walls with considerable effects of axial loads. This unique problem is first identified through a reconnaissance study on existing shear wall tests. RC deep beams are tested to investigate the ultimate strut and associated shear limit in relation to strut angle and concrete strength. Reversed cyclic tests on short shear span RC walls are carried out by varying the parameters of axial load ratio, concrete strength, boundary confinement detailing and reinforcement ratio. Analytical models are developed to estimate the ultimate shear strength, shear failure drift and axial collapse drift limit. Non-linear numerical models are validated against the global results of the tested walls to calibrate the local vertical reinforcement residual tensile strains. The axial collapse instability of cyclic tension-compression load excursion history at the wall edge are found to be controlled by the axial stress to shear stress ratio (p/v). An explicit axial collapse capacity model is formulated based on Mohr’s circle framework with inelastic reinforcement buckling estimation via a two-tier check at wall edge and centre. A two-level secant stiffness response spectrum analysis approach is presented on a shear wall building subjected to low-to-moderate seismic actions. The axial collapse capacity of short shear span walls are checked with the proposed models. It is concluded that high p/v ratio is potentially a devastating failure and axial load should be controlled during the design stage.