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Protection of structural columns against severe damage is of most importance in the seismic design of RC structures. The seismic performance of reinforced concrete (RC) columns can be improved by concrete confinement in the plastic hinge regions, where the required quantity of confining reinforcement is influenced by the axial force ratio (AFR). Nevertheless, different design standards have different treatments on the AFR effect in the ductility design of RC columns. In this regard, this paper presents a scientific revisit, which is conjugated with a statistical analysis of 474 sets of experimental data, on the effect of AFR. It is found that the displacement and hysteretic dissipation capacities of the flexure -critical RC columns can be significantly reduced by increasing AFR, although the lateral strength shows a reversed trend. On the other hand, the displacement ductility of short columns is apparently not influenced by AFR, yet the hysteretic dissipation capacity deteriorates under high AFR. The analytical results also suggest that different AFR limits should be used for the design of slender and short columns due to different failure mechanisms. The presented work sheds light on the considerations of the AFR effect and setting a suitable limit for controlling the seismic perfomiance of RC columns.

In this paper, the effects of vertical irregularity of masonry-infill wall arrangements on the seismic performance and lateral stability of reinforced concrete (RC) frames have been investigated using nonlinear response history analysis. Discrete finite element models, which are endowed with a damage-based cohesive crack modeling, were developed for six prototype structures with different arrangements of infill panels. The analysis of failure modes and lateral force transfer mechanisms of infilled frames shows that the arrangement of infill panels can critically affect the dynamic properties and structural stability under seismic excitations. Diagonal continuity, infill ratios, and eccentricity of infill walls play essential roles in the lateral-load and displacement capacities of the structures. The hysteresis behavior, failure modes, and force transfer mechanisms of RC frames with concentric and eccentric arrangements of infill panels are compared and their implications on the seismic resistant design are discussed.

In assessing the structural performance of infilled frames, in particular those with irregular and discontinuous infill panels, under bi-directional seismic excitation, the interaction effect of in-plane and out-of-plane lateral loads should be properly considered. This paper presents an investigation into the effect of bi-directional horizontal loading on the nonlinear static and dynamic behaviour of masonry-infilled reinforced concrete frames with openings in association with discrete-finite element modelling techniques. Out-of-plane loading and openings can significantly soften the bracing action provided by infill walls to the bounding frame. Under static load, the lateral strength of the infilled frames can reduce by 20-50 % when the applied out-of-plane load increase from 0.5 times to 2.0 times the unit weight of infills. The out-of-plane effects are intensified in dynamic loading cases. It is found that the peak base shears of the fully infilled frame under the bidirectional excitations are lower by 24.7 % under the Superstition Hill earthquake (PGA = 0.45 g) and 54.1 % under the Chi-Chi earthquake (PGA = 0.82 g) as compared with the uni-directional load cases. The displacement demands are also greater under bidirectional dynamic loading. For 2/3 height infilled frame, the displacement demands are significantly increased by 99.7 % under Kobe (PGA = 0.65 g) and 111.0 % under Chi-Chi earthquake (PGA = 0.82 g) respectively. For the fully infilled frame, the displacement demands are 84.1 % higher under Kobe and 53.1 % higher under Chi-Chi. Due to the incapability of developing continuous arching action, the infill panels with openings are particularly vulnerable to out-of-plane action and that often leads to progressive collapse of infill components. The worst scenario is that total collapse of infill panels takes place at the first storey, creating a soft-storey that jeopardise the overall structural stability.