Assessing the effect of bi-directional loading on nonlinear static and dynamic behaviour of masonry-infilled frames with openings

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.