Effect of infill walls on mechanisms of steel frames against progressive collapse

In this study, the effect of masonry infill walls on the progressive collapse mechanisms of steel frames is investigated. Three two-story by four-bay steel frames are designed: a bare frame, frame with full-height infill walls, and frame with partial infill walls. The applied vertical load, horizontal displacement, failure modes, stress development, and load redistribution mechanisms are investigated under center column pushdown scenarios. The influences of critical material properties and opening dimensions of partial infill walls are also investigated in this study. The results indicate that masonry infill walls significantly enhance the maximum applied vertical load and initial stiffness; however, they decrease the ductility and can alter the failure modes of the steel frames. The outward movement of the steel frames is a result of the diagonal compressive actions of the infill walls, and the changes in the bending positions of beams are a result of the bracing effects of the infill walls. The mortar joints and opening dimensions significantly influence the progressive collapse performance of infilled steel frames. In addition, different modeling strategies for masonry infill walls are developed and compared. Finally, a preliminary design procedure adopting masonry infill walls to protect the steel frames against progressive collapse is proposed.