Influence of masonry infill on the seismic performance of concentrically braced frames

This paper presents an experimental and analytical study to investigate the effect of masonry infill on the seismic performance of special Concentrically Braced Frames (CBFs). Cyclic lateral load tests are conducted on three half-scale specimens including two special CBFs with and without masonry infill and a moment resisting steel frame with masonry infill for comparison purposes. Companion analyses are performed to study the influence of masonry infill on the potential rupture of gusset plates and top-seat angle connections by using detailed FE models validated with experimental results. It is shown that the presence of masonry infill could increase the lateral stiffness and load carrying capacity of the special CBF by 33% and 41%, respectively. However, the interaction between masonry infill and the frame significantly increased the strain demands and failure potential of the connections. The results of the experimental tests and analytical simulations indicate that ignoring the influence of masonry infill in the seismic design process of CBFs results in a premature fracture of the connection weld lines and a significant reduction in the deformation capacity and ductility of the frame. This can adversely influence the seismic performance of the structure under strong earthquakes. The results of this study compare well with the damage observations after the 2003 earthquake in Bam, Iran

Analytical study on the seismic performance of steel braced frames with masonry infill

Special Concentrically Braced Frames (CBFs) are widely used as efficient lateral-load resisting systems in seismic regions. In this study, experimentally validated FE models are used to investigate the effects of masonry infill and gusset-plate configuration on the seismic performance of CBFs. It is shown that the presence of masonry infill can increase the initial stiffness and ultimate strength of CBFs by up to 35% and 52%, respectively. However, the frame-infill interaction imposes high plastic strain demands at the horizontal re-entrant corner of gusset plate connections, which may lead to premature failure of fillet welds under strong earthquakes. While using tapered gusset plates can significantly increase the fracture potential at fillet welds, gusset plates with elliptical clearance of eight times the plate thickness can lead to up to 54% lower equivalent plastic strain demands at both gusset plate connections and brace elements. While the effects of masonry infill are usually ignored in the seismic design process, the results highlight the importance of considering those effects in the seismic design of CBF elements and gusset plate connections