Moderately ductile concentrically braced frame (MDCBF) is a highly stiff and efficient seismic force resisting system. Hollow Structural Steel (HSS) members are commonly used as the structural steel brace for MDCBF. They have demonstrated good performance in past earthquakes. A common connection for this type of braces to use slotted hole where the HSS is welded to a gusset plate. Due to the lower cross sectional area around the slotted connection, steel cover plates are welded to both side of the HSS to prevent net section failure. In this paper, an advanced finite element model is developed to simulate the monotonic and cyclic behaviour MDCBF. The numerical model is validated against experimental results. The results show that the numerical model is capable of simulating the cyclic hardening, fracture, and buckling of the HSS braces. The validated numerical model is then used to investigate the performance of HSS braces with side plates with different HSS compactness and slenderness ratios. The results show the compactness and slenderness ratios of HSS could have a significant impact to the failure mode when the HSS is connected to the slotted gusset plates and reinforced with side cover plates.