Effect of inverted-V bracing on retrofitting against progressive collapse of steel moment resisting frames

The effect of inverted-V bracing on enhancing progressive collapse resistance of steel Moment Resisting Frames (MRF) were investigated in this study. A series of nonlinear static and dynamic analyses were performed to determine the resistance of four generic MRFs retrofitted by ten inverted-V bracing element. These structures were subjected to an exterior column loss and had a different number of stories and span lengths in order to study the effect of these variations on the structural response. Both force-controlled and deformation-controlled actions were implemented to determine if the column loss would lead to a failure progression. Results showed that structural configuration affects the structural resistance against failure progression and hence the appropriate brace element to retrofit it. Also, it was shown that for the studied 4-story frames, by increasing the span length by 20%, the structural resistance decreases by 42% on average. Finally, it was observed that by decreasing the span length, the Dynamic Increase Factor (DIF) suggested by the UFC, will lead to underestimating the required cross-sectional area of the brace for strengthening the unbraced structures.