Estimating the cross-sectional area of inverted-V braces required for mitigating the progressive collapse of Steel Intermediate Moment Resisting Frames

The use of inverted-V braces is one of the retrofitting strategies engineers employ to mitigate progressive collapse in steel intermediate moment-resisting frames (SIMFs). However, the required cross-sectional area of the bracing member is not only dependent on the structural configurations but also on the seismicity of the region. In this study, a new equation is proposed to estimate the cross-sectional area of inverted-V braces required to mitigate failure progression in SIMFs that are subject to abrupt loss of an exterior column in the first storey. In the proposed equation, the area of beam element in the affected span and the seismicity of the region are considered. For this purpose, six SIMF buildings are designed, considering variation in the seismic base shear and the number of storeys. Then, the structural response of these SIMFs when subjected to an exterior column loss is studied. Using dynamic pushdown analyses, failure load factors of these frames are determined. Then, inverted-V braces of five different sizes are added to the studied frames. Results provide a mathematical correlation between the base shear coefficients, the size of structural elements and the associated retrofitting effect. This approximate equation is then validated by analysing two extra case study structures.