Progressive collapsed zone extent estimation in two-way slab floors by yield line analysis

Loss of an external facade column is among the most probable scenarios in evaluating the vulnerability of a structural system to progressive collapse. In the case that the column loss causes adjoining panels to fall down in a two-way slab reinforced concrete structure, the capability of the lower floor to prevent formation of any collapse mechanism plays an important role in averting progressive collapse. This paper introduces relevant beam–slab collapse modes of the impacted storey, as well as developing a formula for the ultimate load of each mechanism using yield line analysis. The formulation includes beam and slab main properties and could be used as a way to roughly estimate the extent of collapsed area in lower storeys. A study is also performed to determine the influence of some parameters on the predicted ultimate load. It is concluded that the ratio of beam to slab flexural strength and existing dead and live loads on panels in the vicinity of impacted panels could change the yield line pattern and subsequently control the prevailing collapse mode.