Efficiency of the alternative load path method to evaluate the progressive collapse potential of 3D building under blast load

Abstract

the work in the progressive collapse of three-dimensional composite buildings quite limited due to the cost and difficulties of the full-scale experimental tests and the complexity of the modelling technique. Researchers and design engineers are following The Alternative Load Path method (ALP) recommended by GSA and DoD guidelines to mitigate the progressive collapse potential. The commonly used ALP method, which considers as a threat-independent methodology, taking into account single column removal scenarios only regardless of the threat load type. This paper investigates the efficiency of the ALP to determine the progressive collapse potential of 3D composite building when subjected to 500 kg of TNT. Two-stage nonlinear dynamic analysis of 3D composite 9-story building with top and seat double web angle connection is carried out. In the first stage of analysis the axial load carried by the columns under gravity load is calculated. The second stage devotes to find out the number of columns likely to be involved in the progressive collapse using the damage index criteria through three-step of loading. A precise analysis is implemented to find out the effect of column location in the remaining capacity of the column. The influence of the boundary condition on the column response under blast load is highlighted. The results show that the ALP methodology is not usually conservative and may be more than one column wiped out under 500 kg of TNT which may be carried by a small vehicle. The commonly used damage criteria (stability, yielding and fracture) cannot capture the realistic damage of axial load bearing column, and the damage index which is related to its global behaviour has to be used. The structure response drastically increased when two columns removed at a time under blast load compared to the single column removal recommended by ALP of the DoD.