Energy-based pulldown analysis for assessing the progressive collapse potential of steel frame buildings

• A new nonlinear static analysis for fast, reliable progressive collapse assessment.
• Prediction of peak responses by balancing the external work and internal energy.
• Incremental application of a single downward force at the column removal location.
• Immediate advantages of significantly reduced computational complexity and cost.
• Responses predicted with at least the same accuracy as those by pushdown analysis.

This paper presents a new nonlinear static analysis procedure for predicting the peak structural responses of a building frame upon the sudden removal of a column. Based on the pulldown analysis technique that was originally developed using the empirical dynamic increase factors, the present procedure is derived by checking the condition of energy balance between the external work done by the unbalanced gravity loads and the internal energy stored in or dissipated by the deformed frame following the column removal. In contrast to the existing energy-based pushdown analysis that needs to incrementally apply the distributed gravity loads over all directly affected bays at every floor level above the removed column, the new energy-based pulldown analysis only requires the incremental application of a single downward force at the column removal location. As a result, only a single force–displacement response curve is needed in the energy-based pulldown analysis, compared to a large number of force–displacement curves for multiple nodes along all directly affected beams above the removed column, as required in the energy-based pushdown analysis. Hence, the computational complexity and expense is significantly reduced. Taken a steel frame structure as an example, the accuracies of energy-based pulldown analysis and energy-based pushdown analysis in assessing the building potential for progressive collapse after the notional column removal are systematically compared, using the nonlinear time history analysis results as a baseline. Numerical study shows that values of key structural responses (e.g., peak vertical displacement, maximum plastic hinge rotation, peak axial forces and moments) predicted by the energy-based pulldown analysis agree generally well with those by the time history analysis and are at least as accurate as those by the energy-based pushdown analysis. Therefore, the energy-based pulldown analysis holds great promise for fast, reliable assessment of building progressive collapse potential.