Stability degradation and redundancy in damaged structures

The objective of this paper is to assess the promise of a novel tool for structural safety decision-making in severe unforeseen hazards. A straightforward measure is put forward for measuring the intensity of unforeseen hazards: the number of connected members “removed” from a structure in a brittle fashion. The tool selected for assessing this severe demand is the reduction in the buckling load capacity (stability) of the structure under service loads. Two examples of planar steel frames quantitatively demonstrate how the stability of structures degrades under this severe damage. Degradation of the buckling load, as members are removed, provides an efficient and unique tool for assessing fragility against unforeseen hazards in buildings. Further, a coarse, but efficient measure of progressive collapse is provided by utilizing the condition number of the stiffness matrix for the damaged structure. Comparison of a moment frame with and without cross-bracing indicates the beneficial role of redundancy as damage increases. Challenges remain, particularly in determining reasonable distributions for the intensity, and improving the computational efficiency of the analysis. A framework for incorporating stability degradation analysis into decision-making, similar to that developed in seismic performance-based design, is provided.