Evaluating the structural safety of steel frames using a probabilistic robustness index

During the design of buildings and structures with steel frames, the issue of ensuring their mechanical safety under accidental actions arises. A number of such actions are not accounted for in the normal operating conditions of buildings and are therefore classified as emergency events. To assess the degree of resistance of load-bearing structures to these actions, the concept of structural robustness has been introduced in modern scientific literature. However, due to the insufficient study of the problem, there are relatively few works devoted to the quantitative assessment of robustness in the form of specific indicators, and those that do exist are primarily focused on reinforced concrete structures. This paper proposes a method for evaluating a probabilistic robustness index for steel frame structural systems. Its calculation uses a modified form of classical reliability theory equations, based on the premise that the failure of a frame system occurs through the formation of a mechanism with the minimum number of plastic hinges. When determining the probability of failure, the deformation behavior of frame elements is considered, with their sequential or parallel inclusion in the failure mechanism scheme, analogous to electrical circuits with series or parallel connections. To evaluate the dispersion of random variables, the statistical simulation method (Monte Carlo) and experimentally observed data are used. Examples of robustness index calculation are provided for various accident scenario types. The specific practical implementation of the method demonstrated its applicability, allowing conclusions to be drawn about the mechanical safety of structures with steel frame systems that are subject to heightened responsibility levels or other special requirements for resistance to progressive collapse.