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University of Sheffield

Spyros I. Spyrou

Published 2018 in The Grants Register 2019

ABSTRACT

Structural engineers have a responsibility for incorporating fire safety into their building designs in order to minimize loss of life and property. The Cardington fullscale fire tests demonstrated that member-based structural fire engineering simply does not work for large, complex buildings. Performance-based structural fire engineering building design inevitably has to depend on scenario-based non-linear numerical modelling of large sub-frames of the structure. If the building is to avoid the possibility of disproportionate collapse in fire, this numerical modelling must be capable of predicting real structural collapse, rather than just the first loss of stability. In research described in this thesis a numerical procedure in which the whole behaviour of steel and composite building structures, from local instability to total collapse, can be modelled effectively, is developed and implemented in Vulcan. The model combines alternate static and dynamic analyses, useing both to best advantage. Static analysis is used to trace the behaviour of the structure at changing temperature until an instability happens; beyond this point an explicit dynamic procedure is initiated to track the motion of the system until stability is regained. This has been extensively validated as an effective tool against practical cases, and has been shown to work well in a large number of robustness analyses in fire scenarios. The different forms of global (or severe local) failure are identified, and the sequence of progressive collapse mechanisms is captured. Several factors which either contribute to or mitigate a fire-induced progressive collapse are studied, including the loading level, structural configuration and detailing, and the location of bracing systems. The behaviour of a column in a complete building differs from that of an isolated column, because of the effects of structural continuity. Based on the sequence of behaviour of frames, a simplified model is proposed to model the behaviour of a column in localized fire, considering interactions between the directly affected column and its structural environment. Modelling the progressive failure of connections is within the scope of this research. A connection may be able to retain its robustness after the initial fracture of a component, or the first failure may trigger a cascade of failures of other components, leading to complete detachment of the members. This possibility should be considered in performance-based design when the structure is being tested for robustness. Combined with the parallel development of general component-based connection elements, this procedure is shown to be able to effectively trace the behaviour of connections, from the initial fracture of components, via the failure of successive bolt-rows, to final detachment from columns. In fact the analysis carries on beyond connection fracture, until final structural collapse occurs at a higher temperature. This extension to Vulcan which allows all the states of the structure, including both its equilibrium and its dynamics, to be tracked as its loading or temperature increases, has created an effective tool for practical performance-based global analysis, including the prediction of progressive collapse.

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