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A fundamental aspect of the progressive collapse behaviour of building structures is the response of axially restrained beams following partial or total loss of the loadbearing capacity of a supporting member. Owing to the various complex effects involved such as material and geometric nonlinearity, advanced numerical approaches tend to be the most effective tools for modelling performance. Such approaches, however, lack the simplicity needed for common use and may provide only limited capability for understanding structural behaviour. For such purposes, more limited analysis approaches that can address adequately the basic features of performance are likely to be more productive. One such method for modelling the response of axially restrained steel and composite beams following column loss is presented in this paper. The method involves explicit modelling of the connection behaviour and employs conventional structural analysis principles to describe beam performance using accessible spreadsheet calculations. Following careful verification against detailed numerical analyses and validation against available experimental results, the proposed method is deemed capable of modelling the various complex features of response with excellent accuracy. Therefore, it may form a promising advance in studying and understanding the basic mechanics of the problem.

The topic of progressive collapse of structures has increasingly received particular consideration over recent years, with numerous studies of various forms having been seeking to explore the complex mechanics of the problem and define ways of minimising its effects. Through a continuing research program at Imperial College London, considerable advance has been made in understanding the physical features of the progressive collapse response of frame structures, identifying the role of the several different controlling parameters and compiling simplified methods for simulating particular effects. Knowledge of the subject has, therefore, been improved to a considerable extent, that is now possible to make quantitatively justified assessments of structural robustness by specifying the reasons why a structure might be susceptible to progressive collapse as well as to identify potential methods for enhancing performance. The present paper reviews previous developments at Imperial, evaluates information obtained from relevant experimental studies conducted by other research groups and makes an assessment of the progress made both in developing a scientific understanding of the subject and establishing improved ways of tackling the problem in practice.Several factors that should be considered in the robustness design of steel and composite frame structures are defined as well as needs for further research into specific aspects of the problem are identified.