【正文】 ble benefit, it has also led to increased fire safety through better understanding of the governing principles and the ability to act intelligently in designing suitable arrangements based on a proper assessment of need. Prior to the Ronan Point collapse in London in 1968 the terms robustness, progressive collapse,disproportionate collapse etc., were not part of Structural Engineering vocabulary. The consequences of the damage done to that 22 storey block of precast concrete apartments by a very modest gas explosion on the 18th floor led to new provisions in the UK Building Regulations, outlawing for many years of so called system built schemes, demolition of several pleted buildings, temporary removal of gas in high rise construction and the formation of the Standing Committee on Structural Safety. Eventually, the benefits of properly engineered prefabrication were recognised, safe methods for the installation of gas were devised and the industry moved on. However, the structural design guidance produced at that time that still underpins much present day provision was essentially prescriptive in nature with no real link to actual performance. Subsequent incidences of progressive collapse such as the Murragh Building and the World Trade Centre brought increased attention to the actual phenomenon and issues of how it might reasonably be taken into account for those structural designs where it was considered appropriate. In doing this it is, of course, essential to include both the risk of a triggering incident and the consequences of a failure so that the resulting more onerous structural demands are used appropriately. Arguably, a disproportionate response in terms of requiring costly additional provisions in cases where the risks/consequences are very low/very minor may be as harmful as failing to address those cases where the risks/consequences are high/severe. This paper will review current approaches to design to resist progressive collapse and contrast these with work undertaken over the past seven years at Imperial College London, where the goal has been the provision of a realistically based method suitable for use in routine design. The essential features of the method will be presented, its use on several examples described and results presented to illustrate how it is leading to a better understanding of both the mechanics of progressive collapse and the ways in which structural engineers can best configure their structures so as to provide enhanced resistance to resist progressive collapse The two most frequently used design approaches intended to address the issue of progressive collapse are:*c Providing tying capacity *c Checking alternate load pathsFigure 1: Tie Forces in a Frame Structure The first is essentially prescriptive and consists of ensuring that beams, columns, connections and floor (or roof) can act together to provide a specified minimum level of horizontal tying resistance。 the actual values required are normally related to the vertical loading. Figure 1, which is taken from recent US Guidance (SEI 2010), illustrates the principle. The approach is simple to appreciate, requires minimal structural calculation and, in situations where the original provisions are found to be inadequate, can be made to work by providing more substantial connections and/or additional reinforcement in floor slabs In an interesting recent development, that recognizes the link to the generation of catenary action, US Guidance has restricted the use of tying between the structural members to situations in which it can be demonstrated that the associated connections can carry the required forces whilst undergoing rotations of radiance. Where this is not possible, tying should act through the floors and the roof. However, recent studies (Nethercot et al 2010a。 Nethercot et al 2010b) have suggested that tying capacity correlates poorly with actual resistance to progressive collapse. Moreover, being prescriptive, it does not permit the meaningful parison of alternative arrangements a fundamental feature of structural design. In its most frequently used form the alternative load path approach presumes the instantaneous loss of a single column and then requires that the ability of the resulting damaged structure to bridge the loss be demonstrated by suitable calculation (Gudmundsson and Izzuddin 2010). The approach may be implemented at varying levels of sophistication in terms of the analysis。 for example, recent thinking in the United States (SEI 2010) makes provision for any of: linear static, nonlinear static or nonlinear dynamic analysis and provides some guidance on the use of each. It may also be used as the basis for more sophisticated numerical studies of particular structures and particular incidents . forensic work。 the best of these–which are likely to be putationally very demanding–have demonstrated their ability to closely replicate actual observed behaviour.3. Essential features of progressive collapse Three features have previously (Nethercot 2010) being identified as essential ponents of any reasonably realistic approach to design against progressive collapse:*c Events take place over a very short timescale and the actual failure is therefore dynamic.*c It involves gross deformations, generating large strains, leading to inelastic behaviour as well as change of geometry effects.*c Failure essentially corresponds to an inability of the structure in its damaged state to adopt a new position of equilibrium without separation of key elements.Figure 2: Simplified multilevel approach for progressive collapse assessment Additional features, designed to make the approach attractive for use by practicing Engineers have also been proposed (Nethercot 2010):*c Process sh