Proceedings of the Institution of Civil Engineers Structures & Buildings 160 December 2007 Issue SB6 Pages 317–325 doi: 10.1680/stbu.2007.160.6.317 Paper 700049 Received 19/10/2006 Accepted 10/09/2007 Keywords: design methods & aids/ failures/safety & hazards Uwe Starossek Professor of Structural Engineering, Structural Analysis and Steel Structures Institute, Hamburg University of Technology, Germany Disproportionate collapse: a pragmatic approach U. Starossek Dr.-Ing., P.E. Probability-based design methods face certain problems in making structures sufficiently collapse resistant and thus preventing disproportionate collapse. These problems are addressed and a discussion is given to outline how they might be overcome both within and outside a probabilistic framework. Definitions for the terms ‘robustness’ and ‘collapse resistance’ are proposed, the former being a property of the structure alone, the latter including possible causes of initial local failure. A pragmatic approach for designing against disproportionate collapse is suggested and a set of corresponding design criteria is presented. Design strategies based on preventing or presuming local failure are compared. In addition to the better-known design methods providing specific local resistance or alternative paths, an approach based on isolation by segmentation is examined. It is found that the applicability of the various design methods depends on the design objectives and on the type of structure. 1. INTRODUCTION Research on progressive and disproportionate collapse was stimulated by the Ronan Point failure in 1968. It has intensified since the 1995 bombing of the Alfred P. Murrah Federal Building in Oklahoma City and even more so since the events of 11 September 2001, as documented by a large number of conferences 1–4 and papers. 5–8 These efforts have not yet led, however, to a consolidated and generally agreed set of nomenclature and procedures. Guidance for the practising design engineer is gradually evolving but is still limited to particular types of structures. The first code requirements for avoiding the disproportionate collapse of buildings were introduced in the UK—shortly after the Ronan Point incident. The current state of British codification is represented by Approved Document A 9 and EC 1–1–7. 10 In the US, detailed design rules for buildings 11,12 have been issued, although not for the public sector, and are in use. These guidelines include both indirect (i.e. prescriptive) and direct (i.e. performance-based) design approaches, the latter comprising the specific-local-resistance method and the alternative-paths method. For the design of cable-stayed bridges, the Post-Tensioning Institute (PTI) recommends that the sudden rupture of one cable shall not lead to structural instability and specifies a corresponding load case ‘loss of cable.’ 13 Application of such a load case in the design corresponds to the alternative-paths method. The possibility of isolating an incipient collapse through segmentation—a direct design approach used for the Confederation Bridge, Canada 14,15 —is not included in current guidelines. The current paper attempts to make a contribution to nomenclature and procedures. First, it seems useful to distinguish the terms ‘robustness’ and ‘collapse resistance’. Corresponding definitions are proposed. Next, the basis of current reliability-based design codes for general structures is reviewed with particular regard to their suitability to prevent disproportionate collapse. Three deficiencies are identified. One of these results from practical limitations in considering the effect of local failure on the remaining structure. An outline is given of how this problem could be solved within a probabilistic framework on the assumption of improved analysis capabilities in the future. At present, the problems raised here seem difficult to remedy within a purely reliability- based approach. A framework for a pragmatic design approach, with emphasis on performance-based methods and being applicable to any kind of structure, is therefore suggested. A set of corresponding design criteria is presented. These include requirements, design objectives, design strategies and verification procedures. It is discussed how these design criteria can possibly be specified. Design strategies are based on preventing or presuming local failure. When local failure is presumed, the alternative-paths method and the segmentation method can be considered. These design strategies and methods are compared. The factors influencing their respective applicability are discussed. 2. TWO DEFINITIONS The term ‘robustness’ regularly appears in publications and discussions on progressive or disproportionate collapse. Even so, it is used differently and there is no common agreement to date on its exact meaning. 4 Two definitions are given in the next two sections. They prove useful for the discussion of design criteria, which is presented later in the current paper. 2.1. Robustness It is suggested to define the term ‘robustness’ as insensitivity to local failure, where ‘insensitivity’ and ‘local failure’ are quantified by the design objectives, which are part of the design criteria (see section 6 below). According to this definition, robustness is a property of the structure alone and independent of the cause and probability of initial local failure. Structures & Buildings 160 Issue SB6 Disproportionate collapse: a pragmatic approach Starossek 317