Keywords: robustness, progressive collapse, accidental actions, elasto-plastic analysis, alternative load path, structural reliability, vulnerability analysis.

Progressive collapse is defined by ASCE 7-05 [1] as "the spread of an initial local failure from element to element resulting, eventually, in the collapse of an entire structure or a disproportionately large part of it". The failure of the Ronan Point apartment tower in 1968 initiated interest in the topic of progressive collapse, which appears to have reached a peak following the recent terrorist attacks on the Murrah Federal Building and the World Trade Centre towers. A number of guidance documents have recently been produced on the topic [2,3,4], in an attempt to assist professionals in designing progressive collapse resistant structures.

As building designers cannot possibly design for every hazard that a building may be subjected to in its lifetime, a general design approach is required to account for the risks associated with low-probability high-consequence events. There are in general three alternative approaches to progressive collapse resistant design, found in the codes and guidelines; improved interconnection or continuity, the alternative load path method; and improved local resistance. A detailed review of these techniques is provided. Due to its ease of implementation, indirect design methods such as improved interconnection or continuity are favoured by the codes and guidelines [1,2,3,5]. However, these approaches give no consideration to how a structure should behave if local damage occurs, and therefore may not actually increase the resistance of a structure to progressive collapse [6].

Finally, this paper presents a methodology to assess the extent of damage to a multi-storey structure due to localised collapse. This is accomplished through the design of an algorithm, and its implementation as a computer program. The algorithm systematically removes individual elements from the structure. Each altered structure is then analysed in order to predict the extent of the resulting damage. For example, the analysis predicts how much of the structure will collapse if a ground story column is removed. The algorithm works by increasing the applied load and monitoring the formation of plastic hinges and the development of unstable compression members. The formation of such mechanisms can cause localised damage or may lead to progressive collapse. By systematically considering the effects of damage to all members in a structure the algorithm can identify whether a structure is unduly sensitive to the effects of localised damage.

1

American Society of Civil Engineers (ASCE), "Minimum Design Loads for Buildings and Other Structures", ASCE 7-05, Reston, Virginia, 2005.

2

Department of Defence (DoD), "Unified Facilities Criteria - Design of Building to Resist Progressive Collapse", UFC 4-023-03, Washington, DC, 2005.

3

General Services Administration (GSA), "Progressive Collapse Analysis and Design Guidelines for New Federal Office Buildings and Major Modernization Projects", Washington, DC, 2003.

4

Standing Committee on Structural Safety (SCOSS), "16th Biennial Report", London, 2007.

5

British Standards Institution (BSI), "Eurocode 1 - Actions on Structures - Part 1-7: General actions - Accidental actions", BS EN 1991-1-7:2006, London, 2006.

6

G. Kaewkulchai, E.B. Williamson, "Modelling the Impact of Failed Members for Progressive Collapse Analysis of Frame Structures", Journal of Performance of Constructed Facilities, 20(4), 375-383, 2006. doi:10.1061/(ASCE)0887-3828(2006)20:4(375)

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