Structural performance
The structural design of modern tall buildings is governed by the need to efficiently transfer loading, particularly that from wind, whilst providing increasingly complex building functionality. The development of complex, inspired and highly optimised structural framing systems [23-25] (often deemed tall building technologies) has enabled efficient load transfer mechanisms, thus, in the event of a fire, locally induced deformations and resultant loading will be effectively redistributed throughout the structure. While this could help maintain structural integrity, research has demonstrated that these structural systems are particularly sensitive to the size and nature of the fire [4, 12]. Fire resistance has traditionally been defined as a function of a standard temperature time curve [26], with structural elements tested as single elements and their ratings defined as the time to attain a pre-specified failure criteria, traditionally
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a critical temperature. More recently, through the Cardington Tests [27], it has been recognised that this is not a realistic way of determining the performance of structures in fire. Post-Cardington analyses have used parametric temperature vs time fire curves and time equivalence concepts as input to the structure showing significant effect of the heating rates, period and cooling. Furthermore, numerous studies have emphasised that the presumed worst case fire loading imposed by homogeneous heating might not represent the most onerous scenario. Systems with long span light-weight floors where the load is shared by a stiff core and external structure are particularly vulnerable to multiple floor fires [12]. While for regular I-beams homogeneous heating seems to be a worst case condition, it is not for light-weight cellular beams which are vulnerable to localised heating [28]. In the analysis of WTC-7, NIST [4] concluded that long spans can induce progressive collapse if the detailing of the connections and the symmetry of the beam arrangement is not adequately characterised. Finally, the potential for failure during cooling has been identified in many of these modern systems [29], showing the need for a heterogeneous heating/cooling assessment as an essential component of a detailed analysis of the behaviour of a structure in fire. The advocating of performance-based design for tall and innovative buildings acknowledges the inability of furnace testing of individual structural elements to assure the provision of adequate structural fire safety. The survey conducted showed that there was some degree of structure failure in 13 of the 50 buildings. While the literature reviewed was often lacking on the specific details of structural failures, there were numerous mentions of localized failures, such as sagging of beams, failures of connections, collapsing of decking, and deformation of fire rated compartmentation assemblies and some more extensive failures such as the partial collapse observed at the Windsor Tower or in the cases of the WTC buildings, total collapse. Such behavior could be identified at the design stage though true performance assessment. Such an assessment requires an understanding of the likely fire conditions. Continuing to design for a uniform or standard fire when the greatest challenge to the structure might be a traveling fire is potentially flawed, especially when for many tall buildings the latter case could be the most realistic. Thus again it is clear that the correct definition of the fire is essential to maintain structural integrity and preserve the Fire Safety Strategy.
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