The Changing Face of Structural Design for Fire

Introduction Disasters, whether natural or manmade, are a test to design practices and in many cases prove the vulnerability of our infrastructure. Disasters force us to revisit our perception of the safety inherent to the environment in which we carry out our everyday activities. Therefore, associated to disasters there is always anxiety and pressure to revisit those practices that lead to unsatisfactory performance. The behaviour of structures in a fire has faced, in the events of September 11 2001, one of those disasters that have directly challenged our current design practices. Anxiety has spread over those individuals linked to infrastructure that can be considered as potential targets for terrorist activity. As we understand more about what happened with the World Trade Centre Buildings questions are being raised about our current design practices, proposed amendments and the tools that we use to evaluate the performance of structures in the event of a fire. Furthermore, the collapse of the World Trade Center buildings 1, 2 and 7 occurred within a period where design practices were being pushed out of an environment of prescriptive requirements to one where structures will instead be evaluated on the basis of their performance as predicted by engineering tools. To analyse the response of the designers to this disaster it is necessary to pose a series of questions. The first question relates to the actual nature of the disaster that is provoking the reaction. Why did these buildings collapse, carrying the lives of so many people? The answer to this question will be the product of a forensic investigation [1] that we do not intend to discuss it any further here. Nevertheless, from this investigation will result different conclusions, some pertaining to the nature of the event, some pertaining to the nature of the buildings themselves and some pertaining to the design and construction practices involved in the development of these buildings. The latter point is the one of greatest interest to the public since it is associated to the safety of current and future buildings designed under the same principles. The general question then becomes: in which way is fire incorporated into the design of structures? This question is then followed by a series of interrogations that relate to the details of the design practice, which are of a more fundamental nature but still directly concern the safety of our built environments. Deepening into the detailed processes, a fire affects a structure through the heat it supplies to all the constructive elements. Thus the first pillar of a design process is the understanding of the fire, the growth process it undergoes and the heat it supplies to the structural elements. As much as it is clear to everyone that a fire affects a structure, it is not as common to understand how a structure can have an impact on the growth of a fire. Nevertheless, it is the case that as the structure heats-up energy will be provided by the structural elements to the fuels and so enhance the rates of fire growth. Furthermore, deformation and failure of different structural components will affect the air supply to the fire and consequently the heat released. As a result structural and fire behaviour are coupled.