Travelling Fires for Structural Design

Abstract

Traditional methods for specifying thermal inputs for the structural fire analysis of buildings assume uniform burning and homogeneous temperature conditions throughout a compartment, regardless of its size. This is in contrast to the observation that accidental fires in large, open-plan compartments tend to travel across floor plates, burning over a limited area at any one time. This thesis reviews the assumptions inherent in the traditional methods and addresses their limitations by proposing a methodology that considers travelling fires for structural design. Central to this work is the need for strong collaboration between fire safety engineers to define the fire environment and structural fire engineers to assess the subsequent structural behaviour. The traditional hypothesis of homogeneous temperature conditions in postflashover fires is reviewed by analysis of existing experimental data from wellinstrumented fire tests. It is found that this assumption does not hold well and that a rational statistical approach to fire behaviour could be used instead. The methodology developed in this thesis utilises travelling fires to produce more realistic fire scenarios in large, open-plan compartments than the conventional methods that assume uniform burning and homogeneous gas phase temperatures which are only applicable to small compartments. The methodology considers a family of travelling fires that includes the full range of physically possible fire sizes iv within a given compartment. The thermal environment is split into two regions: the near field (flames) and the far field (smoke away from the flames). Smaller fires travel across a floor plate for long periods of time with relatively cool far field temperatures, while larger fires have hotter far field temperatures but burn for shorter durations. The methodology is applied to case studies showing the impact of travelling fires on generic concrete and steel structures. It is found that travelling fires have a considerable impact on the performance of these structures and that conventional design approaches cannot automatically be assumed to be conservative. The results indicate that medium sized fires between 10% and 25% of the floor area are the most onerous for a structure. Detailed sensitivity analyses are presented, showing that the structural design and fuel load have a larger impact on structural behaviour than any numerical or physical parameter required for the methodology. This thesis represents a foundation for using travelling fires for structural analysis and design. The impact of travelling fires is critical for understanding true structural response to fire in modern, open-plan buildings. It is recommended that travelling fires be considered more widely for structural design and the structural mechanics associated with them be studied in more detail. The methodology presented in this thesis provides a key framework for collaboration between fire safety engineers and structural fire engineers to achieve these aims.