Travelling fires for structural design-Part II: Design methodology

Abstract

Close inspection of accidental fires in large, open-plan compartments reveals that they do not burn simultaneously throughout the whole enclosure. Instead, these fires tend to move across floor plates as flames spread, burning over a limited area at any one time. These fires have been labelled travelling fires. Current structural fire design methods do not account for these types of fires. Despite these observations, fire scenarios most commonly used for the structural design of modern buildings are based on traditional methods that assume uniform burning and homogenous temperature conditions throughout a compartment, regardless of its size.This paper is Part II of a two part article and gives details of a new design methodology using travelling fires to produce more realistic fire scenarios in large, open-plan compartments than the conventional methods that assume uniform burning. The methodology considers a range of possible fire sizes and is aimed at producing results consistent with the requirements of structural fire analysis. The methodology is applied to a case study of a generic concrete frame by means of heat transfer calculations to infer structural performance. It is found that fires that are around 10% of the floor area are the most onerous for the structure, producing rebar temperatures equivalent to those reached from exposure to 106min of the standard fire and approximately 200°C hotter than that calculated using the Eurocode 1 parametric temperature–time curve. A detailed sensitivity analysis is presented, concluding that the most sensitive input parameters are related to the building design and its use and not the physical assumptions or numerical implementation of the method.