Examples of fire engineering design for steel members, using a standard curve versus a new parametric curve

摘要

This paper presents examples of the differences that can occur when a standard time-temperature curve and a parametric time-temperature curve are used to determine temperatures likely to be reached by uninsulated and insulated steel members during a fire. For low and moderate structural fire severity situations, determination of the adequacy of a steel member by comparing the temperature reached in a 'design fire' with the limiting temperature based on the member heat sink characteristics, extent of insulation and utilization factor is becoming increasingly common fire engineering design practice. For this it is important to have an accurate and widely applicable parametric fire model as is practicable. The standard time-temperature curve used in the examples is the ISO 834 curve. The two parametric time-temperature curves used in the paper are the Eurocode parametric curve and a recently developed one termed the 'BFD curve'. The latter has been found to fit the results of a wide range of actual fire tests more closely than do existing parametric curves and is mathematically simpler in form. The shape of the BFD curve and the parameters used to define it bear a strong relationship to both the pyrolysis coefficient (R/A_vh_v~(0.5)) and the opening factor, F_(02). The curve also models the development of fire without the need for time shifts. It uses a single and relatively simple equation to generate the temperature of both the growth and decay phases of a fire in a building and only three factors are required to derive the curve. These factors are (ⅰ) the maximum gas temperature, (ⅱ) the time at which this maximum temperature occurs, and (ⅲ) a shape constant for the curve. If desired, the shape constant can be different on the growth and the decay sides to model a very wide range of natural fire conditions and test results. This paper presents an overview of the background to the BFD curve. It then illustrates its use in a simple fire engineering design application, where the adequacy of a steel beam is checked using the Eurocode parametric curve and the BFD curve to represent the fire.