Steel Buildings in Europe

Part 6: Fire Engineering 6 - 29 4.2 Heat transfer This Section explains the fire actions and the evolution of the temperature in structural members under a standard fire exposure, with an emphasis on the concepts of standard fire and section factor. Section factor A m / V An important parameter in the rate at which the temperature of a member increases is the ratio A m / V of the member, commonly known as the section factor. The section factor is defined as the ratio of the exposed surface area of a member to its volume per unit length. The influence of the section factor is shown in Figure 4.1(b) for protected and unprotected members. A larger section factor leads to a faster heating of the member. For example, after 15 minutes of fire exposure, the temperature of an unprotected member with a section factor of A m / V = 200 increases to about 580  C, while that of the unprotected member with A m /V = 100 only reaches 380  C. This difference is due to the fact that a large value of the section factor represents a large exposed surface area compared with its volume, and therefore, the member receives more heat than that with a low section factor, which represents a small exposed surface area. This is illustrated in Figure 4.4. Figure 4.4 Definition of section factor A m / V of a member in fire Member temperature at time t The temperature θ a,t of the member at time t can be calculated using the simple models given in §4.2.5 of EN 1993-1-2 or §4.3.4.2 of EN 1994-1-2. If the critical temperature exceeds the design temperature, i.e . θ cr > θ a,t then the fire resistance of the unprotected member is adequate for that duration.