Load Combination Requirements for Fire-resistant Structural Design

Abstract

Fire protection of building structural systems traditionally has relied on component qualification testing, with acceptance criteria based on component survival during a “standard” fire for a prescribed rating period. These test procedures do not address the impact of the fire on a structural system. With advances in fire science and the advent of advanced structural analysis, the routine use of the computer as a design tool and limit states design, it is becoming possible to consider realistic fire scenarios and effects explicitly as part of the structural design process. In this modern engineering design approach, load requirements for considering structural actions due to fire in combination with other loads are essential, but have yet to be implemented in standards and codes in the United States. This paper provides a probabilistic basis for appropriate combinations of loads to facilitate fire-resistant structural design and recommends specific load combinations for this purpose. The probabilistic basis is essential for measuring compliance with performance objectives, for comparing alternatives, and for making the role of uncertainty in the decision process transparent.

Reliability Analysis of RC Beams Exposed to Fire

“…This perhaps suggests further consideration of the need for a fire load combination in design, similar to that presented by Ellingwood (2005) or Section C2.0 of ASCE 7.…”

“…Based on an analysis of the frequency of occurrence of these loads relative to that of a structurally significant fire, it can be determined that some load combinations with fire can be practically neglected for calculation of reliability indices β that are at or below typical code targets (approximately when β ≤ 3.5 -4). By examining the coincidence rates of various extreme loads (in the United States) with a structurally significant fire, Ellingwood (2005) …”

“…Based on an analysis of load frequency, Ellingwood (2005) summarized practical design load combinations that need to be considered for fire design.…”

Reliability analysis of reinforced concrete columns exposed to fire

“…However, Ellingwood examined the coincidence rates of the extremes of these loads in the United States with a structurally significant fire [19], and determined that many of these load combinations can be practically neglected when considering reliability indices β that are close to or below typical code target levels (i.e. approximately for β ≤ 3.5 -4.0).…”

“…Some of this research includes Beck [14] and Teixeira and Soares [15], who modeled the reliability of steel members and plates subjected to fire; Vaidogas and Juocevicius [16] as well as Hietaniemi [17], who conducted probabilistic analyses of timber components exposed to fire, and Shetty et al [18] who assessed the reliability of offshore structures under fire load. Other recent contributions include Ellingwood [19], who developed relevant design load combinations for fire design based on an analysis of load frequency, and Au et al [20], who used subset simulation to assess fire risk analysis in a compartment.…”