Abstract
Evaluation of structural fire protection may be considered within two distinct domains: the time domain and the strength domain. Time is conventionally used for standard fire resistance requirements in building codes, in which a particular fire resistance assembly is shown to provide adequate resistance to a standard fire exposure under test conditions for a period of time. Strength compares applied gravity loads and fire effects (e.g., thermally induced forces) to the capacity (e.g., temperature-dependent strength) of the structural system.
In the context of structural engineering, the fundamental philosophy used to design structures can be simply expressed as capacity > demand. The demand refers to loads that are imposed on a structure including its self-weight. The capacity refers to the global and local ability of a structure to carry an imposed demand. The conventional design of a structural system evaluates demand and capacity with respect to specific performance objectives including strength, stability, and stiffness.
Evaluation of structural fire protection within the time domain primarily considers the demand on the structure due to heating, which can be reduced with the application of protective insulation. However, this type of evaluation does not necessarily credit nor discount the capacity of the structure itself to endure fire exposure nor does it properly evaluate all aspects of the demand (e.g., thermally induced forces; see Sect.
2.1.4). Also, the consideration of demand due to heating is not with respect to specific performance objectives, but rather superficial failure criteria enforced during standard fire testing.
Evaluation of structural fire protection within the strength domain accounts for all contributions of the demand on a structural system, as well as its capacity to endure these demands with respect to required performance objectives.
There is no practical method for a designer to quantitatively compare the level of safety provided by an analysis conducted within the time domain compared to one conducted with the strength domain. Hence, the designer should adopt one or the other exclusively for a given analysis.