Structural Fire Engineering
Structural Fire Engineering is the application of heat transfer and structural principles to analyze the thermal and structural response of buildings in fire. The Cardington Fire Tests (as well as considerable research conducted over the last 10+ years) demonstrated that failure of one structural element in a fire does not necessarily lead to total structural collapse. Structures usually have some degree of redundancy, meaning that when a structural member is compromised, its load is transferred to other structural elements without catastrophic failure. However, most AHJs in the US are (rightfully) reluctant to accept the idea that failure of a load-bearing beam or column in a building during a fire is “OK” due to inherent structural redundancies. For this reason, most structural fire engineering in the United States is accomplished by designing active and passive fire protection systems to prevent structural members from reaching a critical failure temperature for a specified time interval. Very rarely does structural fire engineering in the US actually involve any structural analysis other than determining expected temperatures in structures exposed to fire. Essentially, structural fire engineering in the US boils down to a heat transfer analysis to determine the maximum temperatures that would result in structural members, usually steel, exposed to fires. If structural redundancies are to be considered, as is sometimes done outside of the US, then the structural fire engineer must also be knowledgeable in structural analysis. Common applications of structural fire engineering include:
- The omission of fireproofing from structural steel. Since prescriptive building codes do not consider unique aspects of buildings, there are some situations where structural fireproofing can be omitted with no reduction in safety. For example, if a structural fire engineer can show that the temperature of an unprotected steel member exposed to fire will not exceed a critical failure temperature, then fireproofing can be safely omitted. This not only reduces cost but can also have positive impacts on building aesthetics.
- Structural fire engineers are sometimes asked to determine the required thickness of spray-applied fire-resistant materials (SFRM) or intumescent coatings. Heat transfer principles can be applied to determine the thickness of SFRM or intumescent coatings required to provide a certain level of fire resistance. SFRM is usually a cementitious material that reduces to rate of heat transfer to steel structures exposed to fire. It is usually only applied to structural members in spaces where aesthetics are not a major concern. Intumescent coatings are more expensive than SFRM but are more aesthetically appealing. They are usually applied to structural members as several layers of paint, sometimes with a final protective gel coat. Intumescent coatings swell under heating, often by a factor of 20 to 50, to produce a low-density porous residue that insulates structural members from the heat of a fire. Determining the appropriate type of fire protection and the required thickness is an important aspect of structural fire engineering.
- Determination of equivalent fire resistance ratings. In the US, fire resistance ratings are determined by the ASTM E119 furnace test, often at a cost of $10,000 to $20,000 per assembly. Under prescriptive codes, slight variations of the same basic assembly must be separately tested in ASTM E119 to determine its fire resistance rating. For example, a concrete assembly with two different sizes of rebar would be considered two different assemblies under prescriptive codes; therefore, it would be necessary to test each assembly separately in the ASTM E119 furnace. From a practical standpoint, this adds unnecessary cost since a structural fire engineer can apply basic heat transfer principles to determine the expected fire resistance rating of a variation of an assembly having a known fire resistance rating. AHJs may accept this type of structural fire engineering analysis in lieu of ASTM E119 testing if the analysis is stamped by a licensed Fire Protection Engineer.