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11.4. Fire Ratings
11.4.1. Are there any structural systems that should be avoided when designing for an area of a building that contains a large fire load?
Building and fire codes usually restrict excessive fire loads or impose restrictions on the height and area of buildings with large fire loads. These restrictions are especially severe for buildings of combustible construction. In some instances, combustible construction is not permitted for high-hazard occupancies. Even for buildings that are deemed to have a minimal fire load, such as an office building, codes apply limits on the allowable construction types and increase the fire-resistance rating requirements as the building height and area is increased.
11.4.2. What are the general rules-of-thumb for interpreting model building codes when it comes to fire ratings in mixed-use buildings?
Building codes have special requirements for buildings that house more than one occupancy group. Where different occupancies are separated by fire walls, fire barrier walls, and/or floors, each portion of the building may be considered as a separate building in establishing allowable heights and areas and fire resistance requirements (some conditions/exemptions apply). Without this degree of fire separation, a building of mixed occupancy is limited by the most restrictive height and area requirements specified for any of the occupancies in the building. (again, there are several exemptions). In establishing the fire resistance requirements for non-separated uses, the regulations applicable to mixed occupancies govern the respective portions of the buildings, but where the requirements conflict, those that provide greater safety would prevail.
A building or system with a 2-hour fire rating means that the system has satisfied the requirements for a 2-hour rating specified in a relevant standard test. In the case of a building, it could mean that some of the construction elements and/or assemblies in that building have achieved a 2-hour rating in a standard ASTM E119 fire resistance test.
No, a 2-hour fire rating does not mean that a building will only last for 2 hours in a fire. The ratings relate only to the ability of individual components and assemblies in a building to meet the required performance in the standard test.
Buildings are classified by types of construction, each with specific requirements pertaining to the combustibility of construction materials and the fire-resistance ratings required for the various building components (members, elements) and assemblies (systems). In some fire events, the performance may be in excess of two hours. In a severe challenge fire (usually, an extreme event that is not anticipated), the performance may be less than 2 hours.
Specifications/methods to determine fire resistance ratings for generic designs can be found in most building codes, e.g.,
- Chapter 7, “Fire and Smoke Protection Features", International Building Code, International Code Council 2021
- Chapter 8, “Features of Fire Protection”, NFPA 5000, Building Construction and Safety Code, NFPA
The following ASCE Standard also specifies similar methods to determine fire resistance ratings for generic construction.
- ASCE/SFPE 29-05, Standard Calculation Methods for Structural Fire Protection, Structural Engineering Institute of the American Society of Civil Engineers
Fire resistance ratings for proprietary designs could be found in special directories published by respective testing laboratories, e.g.,
Restrained and unrestrained classifications pertain to ASTM E119 tests on beams, floors, and roofs, and depend on whether the test arrangements allowed. The unrestrained test allows for the free thermal expansion of the tested specimen. The restrained test does not allow for free thermal expansion of the tested specimen.
Unloaded Structural Steel Beams ASTM E119 Test
ASTM E119 test on unloaded structural steel (and composite steel/concrete) beams could be restrained or unrestrained, but always requires the longitudinal expansion of the applied fire protection material to be restrained. This conservative requirement can result in earlier fall-off of the fire protection and faster heating of the tested steel beam.
This test results in a single Unrestrained Beam Rating based on the period of fire exposure where the average measured temperature at any section of the steel beam remains under 1000°F and the measured temperature at any single location of the steel beam remains under 1200°F. This type of test is rarely conducted, usually only when the loading device has lower capacity than the required test load.
Loaded Structural Steel Beams ASTM E119 Test results in a Restrained or Unrestrained Beam Rating
ASTM E119 tests on loaded structural steel (and composite steel/concrete) beams are always restrained and result in 2 ratings:
- Restrained Beam Rating based on the period of fire exposure where the beam sustains the applied design load, but not more than twice the corresponding Unrestrained Beam Rating, and provided the latter is 1 hour or more; and
- Unrestrained Beam Rating based on the period of fire exposure where the average measured temperature at any section of the steel beam remains under 1100°F and the measured temperature at any single location of the steel beam remains under 1300°F.
Loaded Floor and Roof Assemblies ASTM E119 Test results in a Unrestrained Beam Rating in addition to Assembly Ratings
ASTM E119 tests on floor and roof assemblies are always loaded. The assemblies could be tested in the unrestrained condition or in the restrained condition all around the floor/roof perimeter.
- Whenever the tested floor/roof assembly contains a structural steel beam, both restrained and unrestrained assembly tests will result in an Unrestrained Beam Rating (based on the same temperature criteria specified for loaded restrained beam tests) in addition to Assembly Ratings.
- For any Assembly Rating period, the unexposed surface of the tested floor/roof should neither develop conditions that will ignite cotton waste, nor exhibit an average temperature rise in excess of 250°F.
- An unrestrained assembly test will result in an Unrestrained Assembly Rating based on the period of fire exposure where the assembly sustains the applied design load.
- A restrained assembly test will result in 2 assembly ratings:
- Restrained Assembly Rating based on the period of fire exposure where the assembly sustains the applied design load, but not more than twice the corresponding Unrestrained Assembly Rating, and provided the later is 1 hour or more; and
- Unrestrained Assembly Rating based on the same temperature criteria specified for Unrestrained Beam Rating, except for steel structural members spaced 4 ft or less on center, where the criterion for the average measured temperature of all such members remaining under 1100°F applies.
Appendix X3 and Table X3.1 of ASTM E119 provide guidance on the classification of beams, floor, and roof systems as restrained or unrestrained. Structural steel beams and floor systems within steel-framed buildings are classified as restrained. It is overly conservative to assume structural steel beams and floor systems within steel-framed buildings are unrestrained, and this assumption will increase the cost of fire protection in a project. Appendix X3 and Table X3.1 of ASTM E119 provide guidance on the classification of beams, floor, and roof systems as restrained or unrestrained.
11.4.8. How can one determine a fire rating for a system that has not been prequalified, such as a concrete-encased steel column?
Concrete-encased steel columns have been "pre-qualified" by many fire tests. These columns are of generic design (non-proprietary); therefore, they are not listed in the UL directory.
However, most building codes, e.g., IBC 2021 (Table 721.1(1) and Article 7184.108.40.206), and ASCE/SFPE 29-99 (Article 5.2.4) contain formulas/specifications to determine the fire resistance of concrete-encased columns. These formulas/specifications are based on extensive experimental data from standard (ASTM E119) fire resistance tests.
Concrete-filled HSS columns are another example of generic construction that has not been listed in the UL directory. Article 5.2.3 of ASCE/SFPE 29-99 specifies a simple method to determine the fire resistance of concrete-filled hollow steel columns.
V. K. R. Kodur, and D. H. MacKinnon, "Design of Concrete-Filled Hollow Structural Steel Columns for Fire Endurance", Engineering Journal, First Quarter, 2000, pp. 13-24.