AISC


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.

Last modified 25 March 2003

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.

last modified 25 March 2003

11.4.3. When a building or system is said to have a 2-hour fire rating, what does that mean?

It 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.

11.4.4. Does a 2-hour fire rating mean that a building will last for 2-hours in a fire?

No. 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.

last modified 25 March 2003

11.4.5. Where can one find fire ratings?

Specifications/methods to determine fire resistance ratings for generic designs can be found in most building codes, e.g.,

  • Chapter 7, “Fire-Resistance-Rated Construction", International Building Code, International Code Council, Falls Church, VA, (2003).
  • Chapter 8, “Fire Resistive Materials and Construction”, NFPA 5000, Building Construction and Safety Code, NFPA, Quincy, MA (2006).

The following ASCE Standard also specifies similar methods to determine fire resistance ratings for generic construction.

    • ASCE/SFPE 29-99, Standard Calculation Methods for Structural Fire Protection, Structural Engineering Institute of the American Society of Civil Engineers, Reston, VA, (1998).

Fire resistance ratings for proprietary designs could be found in special directories published by respective testing laboratories, e.g.,

  • Fire Resistance Directory, Volume I, Underwriters Laboratories Inc., Northbrook, IL, (2002, Updated annually), (online directory atwww.ul.com).
  • Directory of Listed Products, Intertek Testing Services NA Inc., Cortland, NY, (2005), (online directory at www.intertek-etlsmeko.com).

last modified 1 January 2006

11.4.6. What is the difference between “Restrained” and “Unrestrained” ratings?

Restrained and Unrestrained classifications pertain to ASTM E119 tests on beams, floors and roofs, and depend on whether the test arrangements allowed for the free thermal expansion of the tested specimen (unrestrained test) or not (restrained 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.
  • ASTM E119 tests on loaded structural steel (and composite steel/concrete) beams are always restrained and result in 2 ratings: 1) 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 later is 1 hour or more; and 2) 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.
  • 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: 1) 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 2) 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.

last modified 25 March 2003

11.4.7. Are structural steel systems restrained or unrestrained?

last modified 25 March 2003

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 2003 (Table 720.1(1) and Article 720.5.1.4), 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. The relevant background information could be found in 

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.

last modified 1 January 2006