Engineering FAQs


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