One of the most critical issues in the fire protection industry today is the topic of Restrained vs. Unrestrained assemblies. It is important because misunderstanding of this topic can lead to inadequate fire protection and premature failure of the protected structural steel framed building. Many are unaware that the fire test procedure used to design passive fire protection for structural steel recognizes two different forms of end support for beams tested in roof and floor assemblies. The ends of the beams can be tested in a fully restrained or fully unrestrained condition.
From a structural engineer’s point of view, the most common forms of steel framed construction are considered structurally restrained to some degree, and certainly at ambient temperatures, this is correct. The main question, however, is whether the same assembly is Thermally Restrained at elevated temperatures as defined by Underwriters Laboratories Inc (UL). In other words, will the structure remain restrained and be able to support the design loads as the steel temperature approaches 1100°F. When worded in this fashion, few architects or engineers are able to answer this question with an affirmative yes.
In a truly restrained assembly, the structure surrounding the floor beams (primary framing members and adjacent floor areas), will be able to resist the thrust placed upon them caused by the thermal expansion of the floor beams. Since the beams in a restrained assembly, have a rating less than the assembly rating itself, they will start to fail in a particular mode that still allows the structure to support the design load, as well as prevent the passage of heat to the unexposed surface of the floor or roof.
An assembly that is Structurally Restrained at ambient temperatures, but not Thermally Restrained under fire conditions, may not be able to support the design load of the structure for the required period of time, or may transfer too much heat to the unexposed surface of the assembly. To understand the difference between Restrained and Unrestrained assemblies, you must first understand a little about the role of spray applied fireproofing, how it works and how it is tested.
Obviously, structural steel is a non-combustible material; however, the high-sustained temperatures of a fire can severely damage unprotected steel. Structural steel will lose approximately 50% of its load carrying capacity as temperatures approach 1100°F. Fireproofing works by encasing the steel and insulating it, keeping the steel temperature below the point where design strength is compromised. In order to determine the amount of fireproofing required in to achieve this goal, UL tests fireproofing products in accordance with ASTM E119 (UL 263). The results of the test are published in the UL Directory, which specifies the thickness and density of the material, as well as how the assembly is to be constructed in order to achieve various levels of hourly rating.
It is important to note that the ASTM E119 test procedure was developed in 1918, and although construction types and building contents have changed over the years, with the exception of a few minor refinements, the basic ASTM E119 overall test criteria has not changed. ASTM E119 is a standardized test method, which provides a means by which the performance of one material can be compared with that of another, under very defined conditions.
Many designers are under the impression that if an assembly on a project site is sprayed in accordance with a UL tested two-hour assembly, that the structure will remain intact for two hours, sustaining little or no damage. This is not necessarily true. How well an actual structure performs relative to the design assembly greatly depends on how closely the fire mimics the ASTM test standard, the actual condition of restraint, and the amount of fireproofing actually applied to the steel. Unfortunately, the amount of fireproofing required to properly protect a building is the subject of some debate.
