How Fire Rated Assemblies Affect Structural Engineering
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It’s often expected that the duty to fulfill fire rating requirements falls on the Architect or Code Consultant. However, there are certain instances where the Structural Engineer should be consulted to ensure that the approach selected is applicable and/or does not interfere with the structural design. The intent of this article is to look at some of those, including instances where:
the assembly performance differs for load-bearing and non-load-bearing walls;
the assembly is a load-restricted design; or
the assembly performance differs for restrained and unrestrained assemblies and beams.
The Assembly Works for Non-Load Bearing Walls Only
Some assemblies have reduced fire ratings if they are load-bearing. What does this mean? If a wall is load bearing, it may be supporting roof or floor structure from above. For example, National Building Code of Canada (NBC) assembly W2d is a common wood-framed assembly with 2 layers of 5/8” Type “X” gypsum board on each side. If it is non-load-bearing, it has a 2 hour fire rating. However, if it is load-bearing, it has a reduced fire rating of 1.5 hours. In this instance, it is important that the fire rated assembly aligns with how the building works structurally to ensure that the assembly selected achieves the required fire rating.
Similar assemblies also exist within the UL assembly library, such a Design No. U357, with a 2 hour fire rating as a non-load-bearing wall and a reduce fire rating of 1 hour as a load-bearing wall.
Always check assembly documentation to verify if there is a reduced rating for the assembly if it is load-bearing. If there is a reduced rating, confirm that the components comprising the assembly are not load-bearing. If the assembly includes load-bearing components, then you may need to select an alternate assembly to obtain the required fire rating.
The Assembly is a Load-Restricted Design
The NBC prescribes use of Limit States Design (LSD) for structural design. Alternatively, some UL designs have been evaluated based on Working Stress Design (WSD). The difference between WSD and LSD is that WSD remains within elastic limits, whereas LSD allows for exceeding elastic limits and entering plastic limits. An assembly that has been evaluated based on WSD may have been evaluated at a loading less than the maximum loading determined using the LSD. In this instance, it may not have been verified how the assembly would perform at the maximum loading determined using LSD.
If a UL design is based on WSD but the structural design is based on LSD, then the UL design may be load-restricted. This means that the UL design can still be used, but only if the structure is determined to perform with a load capacity reduction applied to it. The intent of the load restriction is to ensure that loading remains within evaluated limits of the assembly.
Read the UL listing closely and look for phrasing such as “Load Restricted for Canadian Applications.” If such a warning is present, read the rest of the listing and refer to any applicable reference documentation. For each structural member or component included within the assembly, confirm with the Structural Engineer if there is an applicable load restricted factor. If there is, the structural design must be revised to incorporate the load restriction factor(s). Depending on the material type and the value of the load restriction factor, modifications to the structural design may be required.
Thermally Restrained Assembly Ratings
Some UL listings will identify different fire ratings for restrained and unrestrained assemblies. This is another condition that should be sought out in the UL listing and confirmed with the Structural Engineer.
From NBC D-1.12.1, “A restrained condition is one in which expansion or rotation at the supports of a load-carrying element resulting from the effects of fire is resisted by forces or moments external to the element. An unrestrained condition is one in which the load-carrying element is free to thermally expand and rotate at its supports.” Increased temperature may cause thermal expansion, which may cause damage to the structure if unrestrained. For this reason, unrestrained assembly ratings are generally less than or equal to restrained assembly ratings. If the restraint condition of a floor or roof assembly is unknown, it is acceptable to use the unrestrained assembly rating.
Restrained assemblies may be assemblies that are designed with continuity or that are otherwise restrained against thermal expansion. An example of a restrained structure is steel beams supporting a continuous concrete slab complete with shear studs. Steel beams are also generally considered to be restrained if they are welded or bolted to framing.
Unrestrained assemblies may be assemblies that are simply supported and unrestrained against thermal expansion. An example of an unrestrained structure is steel beams supporting precast concrete units. Wood framing is also generally considered to be unrestrained.
For more information, check out these references:
CAN/ULC S101-14 “Standard Method of Fire Endurance Tests of Building Construction and Materials”
UL 263 “Standard for Fire Tests of Building Construction and Materials”