Two Hour Assembly Ul Number
We have a local builder wishing to use floor trusses with a 2-hour assembly. I have shown him the 2-hour design on pages 17-27 of the second edition of SBCA’s Metal Plate Connected Wood Truss Handbook. Is this assembly UL approved? If so, what is the design number?
This two-hour assembly does not have a UL number. However, it is identical to UL L538 with the exception that L538 uses I-joists instead of wood trusses. Both of these assemblies were developed using the Component Additive Method (CAM), which adds up the known fire resistance of each of the assembly elements to determine the resistance of the whole assembly.
The UL L538 report does not explicitly state that it is a calculated assembly, but generally when finish ratings are given in broad terms (e.g. more than 90 min.) that indicates a calculated assembly rather than a tested one. Tested assemblies usually list a specific finish rating (e.g., UL L537 lists 21 minutes).
For your information, there is another two-hour assembly available (FR-SYSTEM 1TM, FR-SYSTEM 2TM, AND FR SYSTEM 3 TM, FIRE-RESISTANCE-RATED FLOOR-CEILING AND ROOF-CEILING ASSEMBLIES WITH WOOD TRUSSES.) It was donated to SBCA by Alpine and it uses the FR Quik metal channel for blocking at all drywall edges. For details on this and other rated assemblies using trusses, see SBCA Research Report 1509-01: Fire Resistance Rated Truss Assemblies.