Many construction professionals,(and unfortunately some less knowledgeable fireproofing contractors), do not understand the modifications that have happened back as early as 2005 regarding changes in the flexural strength of beams being designed into modern buildings and how that relates to fireproofing of the steel structure.  The bottom line is that buildings are now built with lighter steel than they did previously.  As a result, most steel designs are based upon the newer LRFD (Load & Resistance Factor design) instead of the olderASD (Allowable Stress Design) method. These are structural engineering designs that are quite frankly, “above all of our pay grades”.  The bottom line is that the older ASD designs are now considered inadequate in order to maintain current AISI/AISC methods and building code requirements, and would now be considered “load restricted” when restrained assemblies are used.  As a result, back in the end of 2005 (and clarified approx 2016), UL updated their requirements for the fire protection of steel floor beams with ratings greater than 1 hour.

UL now requires only the use of the newer restrained “fully loaded/non load restricted” floor beam designs and corresponding greater thickness’, or fully unrestrained UL designs.  The use of the older“restrained load restricted” UL floor beam designs cannot be used for fire ratings greater than 1 hour without approval from the structural engineer. Our recommendation is that when in doubt- Just specify unrestrained conditions and then there are no questions regarding load restrictions.

The specific issue is the misuse of restrained assembly ratings where unrestrained beam thicknesses of half the restrained assembly rating are submitted. The code (IBC) requires that the beam rating must equal the assembly rating. In other words you cannot have a 2 hour restrained assembly with a 1 hour unrestrained beam.