Q: How are fire ratings of Concrete Masonry assemblies calculated, and for fire code compliance, are listing services such as UL® required?
-Fire Rating Expertise Desired
Section 721 of the IBC recognizes several methods to calculate the fire resistance for CMU walls. The equivalent thickness method is the most commonly used, and the most cost effective. It takes into account extensive industry research, the results of ASTM E-119 testing and aggregate type. Equivalent thickness (ET) is determined in accordance with ASTM C140 and is the solid thickness of the concrete if it were to be recast without the cores. For example, if an 8x8x16 cored unit were to be compacted to fit the same height and length profile, it would have an ET (depth) of 4.04”. Based on the tables in the IBC, for normal weight density, the fire rating would be 1 3/4 hours. If enough light weight aggregate was added to the mix to make it medium weight density, the fire rating would be 2 hours. This is calculated using linear interpolation based on the percent of each aggregate type used in the manufacturing process, as outlined in the IBC. The ET of a 100% solid unit, a solidly grouted unit, or unit with its empty cores filled with a material such as Perlite (an aggregate produced from a volcanic rock), is equal to its actual thickness. For example, an 8” partially reinforced wall with the empty cells filled with Perlite has an ET of 7 5/8”. The fire rating for this assembly would be 4 hours. Listing services such as UL, although not explicitly recognized by the IBC, essentially use this same calculation method to determine their fire ratings. In practical terms, there is little difference in the resulting fire resistance ratings. For specific fire ratings for Jandris CMU click here, and for more information of CMU Fire Ratings, see NCMA TEK 7-01. For a further explanation of UL® and fire ratings, check out this FAQ put out by the National Concrete Masonry Association.
Heidi Jandris is a technical expert and trusted voice of the industry. She part of the family business’s 3rd generation, grew up immersed in all things concrete block and worked as a welder at the plant before getting her BArch at Pratt Institute in Brooklyn NY. She is the Sustainability Manager and provides technical and design services for A. Jandris & Sons.