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Calculated Fire Resistance Ratings for Masonry

Clay Masonry

Test results of clay masonry wall elements per ASTM E119

Plaster Finishes:

The fireresistance rating of sanded gypsum plastered clay masonry walls shall be determined by the following formula:

R = (R_n^0.59 + Pl)^1.7

R = fireresistance rating of assembly, hours

R_n= fireresistance period of individual wythe, hours

Pl = thickness coefficient of sanded gypsum plaster (see table)

Concrete Masonry

While ASTM E119 test data is often available for several of the most common concrete masonry units, the calculated equivalent thickness method is usually used. This is because concrete masonry units can be made with any combination of aggregates and volumetric densities. An 8" block which is 72% solid containing limestone aggregate will have a higher rating than a 54% solid expanded slag unit of the same thickness. 

We will look at the determination of CMU Fire ratings in two steps:

Step One - Determining equivalent thickness

Equivalent thickness is the effective fire resistive thickness of a hollow unit. It is determined by multiplying the thickness of the unit by the percent solid of the unit.

For Hollow ungrouted units:

T_e = T_{n *} P

T_e = Equivalent Thickness

T_n = Nominal thickness of unit

 P = Percent solid

For solid grouted construction the equivalent thickness shall be the taken as the specified thickness of the unit

Finishes:

The equivalent thickness of portland cement plaster applied over concrete masonry shall equal the specified thickness of the plaster finish. This thickness can then be added to the equivalent thickness of the unit, thus increasing its fire rating. The equivalent thickness of gypsum wallboard or gypsum sand plaster applied over concrete shall be equal to 1 1/2 times the specified thickness of the finish.

Step Two - Identify aggregate type and determine rating

Given the aggregate type for the unit in question, use the chart to find the minimum equivalent thickness necessary to achieve the indicated rating.