Masonry (CMU) fire resistance

For CMU walls – in IBC table 720.1(2) and table 721.3.2 you can calcuate the fire resistance of CMU walls. Here’s a cheat sheet. Note that your actual fire resistance may be higher – these are minimum values, based on calcareous or siliceous gravel aggregate:

Hollow CMU units

16″ = 6″ equivelent thickness = 3.75 hour
14″ = 5.5″ equivelent thickness = 3.25 hour
12″ = 5.1″ equivelent thickness = 2.75 hour
10″ = 4.5″ equivelent thickness = 2.25 hour
8″    = 4.0″ equivelent thickness = 1.75 hour
6″   = 3.1″ equivelent thickness = 1.00 hour
4″   = 2.7″ equivelent thickness = 0.75 hour

Fully grouted CMU units

16″ = 15.6″ equivelent thickness = 4.00 hour
14″ = 13.6″ equivelent thickness = 4.00 hour
12″ = 11.6″ equivelent thickness = 4.00 hour
10″ = 9.6″ equivelent thickness = 4.00 hour
8″    = 7.6″ equivelent thickness = 4.00 hour
6″   = 5.6″ equivelent thickness = 3.25 hour
4″   = 3.6″ equivelent thickness = 1.50 hour

Note that siliceous aggregate is common in the Seattle area.



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