The answer depends on what, if anything, is in the cells that are not grouted. If the ungrouted cells are fully filled with one of the code-approved materials, then the equivalent thickness of the wall is equal to the actual unit thickness. Refer to FAQ “Can the fire resistance rating of an existing, single wythe, hollow concrete masonry wall be improved by filling the cells?” for a list of the code-approved fill materials and an example.
If the ungrouted cells are void, or are not completely filled with one of the code-approved materials, then the fire resistance rating is calculated in the same way as for an ungrouted single wythe concrete masonry wall. That is, the effect of the grouted cells is neglected. Refer to the FAQ “How does one determine the fire resistance rating of an existing, single wythe, hollow concrete masonry wall?” for a description of the procedure and an example.
- 2012 International Building Code (IBC), International Code Council, Section 7184.108.40.206
- Code Requirements for Determining Fire Resistance of Concrete and Masonry Construction Assemblies, ACI 216.1-07/TMS 0216-07, Section 3.2.1
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