TMS 402/ASCE 5 - 2013 normalized CMU web area for partitions 1

[Jc67roch]

I am designing several interior cmu partition walls (non load bearing) up to 14 feet in height. 2020 New York Building Code references TMS (The Masonry Society) 402-13/ASCE 5-13 "Building Code Requirements and Specification for Masonry Structures". I am reviewing Chapter 14 - Masonry Partition Walls, and figure I should be able to do these quickly with the prescriptive provisions therein. However I am stumbling over 14.2.3.10 - the cmu shall have a normalized web area of 27 in^2/ft^2 or be grouted solid. I am finding that an 8" cmu has a normalized web area of only 6.5 in^2/ft^2. So it seems that to do an interior partition 14 feet in height, of 8" cmu, it would have to be solidly grouted? What? This doesn't seem right. The commentary adjacent the section explains that this minimum is provided so that web shear stressed don't control the design and doesn't need to be checked. Can anyone provide insight on this? Am I missing an exception?

 

[ProgrammingPE]

A normalized web area of 6.5in²/ft² is a minimum set by C90. It allows the webs to be cut down some and still be acceptable. If your block has standard full height webs, then you will have a higher normalized web area.

In 2011, C90 decreased the minimum web thickness for 8" CMU from 1" to 0.75". For a 1" web thickness, your normalized web area was:

(3 webs/block)*(1"/web)*8" / (16"*8" / (144in²/ft²)) = 27in²/ft²​

I imagine that's where the limit of 27 in²/ft² set by 14.2.3.10 comes from. You would just need to make sure that blocks with 1" web thicknesses are used even though 0.75" is permitted by C90.

If you don't meet the requirements of the prescriptive method, though, then you can always just check the walls using one of the engineered design methods. Once you have a spreadsheet set up to do this, then you can check many interior walls very quickly.

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