CMU Walls - Height Limitation Per Code??

[jpw2913]

Is there a code section (IBC Chapter 21, ACI 530) that specifically gives height to thickness ratios. I have a contractor that changed from 6" cmu to 8" cmu without consulting anyone because the owner raised the wall height from approximately 11' tall walls to 13' tall walls. The reason he gave for switching from 6" to 8" is simply that the code requires it. I don't know which code he looked at or if he just pulled that out of his hat. Just for some specifics...these are only interior load bearing walls so no wind loading. K-series joists are attached to the top of the walls with a metal deck and slab above. The walls don't continue up through the 2nd floor. The walls only carry the one floor above. These walls and 2nd floor were constructed entirely within an existing building. Thanks for any help.

 

[sponton]

There aren't limitations explicitly but there are penalties when you have walls that have large h/r ratios and have a limited strength based on the buckling load ( 1/4 Pe), I would consult the TMS 402-11/ACI 530-11 for masonry information but as I said, there's nothing explicitly stated, all comes in strength reductions. Anyways, in my opinion at least he erred in the safe side, I'd much rather have him augment the size of the block than continue the wall as it is, which would have bad consequences assuming i wasn't conservative with my design.

 

[EngineeringEric]

There are prescriptive heights in residential construction, im not sure about IBC, not handy at the moment.


TABLE R606.9 SPACING OF LATERAL SUPPORT FOR MASONRY WALLS CONSTRUCTION
-MAXIMUM WALL LENGTH TO THICKNESS OR WALL HEIGHT TO THICKNESS
Bearing walls:
Solid or solid grouted - 20
All other - 18
Nonbearing walls:
Exterior - 18
Interior -36

For nonimal block, bearing, solid:
6" => 10'
8" => 13.33'

Maybe that is what the contractor is referring too? i don't really like an 11' tall cmu bearing wall made out of 6" anyways

 

[Guastavino]

jpw2913,

If I were you I would just email (therefore it's in writing) ask the contractor what section of the code he is referring to. It's possible he heard that somewhere like EngineeringEric pointed out and misapplied a portion of the code. In other words, if I were to use the strength chapter of the MSJC along with ASD load combinations, I could show you portions of the code to back up what I was saying, but would miss the point and intent of the code completely.

With that said, at least he thought enough about it to upsize the wall. Granted, he should have asked what to do.

 

[wannabeSE]

For empirical design (not engineered with asd or strength design) the max h/t ratio is in ACI 530-11 §5.5.

 

[manstrom]

+1. There is an empirical limit in ACI 531, but no engineered limit.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller