CMU Lintel Header Depth

[Nor Cal SE]

I have a warehouse-type building, 8"-CMU reinforced and fully-grouted perimeter walls, and one location of a 12-ft door opening on the wall. I'm looking at the bending capacity of the CMU header. The total depth of CMU from the top of the parapet down to the lintel is approximately 8 feet with no interruptions. I presume there are no issues with assuming this full depth in my 'd' assumption for the CMU header's moment capacity calc. If I proceed this way, my typical 2-#5 bars at the lintel opening have more than enough capacity to resist the bending moment.

A colleague of mine wonders whether it's accurate to assume the full depth for the sake of 'd' in the moment capacity equation, thinking the beam might not act in this manner realistically, though I'm unclear why. Is there any reason to not assume the full 'd' as I'm inclined to do, whether due to ACI 530 code or best engineering practice?

 

[JAE]

If you are fully grouted, with an eight foot tall header, then you really have a deep beam at a 12 ft. span.
This means that your traditional bending checks for positive As reinforcement don't really apply.

You would have to use deep beam theory, or perhaps strut-and-tie methodology, to get an accurate take on it.



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[BAretired]

What is the height of the parapet? Can you make it work without using the parapet? If not, I would suggest providing a lintel from top of door opening to bearing point of roof structure, the reason being that someone may want to remove part or all of the parapet at some time in the future.

BA

 

[XR250]

I would use as much depth as you need, but no more than roof deck elevation to keep the top flange braced. Might be good to add vertical rebar even if it is not required for shear.

 

[sandman21]

CMU Beams are required to have lateral support of the compression face ACI 530 5.2.1.2. Section 5.2.2 covers deep beams which allows either the full depth and all the provisions apply or you can select an alternative beam depth and design conventionally.

 

[Nor Cal SE]

Good points on the compression-face bracing. My parapet extends less than 2-ft above the roof, resulting in a beam depth of at least 6 feet if I neglect parapet. My preliminary calcs are showing adequate capacity even with more theoretical depth reduction.