Thread: Corbelled Concrete Block

I understand that standard brick should not be corbelled more than 1", but I have not seen any documentation as to allowable corbelling of concrete block. This photo is of the first (bottom) course of a block wall on the concrete foundation. The block looks to be overlapping the edge of the foundation by about an inch or so. Unfortunately this photo was from the customer and I did not have a chance to see, measure or photograph this first hand. It appears to be standard 8X16 block. I measured the wall thickness of a standard big box store concrete block to be 1-3/8. Standard brick is allowed to be corbelled by 1" or 1/3 the witdth. Brick is obviously not the same width and is basically solid core. The block has a hollow core, so not clear as what rules apply. Anyone have any "concrete" knowledge (sorry couldn't resist) or documentation? If not, any opinions?

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Concrete block must have the face shells fully mortared. That is a requirement.

Thus, hollow concrete block is not allowed to be corbelled out, the face shell must be fully supported.

When I first found that condition many years ago, the engineers came up with all kinds of ways to support the face shells (some face shells were entirely over the edge of the slab and I could stick my hand up inside the block cells).

Then while talking with an engineer one day, the easy solution appeared during the discussion: fill the first two courses full of concrete. That turned the hollow block into the equivalent of solid block, and the face shells no longer carried the entire load of the wall above, the entire filled blocks carried that load, and transferred that load back to the slab.

Ever since then, the easy solution is to fill the bottom two courses of block.

Solid block or filled hollow block is allowed to be corbelled out.

Thats great information, thank you Jerry. Do you know where the requirement that the face shells be fully mortared comes from? Is that a manufacturer's requirement or a code requirement or other source? I would like to have documentation on hand for the client and builder.

Originally Posted by Matthew Barnicle

Thats great information, thank you Jerry. Do you know where the requirement that the face shells be fully mortared comes from? Is that a manufacturer's requirement or a code requirement or other source? I would like to have documentation on hand for the client and builder.

ACI 530.

ACI 530/530.1-11 Building Code Requirements and Specification for Masonry Structures and Related Commentaries

Don't overlook the simpler requirement of the IBC R606.2.4 re "masonry change in thickness" and IBC R606.3 re "corbeled masonry". IBC R606.3.1 specifically requires "solid masonry units or masonry filled with mortar or grout".
I encounter this most often where a builder has laid a standard 12" CMU foundation and as it comes up above grade, switched to 8" CMU with 4" of brick veneer. If discovered during construction, the local government Bldg Inspector should not allow it unless the top course of the 12" CMU is filled solid.
The reasoning is actually simply and intelligent. Draw a 12" CMU looking down from above, and then overlay an 8" CMU on it with the back (inside faces aligned) and one can easily see that the outer face of the narrower CMU bears only on the cross bridges of the lower CMU, thus reducing the bearing surface to about 1" square every 8" (once some block get cut and the horizontal coursing no longer lines up). Since that face of the 8" CMU may well carry half the load of the wall (with the inner face carrying the other half), it's easy to see how, in a a fully loaded wall, perhaps two stories with a roof load, could easily exert enough load downward on that square inch to potentially crush the crossing point of the face and the webs below, eventually allowing the wall to start to lean outward.