Point load distribution through a CMU wall to a footing

[griarch]

When a point load is applied to a typical CMU foundation wall that only has dur-a-wall horizontal reinforcing, how much of the footing captures the load.

For example, If I apply a 10000 pound load on the top of a CMU wall that is 8 feet high, the load spreads outward as it travels down through the wall and is transferred to the substrate or soil by a certain amount of footing. How many linear feet of this footing will capture the load?

 

[larsacious]

It depends on whether your cmu coursing is stack bond or running bond. If it is stack bond then you should treat it as a point load on the footing. If it is running bond then you can use arching action to distribute the point load as a line load.

If you are planning on using arching action be cautious with respect to control joint location since you can not have the arch affect through the control joint.

 

[archeng59]

ACI 530 section 2.1.9 describes the length of wall to be used for calculating compressive stresses due to concentrated loads on the wall laid in a running bond. The length is width of bearing plus four times the thickness of the supporting wall, but not more than the center-to-center distance between the concentrated loads. So for an 8" CMU wall the length of wall supporting the concentrated load is approximately 32". That's how I do it, anyway.

 

[JAE]

archeng59,
That applies ONLY to the wall design itself. The question is how much to put on the footing below.

We use the section you refer to for the wall but assume a maximum 45 degree spread for the footing designs.

 

[archeng59]

I agree that applies to the wall design, but I use it for the footing design also.

 

[cap4000]

The load dipersion angle can vary between 27 and 45 degrees. I normally use 30 degrees for the footing.

 

[archeng59]

I understand the angle issue for load distribution, but IMO the wall cannot transfer more load over an area greater than what is allowed by ACI 530 2.1.9. It is possible that because of the overall vertical dimension from footing to bearing point that when using either the 30 or 45 deg angles, the total length of load distribution from the wall to the footing will exceed the allowable length for the wall design. It is not unconservative IMO to use the length of wall per ACI 530 2.1.9 for the footing design. If the floor or roof framing imposes multiple, closely spaced concentrated loads, then those loads could be applied from the wall to the footing as a uniformly distributed load instead of a series of concentrated loads.

 

[larsacious]

Archeng59,

Does this mean that for a 30' tall x 8" thick cmu wall with joists spaced at 6' o/c you would put a series of spread footings in below the joist bearing locations? Or would you widen the entire strip footing underneath the wall based on the ACI 530 effective width for the design of a wall pier?

 

[JAE]

I think you are confusing the wall design procedures of ACI 530 with good engineering principles. Footings don't care what sort of stability issues a wall might have with concentrated loads.

ACI 530 doesn't apply to footing design in any way.

In reality the footing will see a dispursed load over a very wide area (assuming running bond). I could see 30 degree spread (vs. my 45 degree) but limiting your footings to 4T seems very, very conservative.

 

[a2mfk]

I wouldn't overthink it, most structural engineers and textbooks recommend a 30-45 degree distribution for a point load at the top of a wall (no openings, wall returns, etc), whether its CMU, concrete or tilt-up. You are specifically asking in regards to footing design, the Geotech probably was way conservative on his soil bearing pressure and your point load is likely overestimated, so there is lots of safety factor. Anyone hear of any good footing failure stories? Me neither. But yes, settlement could be an issue....

 

[archeng59]

Continuous strip footing. Check the last sentence of my previous post about using a uniformly distributed load for a footing supporting a wall supporting multiple closely spaced loads. But if I have one concentrated load on the wall, I have no problem widening the strip footing at that load if needed.

 

[BAretired]

If in doubt, provide a bond beam under the load to spread it out as much as you deem necessary. Who's going to argue with you?

BA

 

[dhengr]

Good advice BA. As to your question, 7 or 8 people so far.

 

[shobroco]

Archeng59: 30' tall 8" CMU wall? Holy smokes is all I have to say!

 

[Ron]

For lack of any other defined distribution, 45 deg is a reasonable approach. As others have noted that can vary, but you're not building a watch, so trying to define the load distribution to a greater accuracy is a waste of time.....makes us guilty of measuring with a micrometer, marking with a crayon and cutting with an axe.

 

[a2mfk]

Ron, nicely put, where'd you get that from?

You should have a bond beam at the top of any CMU wall anyway, but I am not sure I count on that to spread the load out much... The stiffness you get from the height of a CMU wall is what is helping you. This is usually more of a concern in lintel design over window and door openings.

I second Shobroco's comment, I'd be way more concerned with the masonry design of a 30ft tall wall, are we talking about laterally unbraced??! A large concentrated axial load with some eccentricity should be causing you all kinds of problems unless we are talking a two story building with a floor bracing at mid-height.

But as far as axial load distribution to a strip footing, I don't give it much thought if the wall is solid (I mean no openings, not solid grouted), and 10ft tall or more, unless the reactions are VERY large, and then you may have a concrete tie column anyway (see below)...

Times to be concerned with widening a strip into a mini-pad footing due to high concentrated loads, in my experience:
-tilt-up panels with small panel legs
-CMU with small piers/jambs created by wall returns, corners, expansion joints, or other openings
-concrete tie column embedded in a CMU wall supporting a large load from a girder (even though there will be load sharing into the adjacent CMU unless you have joints, my gut tells me the tie column will "attract" more axial load so I am conservative there)

HTH..

 

[archeng59]

shobroco, that wasn't my wall example, by the way.