Point load on a prescriptive residential one story foundation wall 1

[FCDG]

Is there a "rule of thumb" for determining when a point load on a one story prescriptive foundation requires design? I have generally thought of the number being 1.5 - 2 kips. I am gathering consensus since I cannot find anything definite from other reliable sources. This condition happens frequently in residential construction and plans examiners are not in agreement to the point of requiring a calc for all point loads. I'm sure liability issues have pushed this to a point beyond common sense but I am interested what other professionals think.

 

[milkshakelake]

If you draw a 45 degrees triangle from the application point of the point load and spread it onto the footings + uniform load from other elements, and the footings don't exceed bearing capacity, it should be okay. The reasoning from this is derived somewhat from how point loads are applied on masonry, but neglecting the maximum width of that point load converted to a uniform load, because presumably you'd be using concrete.

The real behavior of a foundation wall + footings on soil is the foundation wall acting as a long beam over Winkler springs (that in itself is another simplification, but whatever). This is onerous to calculate for a one story. So if we assume the soil to be rigid (which has been done for a long time), it makes intuitive sense to me that the load would follow something like a 45 degrees angle, because a similar assumption is made in masonry.

The concrete bearing stress will never be reached for a 1-story. So there's no limit on the concentrated load from there. The axial stress will also never be reached, even if you assume a column inside the wall instead of a triangular distribution. Your lateral moment (earth pressure) is not affected, and we're not going to get into things like P-M interaction and P-delta effects here for the vast majority of loads you'll see on a 1-story.

Taking all this into account, let's say you have a 9' high foundation wall with a soil bearing capacity of 2 ksf. 2 ksf * 18' * 1' = 32 kips if there's no footing, 64 kips if there is a 2' wide footing. Since you'll have a bit of uniform load, make it 20 kips for no footing and 40 kips with a footing. I don't design anything without a footing, so that makes your concentrated load capacity (with no extra calculations needed) to be 40 kips.

This is a common sense approach from engineer to engineer, not from a liability perspective. If liability comes into play, you should be able to support it with calculations. Maybe do it once so you feel comfortable, and have a standard conservative calculation showing that it's good, and you're all set for 99% of concentrated loads that can come from a light 1 story building (I'm not talking about large ones with big moment frames, that's a different game entirely).

Other considerations:
-Moments. If there's a moment at the bottom of the concentrated load, this reasoning goes out the window. You'll need another set of assumptions to deal with that, but it's very much possible, and dare I say, easy.
-Uplift. Same issue as moments.

 

[jerseyshore]

MSL put together a great summary. Typically I'm only worried about point loads in a more local sense. Crushing of a rim joist/ floor joist/ blocking, including a high load on an unreinforced block wall. Typically for an semi large load I'll specify to fill the top two courses solid with grout. If it's going to be a really large load I'll check the bearing stress.

 

[milkshakelake]

OH yeah, agreed with jerseyshore about the structural elements that are on the foundation itself. Also, I'm not worried about concrete, but the bearing stress would need to be considered if you have unreinforced masonry with the 0.33f'm rule (if I recall that correctly). Let's say you have a 3.5"x3.5" post. 3.5" x 3.5" x 1500psi x 0.33 = 6064#, which is quite low and might potentially exceed the capacity, with 6 kips being realistic for a 4x4 post in a one story. So the answer is completely different in that case. But then again, you wouldn't have an unreinforced CMU foundation wall in general.

 

[FCDG]

As I read your posts, I would like to thank both of you. More importantly it showed me that my lack of information made the question extremely generic and local. Here in the Pacific Northwest we have a 12" frost line and so almost never have basement walls unless gradation is a factor. In an effort to give better information, I have added the following:


Footing: 6x12 Solid Pour
Stemwall: 6x24 Solid Pour
Reinforcement: #4 bars T&B, #4 verts 48 o.c.
Assumed wall length: > 6 ft.
Soil Bearing Pressure: 1500 psf

I understand that I can still use your formula drawing the triangle (Milksahkelake) but you can see the hypotenuse has changed considerably. This is typically generated by a girder truss at 4 ft from corners and frequently in the range I originally suggested (1500-2000).

Sorry for the sketchy details in the first post. Thanks to those that posted

 

[milkshakelake]

Sure, no problem supplying enough info. Like you pointed out, you can still use the same assumptions. It's very generic and will work with your circumstances, just tweaked a bit. In your case, the concrete bearing stress becomes even less of a concern due to being more limited by the soil bearing pressure. Good luck!