Load Distribution from CMU Wall on Footing

[ACivilLife]

Hello everyone

If you have a wall footing supporting a CMU wall, how do you treat concentrated load distribution on the wall to get loading on the footing? I know TMS 5.1.3.1 lets you go down at a 2:1 slope up to half the height of the wall to get the effective length the concentrated load is applied over, but do you utilize that same effective length to distribute that concentrated load over the footing or something different.

Also, if your concentrated load spacing (say 2 ft O.C.) governs over the effective length you get from TMS, would you use the 2 ft length to distribute your concentrated load over the footing or is that too conservative considering the load will most likely have a considerable amount of height to disperse over before reaching the footing.

Thanks

 

[KootK]

As far as I know, this is entirely a matter of designer judgement. I believe that the TMS rule is meant to govern the design of the wall although I can see how it might be extended for consideration of a footing.

Most of the time, if I have a single concentrated load to deal with, I'll just design a pad footing assuming that the load comes down to the footing in a concentrated fashion, maybe matching the dimensions of the wall pier if there is one.

That said, if I were confident that the wall was strong and stiff enough to distribute the load and I felt that there was much to be gained from that approach, I wouldn't shy away from using it. And I wouldn't necessarily limit myself to the TMS rule either with respect to the amount of distribution.

 

[TRAK.Structural]

I think a 2:1 spread from the load application point down to the foundation is reasonable, if you only have (1) point load to contend with. If this were a tall wall, say over 15-20 feet in height, I may put an upper bound on how far to spread the load.

If there are multiple point loads at a regular interval than you have to account for all the loads and you end up with a more or less uniform load equal to the sum of the point loads divided by the length over which the loads are applied.

Don't forget to add in the self weight of the wall and footing.

 

[phamENG]

I usually find it best to extend whatever analysis philosophy I'm using as far through the load path as possible, just to keep things consistent. So if I'm spreading the load out in the wall, I want to spread the load out in the footing, too. If I make the footing too stiff directly under the load, then the load might not spread through the wall as much and I could end up with higher axial stresses in the wall than I was anticipating.

Just be mindful of joints in the wall. A lot of firms put this off on the architect for some reason, but it has big impacts on the structural design.

 

[ANE91]

Dispersion triangles cannot intersect. They stop “growing” once you encounter one of the stops (e.g., a wall opening, another triangle, half the wall height, etc.) My masonry professor called this the “dummy rule,” though I don’t believe it’s as fleshed out in 2011/2016 compared to 2022.

Take your dispersion down to the foundation, like phamENG said. If 2ft oc at the top governs, then I use that at the footing.

 

[ACivilLife]

I usually find it best to extend whatever analysis philosophy I'm using as far through the load path as possible, just to keep things consistent. So if I'm spreading the load out in the wall, I want to spread the load out in the footing, too. If I make the footing too stiff directly under the load, then the load might not spread through the wall as much and I could end up with higher axial stresses in the wall than I was anticipating.

Just be mindful of joints in the wall. A lot of firms put this off on the architect for some reason, but it has big impacts on the structural design.

Dispersion triangles cannot intersect. They stop “growing” once you encounter one of the stops (e.g., a wall opening, another triangle, half the wall height, etc.) My masonry professor called this the “dummy rule,” though I don’t believe it’s as fleshed out in 2011/2016 compared to 2022.

Take your dispersion down to the foundation, like phamENG said. If 2ft oc at the top governs, then I use that at the footing.

View attachment 3481

This makes sense and is along my line of thoughts. Logically to me it would make sense to cap the effective length at what TMS prescribes (i.e. length at half the wall height) or whatever other spacing governs, because the stress you would design wall for would be based on that effective length. It makes sense to me that the footing would need to be designed for the same level of load that produces the stress the wall was designed for (i.e. same effective length).

 

[jerseyshore]

I find that quite often if I'm worried about a point load on a wall footing that much, it usually means that I'll need to add a pilaster/ use a masonry column in the wall. Then the footing answer is easy since it's just a concentrated load with little to no distribution.

In this case you have a series of loads so same idea, as TRAK said about it being a defacto line load.

Usually the masonry strength controls and guides you to what is needed footing-wise.