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.