spread footings + foundation beams

[greznik91]

I have to design spread footings for a barn.
Its a steel barn with pinned supports at columns bases. So that means that the only action on top of a spread footing is vertical and horizontal force/reaction from steel column. Since my footings are 1,20 m deep I get a moment as a result of horizontal reaction at the top of a footing: M = RH x h
Since horizontal forces are pretty big I get a large moment acting on a footing and hence large footing dimensions.

I was thinking about adding another foundation beams as shown in attached picture bellow. Those beams would be connected to RC wall (at the end of a beam).
Because of that moment on footing is no longer a problem and I get much smaller dimensions.

What im asking is:
- do you see any possibly problems caused on a RC wall because of extra foundation beams that are connected to it
- can I consider friction between RC slab and ground/soil and passive soil pressures on footing to take care of horizontal forces acting on a footing?

 

[Old Eng]

Place some rebars between the footing and the slab and also connect the slab to the lower level foundation and you have it licked.

 

[spieng89]

Old Eng I think is referring to hairpins which is an option. Is there any reason the columns can't be set lower on top of the footing to reduce the moment? And why are the footings so thick compared to the plan dimension? IS there a frost depth you need to get below?

 

[CANPRO]

It can also be helpful to offset the footings from the column so that your gravity loads counter the overturning. That is my first choice in reducing footing size. Hairpins work but in my experience contractors would rather not use them because they're in the way from the time they're installed to when the slab is poured. There are also some other potential issues with hairpins, one being the potential for corrosion at the slab/foundation wall interface.

Sounds like you're getting into the fun world of designing foundations for pre-eng buildings (or something similar). The book "Foundation and Anchor Design for Metal Building Systems" by Alexander Newman has been recommended on various threads here. I've purchased the book based on those recommendations and have to say that it was well worth the money.

 

[greznik91]

tnx for replies.

IS there a frost depth you need to get below? YES, thats the case.

What do you guys think of my solution - additional (blue) foundation beams that connect spread footings to RC wall?

 

[Old Eng]

It's called belt and suspenders.