Concrete column embedded in a concrete wall

[StarkInc14]

Hello Colleagues,

I have a concrete column (16"x 48") fully embedded in a 16" concrete wall. The wall is a continuous exterior foundation wall subject to soil lateral forces. I have designed the foundation wall using the lateral force and the axial load that wall receives through tributary width. since column and wall are aligned and in the same plane, they will most likely be cast together.

My question is the following: what is the correct design methodology or theory when designing or checking the footing below this column? For example if the column has an axial force of 100 kips...

A) Does this load spread through out the wall at a 45 degree angle? (if this is the case, i would design the wall footing with its own axial load plus the axial load of the column divided by the width of the spread).
Or
B) Does it come down directly into the foundation wall's continuous footing? Should the wall footing local to the column be designing assuming the load does not spread. I feel like the vertical bars in the column force the axial load to come down directly to the footing and does not allow the load to spread at 45 degree angle with in the wall.

I hope i've made my question clear. You help is much appreciated.

 

[jayrod12]

I personally am partial to providing a discrete widening of the footing at the column, it doesn't hurt anyone and provides a definitive load path. Even if the load spreads, you've got more than enough bearing area.

 

[KootK]

1) Like jayrod, I have a preference for using a footing local to the flush pilaster. That, simply because I feel that it is, as a matter of principle/efficiency, better to keep resistance as close to practical as loading. That, particularly at foundations where minimizing settlement is usually the name of the game.

2) The load absolutely will spread out through the wall. And your 45 degree assumption is pretty common although I typically go 2V:1H myself as that correlates with ACI load spread for point loads on walls which I feel is a bit similar.

3) My preference for a localized footing is slight. On many occasions, I've distributed the concentrated load out to a continuous strip footing in order to simplify things for excavation etc. Sometimes the theoretical has to give way to the practical. Alternately, sometimes I've used an oblong, rectangular footing as a compromise where that has made sense.

4) In addition to excavation considerations, sometimes it's prudent to use the strip footing, or an oblong footing, in order to minimize the projection of the footing out beyond the heated perimeter of the building where it might be affected by frost lenses in the soil.

5) Your point about the axial load being delivered through the rebar to the foundation is a good one. My thoughts on that:

a) this partly depends on just what the rebar was needed for in the pilaster design. If it was provided to satisfy code minimums and/or to improve buckling resistance only, as it usually is, then I'd not worry about this affecting the load spread. If the bars were actually needed for "squash load" axial resistance, then I'd put a bit more thought into it.

b) Even if you used the bars for the "squash load" axial resistance, it's worth remembering that you probably didn't include the effect of load transfer into the adjacent wall via shear in your original pilaster design. In reality, that load transfer will reduce the amount of axial load in the pilaster as it approaches the foundation level. And that shear transfer mechanism will be both massively strong/stiff and in play until you crack the tar out of the wall.

 

[StarkInc14]

Thank you for your inputs, they were very helpful.