Eccentric Column Load on Strip Footing

[helliot]

Hello all,

I have some questions regarding an eccentric column load on a strip footing.

Background: Residential home constructed 5 yrs ago, main floor beams designed to be installed in beam pockets in the concrete foundation wall. At one beam location, beam pocket was not cut out of the foundation wall. The framer built up a 3 ply 2x6 column under the end of the beam instead. The column is directly against the foundation wall, and sits on the footing. The basement slab was cast around the column. Fast forward 5 yrs, City requires a structural check of this column.

I have checked the beam and column itself, they can take the loads applied (main floor joist loads only, a 13.6kN (20kN factored) load on the column).

Drawings indicate the footing is 18"x6" with a 8" foundation wall. So, I am assuming the 5.5" base of the column is resting on 5" of strip footing on the inside of the wall, at an eccentricity to the footing centerline. e>(l/6), a large eccentricity.

I know a procedure for eccentrically loaded square or rectangular footing involving looking at the eccentricity as a point load plus a moment (P/a)+/-(M/S) to calculate the non uniform soil bearing pressure distribution.

However, I am not sure how to apply this to a STRIP footing. My questions to get me started are:

1) How does the load from the column distribute into the strip footing? i.e. what is the bearing area A in the (P/A)+/-(M/S) calculation? Will the load distribute at an angle through the 6" thick footing? Or will a length of footing "b" act as a rectangular footing of 18"xb?

2) Since the column seems to be "hanging" 0.5" off the edge of the footing, can I assume the column load as a point load at the centerline of column?

3) Since the eccentricity e>(l/6), I know the pressure calculated on the outside of the wall at edge of footing will be tensile i.e. the footing can lift. Are there any other checks I can do to confirm this? There is likely some steel in the footing, but drawings do not confirm spacing or grade.

Thanks for reading,

 

[EngineeringAdam]

Our office works mainly in residential work and for a load that minimal I wouldn't worry about eccentricity for a spread footing.

What we have done in cases like this is epoxy bolt through the column into the foundation stem to transfer the vertical load to the stem as quickly as possible. This way you aren't relying on the bearing at the bottom of the post.

 

[EngineeringAdam]

I should mention simpson Titan HD concrete screws, which are perfect for an application like this.

Not sure if epoxy would be my first choice on this.

 

[Once20036]

I wouldn't look at it as a point load on the foundation without also looking at the weight of the concrete wall adjacent to the column. This weight may be enough to counteract the eccentricity of the column and eliminate any uplift.
Also - don't forget about the weight of the soil on top of the outer 5.5"

For a 6" thick footing, I'd start out looking at a 12" width of wall. If it works out, with the load from the concrete wall and soil, I'd call it a day. If it doesn't work out, you could probably justify using a wider width.

 

[JAE]

If the wall is reinforced concrete masonry or concrete there would be significant stiffness in the wall that would mitigate some of the eccentricity of the column. Pretending that the wall is just a load (and not an integral part of the whole system) is an incorrect analysis. So I would agree with EngineeringAdam that it appears to be fairly minimal, and if you account for the wall stiffness you would probably find that you are OK.

I would also double check your column. Are the multiple 2x6's nailed together properly? If not, you have to consider these as individual 2x6's each separately taking a 1/3 of the load and not a composite 4.5" x 5 1/2" member.

 

[EngineeringAdam]

Great point JAE - the multiple 2x6's might be useless without proper connection to each other.

This is especially important if the built up post is of any significant height.

 

[helliot]

Thank you for the replies.

I do see now how the wall is acting to counteract the eccentricity, thank you for pointing that out. Not 100% sure how to do a calculation to show this stiffness action, but I will think about it this morning and come up with some ideas.

JAE/Engineering Adam - I have factored in that I can't see the nailed connections and cannot verify if they are adequate. I calculated once as a built up beam - with the CSA code (Canada, eh?) compressive resistance is 60% of the equivalent gross section for nailed built up - and also as separate members with 1/3 of the load.

Thanks for your help,