Eccentric Footing 2

[sponcyv]

I am dropping a column down 1/2" from the face of a wall for a renovation project. There is an existing strip footing under the masonry wall that I am pouring my isolated column's eccentric footing atop. Basically, the bottom of the bulk of the footing will be level with the bottom of the existing strip footing, but there will be a portion of the footing that is to be poured atop the strip footing. This portion will have a styrofoam strip to keep the new footing from loading the strip footing. Does anyone have a design example for this exact condition? I have attached a link to a pdf image.

 

[SteelPE]

I'm not sure someone is going to have an exact example.... but I'm sure you could modify a strapped footing example to suit this case.

 

[sponcyv]

A strap footing is not applicable...the new eccentric spread footing is atop a strip footing.

 

[larsacious]

Here are some ideas/suggestions:

-Can you drill and epoxy into the existing footing and analyze it as a "T" shaped footing by utilizing a certain effective width of the existing footing?

-Can you move the column away from the existing building and cantilever the joists over the top of the beam with joist seat extensions or bottom chord bearing joists?

-Provide a strap beam like SteelPE said to the next interior footing.

My fear with your approach is that you will be so eccentric that you will have rotation in the footing causing long term settlment.

 

[sponcyv]

Larsacious,

I really don't want to load the existing footing, thus the "T" shaped footing is not ideal.

This is a retrofit to a building....the joist seats were anchored into concrete beams with (2) 2" sleeve anchors, which failed due to concrete break out. I am proposing to put in a beam and column line to support the existing joists. The columns need to be as snug to the wall as possible so that they do not interfere with the building use.

There are no interior footings...this is a clear span condition. Even if there was an interior column, I'm not following the strap beam idea.

I understand your concern about rotation.

 

[DaveAtkins]

I think loading the existing strip footing is the best answer. You could add a very wide (parallel to the strip footing) spread footing.

I can't see your current detail working at all. Too much eccentricity--the bearing pressure under the footing would be enormous.

DaveAtkins

 

[larsacious]

Wow what an interesting dilemma!

What about using (2) helical piers or concrete sonotubes. One would be placed near the existing footing and another would be placed approximately 4 to 6 feet away from the existing footing (both perpendicular to the existing wall). You could then cast a concrete beam on top of the helical piers/sonotubes and cantilever it out to pick up the new column. The reason for using the piers/sonotubes is to be able to take resist tension by using the pier or sufficent weight to counterbalance the overturning force in the case of the sonotube.

 

[sponcyv]

I am attaching a link to see my calcs. 1 of 2...

 

[sponcyv]

1 of 2

 

[sponcyv]

2 of 2

 

[BAretired]

If you can use a slightly larger column, probably a WF, you could use the attached detail.

BA

 

[azcats]

How about a deep (moderately), round footing that uses end bearing to resist the vertical load and lateral bearing to resist the eccentric moments?

 

[TXStructural]

Placing "styrofoam" between the footings will not effectively prevent added loading to the strip. I'd use cardboard void form or taper the spacer so it can be removed.

 

[sponcyv]

BAretired,

Thank you for the detail. I'm really ok with mine. We've done this type of "diving board" eccentric footing several times and basically wanted to double check my design.

azcats,

I see where you are going with this, but I'd rather not undermine the existing footing.

Thanks everyone for the feedback

 

[BAretired]

I found a minor difference in your calcs (see attached). I don't know the bearing capacity of the soil, but this seems fairly high for a clay soil.

You are using the weight of the new footing plus soil plus slab to counterbalance the applied column load, but it is clear from your drawing that you do not really know the dimensions of the existing footing. This could be a problem.

I'm not sure why you do not want to tie the new structure into the old. Is the bearing pressure very high in the existing footing? I would seek to engage the existing footing with the new footing by means of dowels embedded into the existing and epoxied.

Alternatively, if you are adamant about not tying the two together, why not use one or two screw piles at the outside edge of the new footing to provide a tiedown, balancing the column load. If you do that, you might consider a rectangular footing instead of square...or even better, a T shape.

BA

 

[sponcyv]

BA - thanks for the calc check!

You guys have talked me in to doweling into the existing footing. We really don't have much load on the existing footing. I typically don't like to load existing footings unless I know for sure what load the footing.

For those who suggested strap footings...I now see what you are talking about and that would be a good option too, but tying in to the existing would be less construction.

For those who suggested helical piles or sonotubes, I just think the owner is looking for something more simple and as cheap as possible. They are going to bid it out to some really small local contractors.

 

[larsacious]

Also, from your section it appears the architect is going to have a wall at the edge of your slab on grade. You may want to double check the slab capacity for carrying this wall and how you brace the top of the wall. Be careful of the ramifications if they are using this as a firewall.

Good luck.

 

[sponcyv]

larsacious,

That is the existing condition. The only thing new is the column, beam, and footing. This was definitely a strange building. Concrete beam high, grade beam low, and masonry/brick infill between. It was an exterior wall at one time.

 

[sponcyv]

For those who may care, here is what I finally came up with...please see link.

 

[BAretired]

sponcvv,

A projection of 3'-0" produces a substantial moment at the face of the existing footing. This is carried by 5-#4 bottom bars epoxied into the footing. If the existing footing is plain concrete (unreinforced) there is a danger of cracking it under the existing wall. Perhaps a lesser projection would be advisable.


BA