How to widen existing footing 3

[tclat]

Hi,

I have been hired by a contractor to explore options to widen and existing footing.

The contractor cast the the column eccentric to the footing. 12x12 r/c column on 5x5 footing but 1 foot from the edge. Bearing pressures now exceed allowable.

One option is obviously to demolish and another to underpin although I'm not to keen on this option.

I was considering another option of casting a jacket around the existing column (say 5" around) and attach the two with epoxy dowels. I would then consider pouring a new footing over the existing footing and with the the reinforcment in the "jacket" embedded in the footing. This new footing would now be centered on the column. I would also have to expoxy dowel the new footing reinforcing bars into the existing column.

Has anyone seen this type of detail before? Any other options?

Thanks

 

[DaveAtkins]

The easiest solution would be to justify the footing as is--assume uniform bearing pressure under the footing, and resist the resulting moment with the column. Make sure the column bars are adequately developed into the footing and up into the column, and of course make sure the column can take the moment.

If this doesn't work, you could expose the ends of the rebars in the footing, and connect rebars to them with mechanical splices. Then add additional footing as required.

DaveAtkins

 

[JoshPlumSE]

I've done something fairly similar. In my case, I was pouring a new footing that would not be sufficient on it's own. Therefore, I used epoxy dowels to connect the new footing to an existing mat foundation.

That was 10+ years ago and the solution was really my bosses suggestion (he was the EOR). Hence, my memory isn't all that clear about it. But, I don't recall experiencing any major problems with the solution. In fact, I remember the field being pleased with the relatively simple solution.

 

[hokie66]

I seriously doubt that Dave's first solution will work. A 12 x 12 column is much more flexible than the footing, so won't attract the moment required to make the bearing uniform. I would use his second approach, with added epoxied dowels mid-depth to take the shear.

 

[tclat]

Dave,

I don't understand your recommendation. The footing is usually designed to accomodate the column moment. It seems you are suggesting the other way around. Can you elaborate?

I don't have the flexibilty of mechanical splice connectors.

Josh,

I don't fancy extending the footing by epoxying into the side of it. Getting the tension to tranfer across the epoxied joint and into the existing bars seems cumbersome and really like a hit or miss.

Any other thoughts?

 

[hokie66]

Another possible solution is to install ground anchors through the footing on the 4' side. This would provide a reaction to redistribute the load.

 

[TXStructural]

Extend the footing in the needed direction. Demo sufficient concrete to either use mechanical couplers or lap splice rebar. I do not understand any hesitation to using mechanical splices for the rebar (if I read that correctly above.)

Chemical anchorage may be fine, if you carefully select the product for the exposure and continuous loading.

 

[tclat]

Thanks for the comments.

I'm in a remote country and mechnical connectors are simply no available.

 

[hokie66]

My approach would vary depending on the status of construction above the footing. The column has been cast, but is that all? If so, I would just bend the column bars over, hook up to a big mobile crane, and pull out the footing, then do it right. Simple and straightforward. But if the floor above has been cast, or if there is a big area of formwork already surrounding the column, your approach of trying to rectify in place makes sense.

As you have been hired by the contractor, what does the responsible design engineer for the project say about this? It is ultimately his responsibility to approve any solution.

 

[Lion06]

If you have access to epoxy adhesive, I would drill and dowel into the new footing with bars to extend the footing in the direction needed.

I recently did the same for a footing that was poured in the wrong place and it didn't work as the contractor placed it.

 

[tclat]

Structural EIT

I understand the transfer of shear and moment across the joint but what happens across the joint. Do you simply apply the loads to the old section? Can the tension effectively get back into the existing bars? Is there a spacing requirement between the new dowel and existing steel? If we were doing a lap splice then there would be lap lenght requirements and spacing limitations unless it was a mechanical splice. Drilling more than 6-8 inches is probably not practical.

 

[DaveAtkins]

Simply embedding dowels into the side of the existing footing a few inches and setting them with epoxy will not work. The bars in the existing footing will not be developed where the new bars cut off. That is why I recommend mechanical splices.

Regarding my other solution, tclat, draw a free body diagram of the column and footing. Apply a uniform bearing pressure under the footing, equal to the vertical load in the column. The eccentricity between the centroid of the bearing pressure and the center of the column creates a moment that must be resisted by the column.

DaveAtkins

 

[hokie66]

Dave,
Even if the column can resist the moment strengthwise, you are neglecting the flexibility of the column. It will bend, and the vertical load from the column will go straight down.

I agree that epoxied bars are not an appropriate solution for the bending.

 

[Lion06]

I typically do the following.

Determine the actual bearing stress and use that to find the moment at, say 1'-0" from the joint. I check to see if 9" of development is enough for the amount of steel I have compared to what I need. If it is, I provide a 1' embedment (of the bottom bars only) in an epoxy. If it's not enough, I'll try 1'-4", and so on, until it works. You'll get a longer embedment on the bottom bars. I also provide top bar dowels (even if there are no top bars in the footing) just to help with shear transfer. I only use the embedment needed for development of Fu at the top.

 

[BAretired]

Temporarily support the floor currently supported by the column. Jackhammer the bottom 12" height of column, leaving the reinforcement in place. Pour a new footing over the existing through the column leaving a gap of an inch or so. Grout between new footing and column using non-shrink grout.

BA

 

[PEinc]

If the soils are OK, you could possibly install some helical piers around the perimeter of the footing. Attach the helicals to the footing using L-shaped brackets from the helical pier supplier.

http://www.PeirceEngineering.com

 

[dhengr]

tclat:
What about a few helical piers/anchors driven right along side of the footing. The footing is then held up, from below, by one of their canti. brackets off the pier stem?

 

[Lion06]

I just had a contractor drill 1'-3" For this exact situation and it wasn't brought up as a problem at all.

 

[Dad2one]

Helical piers is one good suggestion to increase strength but with that much eccentricity, very little of the footing rebar will be effective and you will likely not have sufficient bending capacity. Also, with that much eccentricity, I would argue that you effectively have a 2 foot wide one-way slab left to work with. Is there another column/wall close by that you can you strap this to and treat it like a property line footing? Otherwise, this is just too far off. I would make the contractor replace it. As soon as you 'fix' it, you've bought it. Good luck.

 

[tclat]

Thanks all.

I decided to go with the option I noted above.

This is a design build project so I am also the design eng.

Mobilising a crane or rig for doing any type of piling is a bit much for one footing in a remote site. The contractor definitely would prefer not to drill more than 6 inches

I would have probably gone with BARetired's option if the floor was already in place since it would be better if the footing bars were continuous through the column as opposed to just epoxied 6" on each of the four faces. Couldn't wait since the client wanted things rectified before we moved on.