Reinforcing foundation 3

[BAGW]

Hi all,

I am reinforcing an existing foundation as the loads on the foundation has increased. Solution I am thinking of is adding more concrete on either side of foundation and attaching them using epoxy rebars to reduce the bearing pressure. Punching and one way is not an issue.

How to make sure the epoxy rebars are good enough to develop the moment resulting from the load at the joint.

I am leaning towards creating a new foundation by digging below the existing.

Any inputs on how to calculate the moment transfer at the joint?

 

[Gerber_EIT]

Do you have data on the reinforcing for the existing foundation? If so, I would check out Hilti's post-installed reinforcing guide. Theoretically, you should be able to epoxy and lap new bars with the existing. You can then calculate capacity of the foundation as if it were new using the increased foundation size and the lapped bars idealized as uniform the whole length of the foundation. While this sounds good, also take a careful look at settlement under the new concrete. Any significant settlement will put the post-installed bars under both tension and shear, so you'll have to make sure that they are sized properly and the foundation can handle the shear load as well.

While this approach has been suggested by a few people I've worked with, I'm not 100% on the validity of it. To lap with the existing bars properly, you need a good idea of how the existing reinforcing is laid out to place the new bars at the correct locations. And most drawings might not show exactly where/how things got done in the field. Feels like throwing darts with a blindfold on to me.

 

[JoshPlumSE]

Theoretically, you should be able to epoxy and lap new bars with the existing. You can then calculate capacity of the foundation as if it were new using the increased foundation size and the lapped bars idealized as uniform the whole length of the foundation

I've done this a few times in the past.
1) Once for new construction... foundation had been poured when we realized it needed to be larger. I was pretty comfortable with this because it was mostly about avoiding uplift by connecting the two foundations together.
2) A number of times for repairing a foundation slab. Again, not much loading on that area of the slab. All soil preparation was consistent in both locations. So, nothing to worry about.

I agree with Gerber that the portion of this that concerns me the most is the potential for differential settlement if the newly loaded soil has been compacted the same way as the existing soil area.

 

[SlideRuleEra]

I am reinforcing an existing foundation as the loads on the foundation has increased.
I am leaning towards creating a new foundation by digging below the existing.

If "digging below the existing foundation" is really an option (while temporarily supporting existing loads???)... I would completely demolish the existing foundation and construct a new foundation of the correct size. Far better and cost effective than trying to modify an existing foundation and expecting it to uniformly distribute loading to the soil.

http://www.SlideRuleEra.net

 

[r13]

Extend the base slab as shown, with flesh concrete at 2:1/1:1 slop from the new slab edges toward the column. Make sure to roughen the new-old concrete contact areas, and provide bonding agent as suggested by the manufacture.

 

[BridgeSmith]

With the poor results we've seen with epoxyed anchors, I'd be very hesitant to trust them for flexural tension reinforcement, unless the installation was observed very carefully. Specifically, ensure that the drilled holes are thoroughly cleaned (especially if core drilled) that the 2 parts are thoroughly mixed.

Another option (one we commonly use for bridge rehabilitation), is to chip away the concrete to expose the existing reinforcing, and either lap splice or mechanically splice new reinforcing to it.

My first question, however, would be whether the existing reinforcing is adequate for the flexural moment of the larger footing.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[WARose]

At the risk of getting off topic.....I wouldn't expand the foundation like you are proposing. See attached for how I have typically done it.

 

[WARose]

A pic of what I am talking about.

 

[phamENG]

Scale is an important factor. What kind of loads, bearing pressures and foundation sizes are we talking about? If it's a 9'x9' and you're trying to make it a 12'x12', I'd pay attention to what WARose has to say. If it's a 3'x3' and you need a 4'x4', then you're probably on the right track with your original post.