Underpinning Foundations 1

[Hercul3s]

Fellows,

Is there an underpinning expert out there?
I was just wondering if concrete underpinning less than 4'-0" deep, for an existing concrete foundation wall, needs to be reinforced (I believe more than 4'-0" deep needs to be?!!).
If so, what is the best configuration of reinforcement: spliced horizontally; vertical reinforcement; other??

Thanks,
Hercul3es

 

[KootK]

There's a member here by the handle PEInc that truly is an underpinning expert. Hopefully he shows. He tends to troll the earth retention forum.

I've never had to reinforce a 4' underpin. They usually end up being about 2' thick and compressed by axial load so it's never been necessary by the numbers. That said, you should run the numbers for your particular situation. Vertical rebar is not a big deal but horizontal reinforcing is pretty terrible constructibility wise.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[PEinc]

Concrete underpinning piers should not need, and very rarely have, any reinforcing steel. The compressive stress in concrete underpinning is very low (usually well less than 100 psi). Unless the underpinning is deeper than about 10 feet so that it needs lateral bracing or tiebacks, there is no bending. When there is bending, the size of the concrete pier is such that the tensile bending stress is minor, usually less than about 150 psi. The unreinforced concrete is most likely better than an unreinforced stone or block foundation wall. The front to back width of underpinning piers usually is at least 2.5 to 3 feet in order to allow access for the guy digging the pit and for providing sufficient bearing area. Placing reinforcing steel in concrete underpinning piers is problematic and expensive. There should be a number of threads on ET about underpinning.

http://www.PeirceEngineering.com

 

[oldestguy]

The only reinforcement we have used is using re-bars to tie the adjacent units to each other. No particular design.

 

[KootK]

@oldestguy: how do you install the rebar such that it laps with the bar in the adjoining segment? I've been trying to come up with something like that for years.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jike]

We also only used horizontal reinforcement to tie the segments together. We would typically would use #6 bars x 4'-0" long each face and drive them into the adjacent soil 2'-0". Lap 2'-0".

 

[Hercul3s]

Thanks guys, this is really helpful, and was pretty much what I expected to hear.

Just one more question: do you recommend formwork (side-to-side) to pour the concrete piers?

Thanks again!

 

[oldestguy]

KootK: As jike says that works if the ground is stable and no sheeting. However, using removable sheeting that can come off after excavation on its "earth side", should work, or just leave that in place.

 

[PEinc]

If an underpinning pier needs to be deeper than about 4 feet, you need wood sheeting to protect the worker in the pit. This wood sheeting becomes the concrete form. If you will be digging down in front of the completed underpinning, the front shoring boards can be stripped. If you are installing continuous, adjacent, underpinning piers; you can strip the side shoring boards. The rear boards cannot be stripped or removed.

Installing reinforcing bars that jike mentioned may work for piers less than about 4 feet deep but they will greatly interfere with excavating for deeper underpinning piers. How can a worker fit inside a 4' wide pit if there are 2' long dowels coming at him from both sides? Also, what purpose is this reinforcing really serving? If the underpinning consists of continuous, adjacent piers, the underpinning is designed for overturning and sliding on a per linear foot of wall basis and there should be no shear between adjacent piers. If the underpinning consists of individual, spaced piers; there are no adjacent piers to dowel into. There is rarely a good reason to install reinforcing in underpinning piers.

http://www.PeirceEngineering.com

 

[Hercul3s]

Yes, I was referring to the side shoring boards. I guess the answer is yes.
Thank you so much for your help guys!

 

[MacGruber22]

I'd say that pit looks to be about 4'-6" or so.

"It is imperative Cunth doesn't get his hands on those codes."