Founding on an Existing Footing 1

[someengineer]

Hello,

Not sure if there is a better section to pose this question but I thought I would start here.

I am running into a situation where we have done a foundation design (piers and footings) at a mill site and issued for construction. The owner is overseeing the excavation and the contractor has unearthed a large existing footing where we are proposing to place our new footing. The owner has contacted us and has suggested that we could possibly use the existing footing instead of demoing the footing and pouring a new one. I have expressed my concerns with the many unknowns regarding using the existing footing (no drawings available) and he has suggested that he would provide a letter taking on the risk of using that existing footing.

My question is - does anyone have an experience with this situation that could offer guidance? I am a bit unsure if what he describes is appropriate. Could we revise the drawing to indicate that that particular footing is "designed by others"? Also, we have a pier that we have designed which would need to be doweled into the existing footing, which may also cause issues of where our liability starts and ends.

Any insight into this would be appreciated! If you have any questions, please reply and I will do my best to answer them.

Thanks!

 

[msquared48]

Is the existing footing thick enough to support the load as an unreinforced footing using a f'c of 2500 psi? I assume it is large enough in width and breadth to possibly work with a 1500 psf bearing value.

Mike McCann, PE, SE (WA)

 

[someengineer]

We are still waiting on geometric information from the client, i think it is only partially exposed at the moment.

 

[oldestguy]

Of course you have test borings to evaluate the soil. Right? Is the fill there a surprise? Maybe it needs your geotech on site for that part at least.

 

[hokie66]

It is not uncommon on old industrial sites to find things underground that you did not expect. More information is needed, which I trust that you are acquiring. I have encountered similar situations, and sometimes have used the old footing, sometimes not. But it is not good enough to accept the owner's letter as relieving you of your responsibility.

 

[msquared48]

Hokie is correct about the letter. Remember that the owner can always sell the property which can leave you legally exposed.

Mike McCann, PE, SE (WA)

 

[someengineer]

Unfortunately there is not much information available in regards to concrete strength or reinforcement. It appears to be part of a strip footing. If we were to use it the new pier would intersect at the edge of the strip, therefore we would likely need to pour a new portion of the footing that ties into the existing footing and have the pier tied partially into the new and existing footing.

Would it be acceptable if I were to do a analysis using conservative properties as msquared48 suggested? Also, I would get the geotech on site to investigate the sub-grade under the footing and give me an appropriate bearing pressure.

Thoughts?

 

[hokie66]

We don't know what you are proposing to support on the footing, so that makes it doubly hard, or rather impossible, to give good advice. But if you are going to need to tie into this footing, it is almost certainly better to demolish the section in your way and cast the new footing separately. One caveat...if the "strip footing" is rather a tie beam of some sort, you may need to restore its continuity.

 

[someengineer]

Sorry for not saying what the footing is supporting. Its supporting a column for a small pre-engineered building. The column is fairly lighty loaded - mostly wind load.

The client has elected to demo the existing footing and install a new one as originally planned.

Thanks for your assistance!