Building on Abandoned foundations

[jhoulette]

I've recently been asked to review structural plans from the 1980s where they plan to start building on abandoned foundations constructed in the 1980s. Does anyone know if this can been done? I can't think of too many major problems, just the minor problems of the condition of the existing foundations.
If the foundation is a post tension slab, I'm concerned about the foundation not being loaded by the actual design loads to resist edge lift. The same if drilled piers, not being loaded to resist uplift soil pressures.
Any advise would be GREATLY appreciated.

 

[csd72]

What type of building?

I would be particularly concerned if there were large concentrated loads on this slab as these would almost definately not be accommodated for.

I would also demand that some investigative work be done to confirm founding depth e.t.c.

Personally this is not something that I would get invoi\lved in.

 

[jhoulette]

The framing would be the same framing that the foundation was originally designed for.
Type of construction is multi-family townhomes of wood framing.

 

[msquared48]

Are the anchor bolts and holddowns already embedded in the foundation concrete? If not, then afterset is needed.

The lateral forces have changed over the years, depending where you are, it could require more lateral load, or even a different governing force than in the 80's. This will mean an upgrade if the hardware is already present in the foundation.

OIf you want to do this, you might have the f'x of the existing foundation tested, as with 25 years or so, it should have risen well above the design f'c. The results Could give you an extra margin in your design, maybe offsetting any increase in Anchor Bolts or Holddowns.

There could be other code change issues here, Architectural in particular, requiring some walls to be adjusted. This may or may not affect the structural framing layout and the foundation.

You have the structural plans. Is there a soils report? Are there structural calculations to review?

Mike McCann
MMC Engineering

 

[jhoulette]

Thank you for your thoughts. I didn't think about having the existing foundation tested to obtain higher f'x.
And yes, our arhcitectural department is looking into the changes needed there.

 

[JLNJ]

A bit from the devil's advocate:

If the foundations are already in and somewhat undocumented, how do you comply with the inspection provisions now mandatory in the code?

We can build on old foundations that were not inspected but we have to inspect new foundation installations?

 

[csd72]

JLNJ makes a very good point that you should investigate before you waste your clients money.

Now that I know more about it, the use of the foundations seems like a reasonable strategy if you can get around the code provisions and the issues noted by others above.

Can you make it work if you treated it as plain concrete? This would get around any lack of inspections for reinforcement. The geotech report may suffice for the soils side and testing of the concrete may suffice from the concrete side.

But I would definately recommend that you meet with your code officials to see if they will acccept this.

 

[45North]

Like most structural re-use projects the real question is; what is the net value to the projsct? The degree of certainty you can obtain varies with price. (not a new concept, eh?)

The degree of certainty that I would require, regardless of the minimum code requirements, would also depend on the potential consequences of a "failure". If a client knew the risks and wanted to accept the potential for some cracked drywall over the years that's a lot different than a potential for total collapse. (Although I would still want a hold harmless agreement)

As for code issues; proof loading is one way. Piles can also be checked using sonic pulses to confirm length and integrity. If its a large foundation you could even do a limited inspection via diamond saw and jack hammer. I would suggest contacting a GOOD construction testing lab in your area and see what they can do for you.

The code typically allows for re-use. I agree with the comments above, start by talking with the code official to see if its an option.

 

[jhoulette]

Yes I agree with checking with the code officials. The problem is, this is a very samll town and I have feeling there was not much code inforcement done or asked of. I have yet to find a phone number for a local code official.
Thank you. I might invest in a really good metal detector as well.

 

[civilperson]

If you can figure the loads from the previous structure, then a comparison to planned loads can be made. If same or less then OK.

 

[Atomic25]

I've been in your position before and I'll offer these words of wisdom:

1. You're not going to do any good with a good "metal detector". You absolutely need to have the client invest in an x-ray or a destructive test to see what the heck is going on. Do not settle for the cheapo RF meter scan that might pick up on bars within 6" of the surface of the concrete. If they don't invest this money, I would walk away from the project. I would also recommend they invest in a geotechnical report of the site.

2. Once you have a feel for the original design, obviously check it against current code.

3. Report to your client the feasibility of what you're doing, AND more importantly document the degree of risk they are taking. There's no way you'll be able to verify every bar in the concrete, nor predict all of the subgrade. Who knows if the previous contractor didnt compact subgrade??

4. Always remember that problematic settlement or cracking is something you can avoid by explaining risk upfront to clients, but a failure that could harm/hurt a person is not....nor would I want that on my conscious.

 

[msquared48]

Atomic25:

We know that you're conscious, but do you have a conscience?

Mike McCann
MMC Engineering

 

[jhoulette]

Thank you Atomic25. I have a kick off meeting with them today and I will talk to them about an x-ray/destructive testing.

 

[BigH]

This has been discussed in the past - foundation forum?. That thread has links to some good references and also good discussion. Do a search on the Eng-Tips site.