New 2 story house on existing foundation

[vitium]

I am working on the design of a ADU (accessary dwelling unit) in the backyard of a main house.
They want to tear down the existing 1 story ADU superstructure (but leave the existing concrete slab on grade foundation) and build a new 2 story ADU wood framed structure on top.

The original ADU was built in 1950ish, so, no idea about the foundation construction quality or design, except that it is a concrete slab that appears to be in decent condition.

My principal says he will not sign off on the existing foundation. The owner is wondering if they can "sign off on it", or, if you will, assume the liability for any future issues that were a result of the moving foundation.

Is this a legal option? If this was, for example, a steel beam, we obviously would not let an owner sign off on something that was 50% undersized, however, for a concrete foundation, I feel like most or all of the issues that could arise would not be life threatening and more of an aesthetic nature.

In the IEBC (International Existing Building Code)it says that any structural element carrying more than a 5% increase in loads shall be brought up to code, essentially. So, checking the code, the IRC references the IBC which basically says that foundations shall be designed such that deflections and racking of the supported structure (due to expansive soils) shall be limited to that which will not interfere with the usability and serviceability of the structure.

I'm not aware of anything in the code that allows (or disallows) an owner to assume liability for excessive "serviceability" issues.

Thoughts?
Thanks in advance.

Matt

 

[Enable]

Not in this lifetime or the next. Owners have a habit of telling you not to worry at the start of things, but watch how hard you are thrown under the bus for not advising them well enough of the potential risks (or some similar BS) when things do happen. Even if you get a waiver signed, it wont matter. You'll be sued and your insurance premiums will increase regardless.

Better to not get involved than to do something like that IMHO.

 

[jayrod12]

I could get behind having a delegated design foundation. But around here doing so would require a professional seal on the foundation from someone. So unless the homeowner was a professional engineer qualified to seal the foundation drawings, I doubt they'd be able to find someone to do so.

Listen to your principal.

 

[phamENG]

How about having the existing footing excavated for inspection? I haven't done one of these (though I have inspected cracks and sloping floors in a house constructed on one), but wouldn't be totally opposed to it if I could get enough information. These would include: excavating around the perimeter to determine current footing size and condition, coring the slab to verify thickness and, if possible, reinforcement (maybe do a GPR scan first). If you're just leaving the slab as-is for the first floor, then there's really no change in usage and I wouldn't worry. The footings, though...those I'd worry about.

As for the liability question, no - you seal it you own it. We have licensing laws for a reason. We can't fight to maintain them one day and claim 'the owner knows enough to decide if their structure is adequate' the next.

 

[skeletron]

I wish the waiver idea was possible, but I think there is always the risk that the current owner won't necessarily be the future owner. And then, who does the responsibility lie with at that point?

Old foundations are hard to make work for increased loading without upgrades, IMO. You can do some exploration as suggested above. But it's a penny-pinching exercise that puts the risk back on the engineer. Not worth it.

 

[dhengr]

We are doing more and more junk engineering to save someone a few bucks, while we will ultimately assume the final liability for no fee increase, and then we have to fight with them to collect our fees because the, extra messing, extra work took longer. Why don’t we just quit the engineering business, at least on these kinds of jobs, and just go into the reinsurance business. But, we get to set the premiums, everyone bitches about them too but they do pay them. And, you can increase them 10-15% each year and claim it was the wind storm in Outer Slovobia which cause the increase.