PT Slab Discovered During Construction!

[ampersand]

Hello All,

I am a licensed CA structural engineer, and I recently designed a rear addition to a residence in the SF Bay Area. The owner did not provide a geotechnical report. I visited the site, took measurements and photos, and I designed the foundation as a conventional foundation system. The contractor began construction, which included cutting some holes out of the back end of the foundation/slab-on-grade, and unfortunately the GC discovered that it was a PT slab system- and he had cut some of the PT strands.

I am trying to help the owner locate a specialty engineer who can design a fix for the PT slab (I cannot design PT). I will offer my client all the coordination help they need. But I am concerned: Did I act prudently in designing the addition? They will probably have to have another company design a slab fix, and then also re-design the entire addition foundation as some kind of PT system.

It is of course highly uncommon for a residence to have a PT slab system. I did not see (nor did the GC) and "Warning: PT Slab" notations stamped in the garage slab, nor did I see any strand end caps in the foundation. I did not receive any as-built drawings, but it is possible that the as-built drawings are on file at the building department (I did not actually travel to the bldg. department to look this up). I feel bad, because this is the last situation I want a client of mine to be in, but there is only so much an engineer can tell from a visual inspection/walk-through.

My question to you all is simply: Do you think I should have acted differently? I would appreciate any feedback, even if it is critical.

Thank you

 

[KootK]

I sympathize. Just last month I very nearly got myself into the same situation. Low fee, limited existing drawings, and no geotechnical report. At the last minute I noticed some things about the existing foundation that led me to believe that the slab on grade was a structural slab of some kind. I then insisted on having some kind of geotechnical input for the project and, as a result, lost the project (I'm not terribly sad about that).

In retrospect, yeah, it would have been good to have be a bit more dogged in your pursuit of the existing condition. That said, I know how things work in the single family residential apace and it kind of invites these sorts of problems from time to time.

For what it's worth, post tensioned slabs on grade are usually designed semi-empirically. And there are plenty of design guides available through PTI etc. If a small number of strands have been severed, you may be able to evaluate whether or not they actually need to be restored fairly easily. If they do need to be restored then, as you mentioned, it's probably a matter of engaging someone familiar with that technology to do the work. Member Inginuity here is bonafide expert at this stuff and may be able to help if you can get his attention somehow.

 

[Enable]

I've moved on from the restoration industry but for most of my working life I have repaired existing buildings, bridges, etc. and I can tell you with 100% certainty that we designed and constructed without ever fully knowing the state of the existing structure. Not only is it not pragmatically possible to do so (no one pays enough for such in-depth investigation) it's not technically possible either, even if people did say money was no object. How the hell do you know what happened over the life cycle of the structure and where the associated stresses ended up being resolved? A guess, sure, but exactness is not a thing.

That relates to your query in the sense that I do not think you should be so hard on yourself as to not have known everything at the stage of design. Learn from it, but don't beat yourself up over it. The only thing I'll say that may be prudent in the future is to require the contractor to GPR scan whatever it is they are coring / making holes in. Even then, scanning companies aren't all the same and shit gets labeled as something it's not or gets missed entirely. Ingenuity had a thread where the GC scanned the slab and cored through a PT tendon even though it was labeled as rebar (and the EOR said it was cool to cut the rebar)! It's hard to be good as a former carpenter / partner of mine once said

BTW: I'll second KootK on if you proceed with a repair that Ingenuity is the person to get ahold of. If you mention PT in the structural forum likely he will come, may be a good bet just to @him in the title.



CWB (W47.1) Div 1 Fabricator
Temporary Works Design

https://www.enable-inc.com/