guidelines for retrofit plumbing holes in concrete of post tension deck?

[Engineerataltitude]

I have a client who would like to cut a couple of holes for new plumbing through an exiting post-tension deck of a condo building. He can't find an engineer who specializes in post-tension deck design (which I recommended he try to find) to advise him on this. I've never designed a concrete post-tension deck, but I think I understand the fundamentals of the design concept. Can anyone give my any guidance or send me to any reference which would give me some guidance as to what to look out for to be able to do this?

 

[MrHershey]

Scan the deck. This is not optional. If owner doesn't want to pay for that then tell him you need to shore under cores and detension cables nearby. If he says he doesn't want to do that either, then walk away.

If you manage to avoid both tendons and rebar, holes <6" can normally be accommodated without any reinforcement. Once you get bigger than that or if you take out any rebar, would like at FRP around holes to 'replace' the rebar. Most FRP companies should be well-versed in doing this.

 

[Ingenuity]

I agree with MarkHirschi that scanning is mandatory before coring. Having the original Structural drawings and/or PT shop drawings is good, but seldom occurs.

It is exceptionally rare to 'shore and detension' for the average coring operation, but it has been done.

For the majority of condo construction incorporating PT the slab systems are flat plates, and assuming you are in the USA, it will be unbounded PT. Plumbing stacks/chases tend to be close to columns and walls - and if your new holes will be close to vertical elements then you need to aware of 1) reduction of punching shear capacity with new cores, and 2) hitting banded or uniform (or both) tendons in this proximity.

Even with scanning, PT shop drawings, and a careful contractor, tendons still get severed. They can be repaired assuming unbounded PT.

We also use FRP to strengthen slabs that warrant such with significant cores in close proximity.

 

[Engineerataltitude]

Client has no problem getting slab scanned. Is there a minimum edge distance that should be maintained between any new hole and existing tendons and rebar?

 

[MrHershey]

For straight (in plan) tendons, you can usually get pretty close. I think we usually call out 2"-3" clear for new construction. I would double that to 6" for coring in existing construction due to uncertainty in where the tendon actually is. Scan machine pictures aren't usually crystal clear, in my experience has usually been a fuzzy area a couple inches wide where the tendon is. Clarity gets worse with thicker slabs and if you've got crap (like conduit) in the way. If tendons are curved (in plan), would vary based on angle of curvature, whether you can tell if there are any hairpins present or not, etc.

Ingenuity - Agree that shore/detension is rarely done. That's more to force the owner into scanning than anything else, as it would in most cases be wildly more expensive and intrusive. I've usually had good owners that understood that scanning was necessary as well as contractors that weren't stupid enough to be willing to move forward without it. But in the event I didn't, I'd move to shore/detension and then if they don't accept that (or don't backpedal to scanning), I'd walk. Too risky. Owner can find someone else to ignore.

 

[Engineerataltitude]

Excellent advice. Thx