Have you ever cut a post-tensioned slab??

If so, tell me about it! Nightmare, or fairly feasible?? I’m spooked.

Any tips? We need to trench for a new toilet, flush threshold shower, and sink...

Side note, possibility of age-in-place house construction should somehow earn you extra development tax dollars or something. Blargh
Aug 7, 19 1:08 pm
Non Sequitur


I have no basis for my comment here, but iI'm thinking you would need to make some sort of additional cuts or add structure to the slab to divert the forces and prevent unwanted cracking.  I'm imagining chipping around the area and inserting rebar before making the cut.

Perhaps even saw-cut a steel angle and bolt to either side of the proposed trench.

Aug 7, 19 1:23 pm

We had a tenant fit out project in a cheap, junky strip mall where the very unskilled GC accidentally cut through multiple post-tension strands in the foundation slab.    The original installer of the post-tension strands came out and repaired it under the supervision of the original engineer.  Both charged fat prices that the GC had to eat.   Our contract stipulated no structural design or investigation by us, so we were not involved.

In other situations, I've seen a radar or x-ray scan used to locate the reinforcement and then the floor plan design gets driven by wherever new utilities can be placed (as determined by your structural engineer) and not compromise the system.

Aug 7, 19 1:25 pm

Get it x-rayed first

Aug 7, 19 1:27 pm

I had to do a dental office tenant improvement on a pre-stressed concrete floor (second floor). We decided to put a sub floor on top on said concrete to run all the plumbing lines. Than we ran an exterior plumbing chase that ran horizontal to pick up all the drain lines from the exam rooms to go around the second floor slab. Didnt touch floor slab at all.


are you saying cut the tensioned rebar, or just the concrete where there isn't stretchy rebar?  i would think post-tensioned rebar has to stay stretched from one end to the other, so if you cut it anywhere, the whole thing becomes useless.  if it doesn't need to be post-tensioned you might be ok, but if it was me i would move the toilet.

Aug 7, 19 1:31 pm

changed my mind.  this video shows them cutting the tensioned rebar, then re-tensioning it. 

Aug 7, 19 1:34 pm

I always trust youtube animations. Particularly ones where they glaze over cutting a steel bar stressed to 33,000 lbs.


I would not take on that responsibility as an architect, but I would ask a structural engineer if it was possible, and if it is possible, is it feasible?


See below... I've seen cables fail. I've seen recommendations to scrape a building with a failing slab on grade PT foundation; there is no way to repair it. PT's rely on the tension of the cables and compression of the concrete to work together to get the spans and strength; if either is compromised, the system fails. Theoretically, what they are proposing might work, but you'll need to relieve the tension on the rod slowly rather than cutting it. Not sure how they'd do that when they cut the pull side after tensioning at the anchor.


Small penetrations are usually ok - not a big deal - as long as you know where the tendons are and you stay far enough away from them.  You have to have testing done - GPR usually - to get a survey of the tendons (obviously don't trust any shops or as-builts).  You should involve an engineer - especially for any bigger penetration, or multiple close-together penetrations, or any that will be close to a tendon...   and of course you need to involve an engineer if any of your penetrations mean you need to cut one or more tendons - it's not impossible to cut them but you don't want to be the one deciding that.

Aug 7, 19 1:35 pm

Best method... xray and core through.  You can not trench. Hopefully, you are above grade PT.  That isn't that bad and I've done that on multiple projects.  Just locate the cables, avoid, and core straight through; sewer gets run below that slab. Get a structural engineer to design it.

If it's a pt-slab on grade, you'll need to cut a hole large enough to dig, then use a directional boring rig to core through the earth to that hole.  Expensive since you'll also need to excavate at the exterior for the boring rig and route the sewer around the building outside the foundation edge.  If you are a zero lot line, you are down to ejector pumps and a pit to pump it up high enough to route overhead.

Another method for slab on grade pt; topping slab, cast it in or provide chases, and change the floor level.  Another I've done is a wall hung toilet where I locate it close enough to the sewer stub you can route through the wall to get there.

How badly do you want to do this?

To put it in perspective, ask your structural if you can cut a 1/4" wide, 3/4" deep reveal in a PT to receive a flash ... they will tell you no. And yes; I've seen one fail as part of the forensics with failing anchors.  Goes off like a shotgun and the rebar shoots out; the one I saw shot through the bathroom, including the toilet at the foundation step. It'll come out at the anchor where they tensioned it since that anchor relies on the tension to clamp down (directional).  Basically you have a steel rod, tensioned to around 30,000lbs, in a greased tube; you don't want to cut it.  Additionally, the concrete of the slab is in heavy compression.  If you cut it or try to trench, you create a weak spot where a structural failure can occur.  

An architect shouldn't be advising or doing this.  It is highly doubtful you'll find a structural to go along with it either beyond a core or small knockout well away from the PT cables..  

Aug 7, 19 7:03 pm
Non Sequitur

I am thoroughly aroused by this answer. Cheers.


Bucket list items for most people include skydiving and seeing the Grand Canyon. Mine is seeing a PT tenon snap. But not in person. On video is good.

Non Sequitur

Good one Tintt... mine is seing one of those concrete filled buckets fall from the top of the crane onto an unoccupied row of portable toilets.


How about this one? Row after row of unbraced trusses flopping over one after another in beautiful succession like those dominos scenes kids set up.

Non Sequitur

Yes, but only if the last one fails to topple and the workers re-set them all up again for another go.


On the forensic side of stuff, I’ve seen a lot of aftermath over the years of resultant “building experiments” I can answer like: ‘what happens if you demo a bearing wall’, ‘if I undermine a foundation, what’s gonna happen next’, ‘can I use a thickened slab instead of a foundation?’, ‘let’s notch this beam’, ‘I wonder what the building is going to do if I slam a car into it’, ‘if I chuck golf ball hail with a tornado at a house do you think it can shatter the toilet inside’, ‘who needs a weather barrier’, ‘screw moment connections connecting two structures’, ‘lets flood the crawlspace for years and see what happens’, ‘I wonder if day laborers can frame and side’, ‘is frost protection really necessary?’, ‘roof vents are underrated’, ‘will a fire spread if I forgo draftstops?’, ‘thermal expansion is a myth’, ‘code minimums work the same in desert as it would in mountain as it would in hot/humid right?’, “does structure really deflect/shrink/expand that much?”, ‘my house/neighborhood actually slide down a hill?..pfft, the Geotech is conservative’, ‘geotech reports are recommendations, which are more like pirate guidelines right?’, ‘can high winds lift the trusses?’, ‘noise won’t pass through a wall’…. And current ones like “If I believe concrete above my podium will shed water, can I still make stalactites through the podium? Yes you can!”, followed by “what happens to saturated concrete over multiple freeze/thaw cycles”, “does rusty rebar really expand?’ and finally “what happens if I terminate dryer vents into the trusses?”. I’ve seen things…. The majority of which seem to be building experiments about proving simple physics is nothing more than opinion you don’t prescribe to; like gravity.


^ i'm just curious: what circumstances require a forensic analysis of a cracked toilet after a tornado strikes?

Non Sequitur

You don't use toilets as part of your load bearing walls?


The forensic side for that tornado one is more of a cya... Normally, they pulled us in when they want to scope the extent and/or the claim size hits some magic number on the books where they feel they need a third party stamped report. As a normally practicing architect, you'll get to do similar sorts of things dealing with existing structures. You see existing conditions that are pretty bad and add repairs to the scope of your remodel/addition. Forensic is just looking at the bad stuff, figuring out why it's bad, and making repair recommendations.


I'm not based in the US, so I don't know what regulations are applicable, but a possibility here in NL (with low ground-water levels) is to use a macerator pump. No need to cut new holes in the slab.

Aug 8, 19 3:47 am

look into stress concentration, deal them as plates, find the postensioned slab values

Aug 10, 19 4:24 pm

I'd do boundary element analysis to or finite element analysis you definitely don't want those tensioning to fly out and smack
the ceiling

Non Sequitur

you would not because you don't know what any of those words mean.


Be serious for once you got to take the axial load on the post-tensioning beam


I'm guessing that this slab is attached to the foundations so you have to be very careful that this slab does not break

Non Sequitur

Off your meds again?


chapter 17 roark's formulas stress & strain 8th edition

Aug 10, 19 5:20 pm

Going through this on a project right now.

The best advice above is to not take this on yourself - this is work for a structural engineer. Architect's are not engineers, and when you're dealing with post-tension, you definitely need someone who understands the stresses to make this call for you. Many factors go into deciding if you can cut the slab:

- How far from the top of the slab are the tendons?

- What is the drape/spread of the tendons?

- How closely spaced are the tendons?

- How many tendons are in a bundle? How large are those bundles?

The rule of thumb we've operated with is 6" away from the centerline of a bundle. So if your tendons are 4'-0"x4'-0" on center, you can get a 3'-0" square cut out of the middle of that, but then you cannot cut anything in any adjacent strand bays. 

Next best piece of advice that you got was to X-Ray. Never rely on existing drawings to tell you where the strands are, because they will almost always be off. The setting of the concrete and the tensioning of the strands causes them to drift within the slab, moving them from their originally drawn locations. 

If you do absolutely need to cut a strand, it can be done but is time consuming and expensive. The strand first needs to be de-tensioned, meaning the ends of the strand at the edge of slab need to be accessed. Once the stresses are out, the strand can be cut, new ends added, and it can be retensioned. Your structural engineer can tell you if additional reinforcing will then be required for your cut, or adjacent concrete. This can be steel plates/angle/supports or we even discussed carbon fiber "mat" to hold the structure together.

Aug 12, 19 11:55 am

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