Cut post tensioned strands

[webotf]

I have a situation where a plumbing contractor sawcut an existing warehouse slab that he later learned was a post tensioned slab. The building is 350' X 80' with grade beams and caissons at 25' bays in the long direction and 26'8" bays in the short direction. The cut extends diagonally across an interior bay. The contractor is proposing cutting the tendons, adding a couple at the cut ends and then re-tensioning the tendons. Then replacing the slab. Is there an issue with retensioning the tendons before replaceing the 5 foot strip of concrete removed to find the ends? I am thining we at lease need to pour strips between the tendons to take the compressive force from the tensioning. I am also planning on doweling in repar at the edge of the patched area to transfer the shear from the new to existing slab. Has anyone dealt with a repair like this. Is there a typical way this repair is done for a full bay length?

 

[MiketheEngineer]

There are companies that specialize in that exact type of repair. Sorry - no names come to mind - but I am sure a good Google search will find them. This is not for the faint of heart.

 

[csd72]

webotf,

I agre, bring the specialists in. Call your local PT contractor and see if they can point you in the right direction.

 

[rowingengineer]

is this bonded or unbonded PT?

ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION.”

 

[hokie66]

Since he is talking about retensioning, it must be unbonded. In some places, they are silly enough to do that in slabs on grade, not to mention in suspended slabs.

 

[rapt]

Hokie,

You have been in Australia too long!!

 

[csd72]

hokie/rapt,

made me laugh and think of my original home.

The whole unbonded PT thing is too entrenched in some countries to change.

I always have the image of an unbonded tendon launching out through the end anchor if cut, suspect that is not really the case though.

 

[hawkaz]

csd72,
The tendons won't go shooting out very far, but the wedges could.

During demo of a elevated un-bonded PT slab, we had them set up plywood baracades in front of the anchorage points. As the tendons were cut, the wedges would dent up the plywood pretty good- but none penetrated it.

 

[rowingengineer]

Hokie,
you could be right, but the sentance about the contractor already cutting the slab would make for a fun day.

ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION.”

 

[OldPaperMaker]

I remember seing some pictures of a lift slab building that were taken after the 1964 Alaska earthqake. The building, which collapsed, was about 6 stories tall and had unbonded, post tensioned tendons in the slabs. Some of the tendons ended up embedded in the sides of buildings that were a few city blocks away.
Most engineers specified bonded tendons after this event.

 

[rapt]

OldPaperMaker,

Too bad the engineers in the other 51 states didn't listen. The post-tensioning world would now be a beter place if they had! Then, Alaska is never really considered to be part of USA except when they calculate their locally sourced oil quantities is it!!

Wonder how all the West Coast unbonded PT buildings are going to fare when the big one hits!

 

[apsix]

other 49

 

[larsacious]

webotf,

So have the tendons been cut yet or not? Your opening post is somewhat confusing. If the plumbing contractor has already cut through the tendons repair is the next step.

If the tendons have not been cut yet than I suggest that you have the slab scanned with ground penetrating radar. You may find that the tendons are bundled and you can avoid cutting them altogether. This may mean that some modifications will need to be made to the plumbing chase but this would be preferred to making structural repairs.

I just finished detailing shear transfer across a pt slab and it is not pretty or cheap and I would definitely avoid it if possible.

 

[TXStructural]

Assuming unbonded:
Yes, you will need to replace the cut out concrete to take the compression along the cut. You will have small leave-outs where the tendons will be repaired. The space required to make the repair connections is relatively small, and they will use "dog-bone" connections. The repair contractor should be experienced and they will have to verify the end anchorages.

It is not all that much trouble when it is a single tendon that is damaged, but this isn't quite as simple.

If they are unbonded, I expect that the anchorages have been unseated. I would also expect to be tendon snaking out of the slab on top or bottom, where the force cracked the tendon from the slab.

 

[hokie66]

I assume since this is a warehouse, the slab is a post-tensioned slab on ground. I don't really know why anybody would use unbonded tendons for that, as if cracks form, the tendons do nothing to control the cracks. Maybe it's bonded? If so, retensioning is not possible or necessary.

 

[webotf]

Thanks for all your input thus far. Slab is unbonded strands. The cut has already been made (completely throught the slab). We have a post tension concrete repair contractor on board. We have been hired by the owner to "keep him honest" and help with scope of work questiones in the planning stages etc. Fotunately there has been no evidence or strands or seats busting through the slab or flying through the air.

Strands are in both directions.

No cracks in the slab anwhere.

The building owner hired a scanning company to locate the tendons in the slab.

As far as we can tell they cut 25 or so tendons and all the top reinforing in one grade beam.

Quite a mistake by the cutter.

 

[Ron]

Another thought is to determine why you have a post-tensioned slab. We see a lot of post-tensioned slabs in our area for no good reason other than the PT Contractor had a lower price than the conventional slab guys. Unless you have structural reasons for the PT slab, I wouldn't worry too much about it; although, you mentioned grade beams, so I assume you might have soil conditions that dictated the PT slab.

 

[hokie66]

The main reason for using post-tensioned slabs on ground is to eliminate joints. If the tendons are bonded, then holes can be cut without much hassle, maybe just resulting in a bit of cracking at reentrant corners.

 

[larsacious]

Another reason for a post-tensioned slab on grade is when you construct on expansive soils. What region is your project located?

 

[webotf]

The soil conditions are very poor at this particular site. From what I can tell the slab was cast on the ground (about 20 years ago). There is now a 3" space between the bottom of the slab and the top of the soil. This area is know for soils comprised of marine clays and or poorly compacted fills.


This really is not a post tensioned slab on ground but a structural post tensioned slab supported on grade beams which are supported by caissons.

The project is in the north east where this type of construction is cost effective for poor soild situations when there is a high live load(250psf)on the slab.

Thanks TXStructural for your post. This is the sort of information I was looking for. Has anyone else seen repairs done in the manner TXStructural mentioned?