Plumbing holes cut in post-tensioned concrete slab. Repairs?

[Screwtape]

Background:
The building is a 3-story building for elderly and handicapped on the Mississippi coast that was built in the early 1980's. The construction consists of 7" post-tensioned concrete slabs supported by CMU walls. The typical floor layout is similar to a hotel, with individual rooms on each side, with a central non-load bearing corridor of metal studs framing along the length of the building.

The typical interior bay strand layout consists of 1/2-inch diameter strands at 24-inches on center, and the exterior bays have approximately 6 additional strands. The strands are "draped" and the depth varies.

Current Construction:
The building is currently being renovated and as a result of the addition of new plumbing lines, 9 holes approximately 6-inches in diameter were cut through the second floor concrete slabs. Somehow, each core cut through a strand, but since there were only 2 rows of holes, only 2 total strands were cut. IE, one complete strand was cut 5 times on one side of the hallway and another strand was cut 4 times on the opposite side of the corridor.

We have been asked by the construction company to examine the situation and determine what repairs (if any) are needed. We have the original construction drawings that include the stressing forces in the strands.

There is no structural damage or cracking, but one option we have looked at would be to size a steel flat plate with Hilti bolts to place on the underside of the floor that can handle the temporary (max) stress of approximately 33-kips.

Any suggestions for repairs?

 

[hokie66]

I would initially guess from the information that you have given that no rectification is required. But you need to compare the capacity of the floor in bending with the design bending moments.

What do you mean by the "temporary max stress of 33-kips"? Do you mean the force in one strand?

Do you really mean there are individual strands at 24" centers? Very unusual in posttensioned work for the strands not to be grouped in tendons. Are you sure the floor is not prettensioned hollow core planks?

 

[Screwtape]

Sorry for the confusion in the first post. I had only briefly reviewed the original drawings and I'm new to post-tensioning.

No, there are not individual strands at 24" centers. The PT plans show "strands at 24" OC" and I previously thought that strands and tendons could be used interchangeably. After looking again, the drawings state each tendon is composed of 1/2" diameter 270KSI strands conforming to ASTM A416.

The plans had a note that said "Stressing Forces" and were listed as:

- Maximum temporary = 33k
- Maximum anchor = 29k
- Maximum effective = 25k

 

[csd72]

Could possibly be a solid precast slab, I have heard of a few places that made those in the past.

I wouldnt think that the straps and anchors would work, these 'give'(move) a lot more than a PT strand and would really not take much weight.

difficult one.

csd

 

[mfrad]

You need a structural engineer familiar with post tension design and construction. The tendon is a seven wire strand with an ultimate tensile strength of 270,000 psi. The final stress (force) indicated is 60% x 270 ksi x .153 in^2 is 25 Kips. The strands that were cut are still highly stressed and can fail at any time in service. Typically a large sound is heard and a large loop of strand snaps through the concrete cover spalling concrete. This scares everyone since it could occur anywhere along the strand length. Therefore this condition needs repair.

 

[csd72]

You may want to give the contractor the following link for next time they drill into a pt slab.

http://www.undergroundsurveying.com/concreteimaging.html

mfrad made a very good point, the strands do not have any end anchorages and could therefore overstress the grout. It would be worth consulting a PT contractor for suggested repairs.

Even more important than knowing what you are doing is knowing what you dont know.

csd

 

[hokie66]

JD,
The information that Mfrad gave is good for a single strand. Just to be clear on nomenclature, a tendon consists of a group of strands, which can vary in number from one up to a lot of strands, but a 7" slab would usually have 2 to 4 strands in a tendon. But as you stated that the strands were at 24" centres, there may be only one strand per tendon, which is better than cutting a tendon with multiple strands.

I am not confident that you can replace the capacity of the strand(s) which have been cut without doing it externally to the slab. But the first step should be to determine if the cut strand (s) are necessary, and this can only be done by detailed analysis of the structure.