Post tensioned slab on grade remodel

[roberto222]

I am doing remodel where is post tensioned slab on grade.
I need to add some square spread footings below to support new columns. How can i get thru the slab? Can I cut the tendons? How would I retrofit them if i cut them?
Thanks.

 

[msquared48]

I would try to work around the existing tendons here. If you do not have any plans, you will need to mark the tendons on the slab to avoid cutting them.

Are the tendons bonded or unbonded?

Is this a PT Matt footing? If it is a matt footing, I would try to keep the loads as similar to the original as possible.

Mike McCann
MMC Engineering

 

[roberto222]

3 story, wood framed, 50'x300'. PT slab on grade with unbonded tendons. On asbuilts are shown continuous footings under bearing walls and regular slab on grade between.
The contractor cutted one tendon when he was cutting the slab for new square ftg.

 

[msquared48]

So you have a mix of "regular", or non post-tensioned, and post-tensioned slabs on grade?

If so, I would look at the geo tech report as there may be a bearing, excessive settlement problem with the subgrade in the PT area. In other words, more settlement possible in the area of the PT than the non PT slab.

If not, then I guess I am all wet here...

Mike McCann
MMC Engineering

 

[roberto222]

The footings are most probably grade beams at 15' o.c. with perpendicular tendons over them for the slab. But the slab is sitting right on the soil, there is no any gap between.

 

[msquared48]

Perhaps not when it was designed, but maybe not now. Whar did the hole the contractor made show?

Mike McCann
MMC Engineering

 

[roberto222]

here is the link to the photo

 

[roberto222]

Has anybody experience with strand splice chucks? Is it possible to use them inside of the slab cut without exposing the ends of the strands at the ends of the slab?

 

[hokie66]

You could put a coupler on, but you can't restress the strand internally...not to my knowledge. That's one (of the many) disadvantages of unbonded prestress.

If you have only cut one strand, as long as you are careful not do damage any more, I would tend to just leave it.

 

[Ingenuity]

It is possible to internally splice AND re-stress a severed monostrand tendon in slabs, WITHOUT accessing the original stressing end anchorages. Re-stressing is undertaken using a center-stressing anchorage (analogous to a belt-buckle) - similar to that used on circular tank construction. See LOKCOUPLERS - developed by LANG TENDONS a long time ago.

I have repaired many severed unbonded tendons INTERNALLY without accessing the existing end anchorages, but best to check with a local PT contractor in your area to see if they have the experience to do such repairs.

Or, if the tendon is not excessively long, use a "GRABB-IT" combination splice chuck (see

http://www.precisionpt.com)

that incorporates a turn-buckle thread-arrangement that enables the strand to be re-stressed using a calibrated wrench but these are a pain to use.

The problem with both of the above stressing/splicing combination options is that the stressing hardware is bulky and for thin slabs it is difficult to accommodate, and also provide long-term durability to the hardware whilst also maintaining unbonded construction. Often the re-stressing hardware is cast-in and therefore the hardware becomes "bonded" - which I personally think is a benefit.

 

[Ingenuity]

roberto222,

Judging by the photo, it looks like the tendon depth at the cut location is very near the bottom of the slab. This makes splicing/stressing at this location near impossible to accommodate the necessary hardware. If the tendon depth varies along its length it may be possible to accommodate the hardware where the tendon is closer to mid-depth of slab on either side of the cut, and therefore replace a segment of strand too, to 'link' the splice chuck to the center-stressing anchorage.

 

[roberto222]

Here is a better photo.

Does anybody know how can i set the prestressing load into the splicer if i don't know what was the stress before?

Thanks.

 

[roberto222]

Also I was thinking if the wedge effect at the end anchors couldn't be lost when it lost it's tension. So if the splicing with the splicer couldn't be a little without effect?
And there is no way to check the original ends of the strands.
Any thoughts?

 

[hokie66]

My thought was...why do you need the strand? What was it doing? What are the consequences of ignoring one cut strand? If this were a suspended slab, and the strand was required by analysis, then Ingenuity has given useful advice, but for a slab on ground, the decision making process is different.

 

[Ingenuity]

I agree with HOKIE66. Slab on grade only, consequences of one strand loss is very minor.

When an UNBONDED tendon is cut the sudden release of energy does send a shock load to the end anchorages and there is always a possibility that the anchorage wedges may be dislodged. Is is more probable that the wedges will NOT be displaced.

As far as what would be the re-stressing load to place back into a cut strand, you simply re-stress to NOT exceed 80% of MUTS, and you could elect to re-stress to say only 65-70% based upon the fact that long-term PT losses have taken place, so adjacent strands are probably at a prestress level of 65%.

Anyway, I would do what HOKIE66 stated. And very quick and inexpensive

 

[Margaretperron]

As post-tensioned foundations have become more common place in residential construction, remodels and additions on these foundations have also become more frequent. If a note or plaque isn’t available, the governing building department may have the existing structural drawings which should contain the design or other indications of the foundation system. If a design/build contractor was used which is fairly common in tract home renovations, the actual foundation drawings may not be available at the city, since they were most likely a deferred submittal.