Repair of Double-T Bottom Tendon?

[1SEngineer]

A plumbing contractor has drilled and epoxied 3/8" dia. dowels into the bottom of the stems of Double-T's. If the bottom strand has been severed, what would be a solution to repair the problem?

I am considering having GPR performed to determine the actual depth of the bolt penetration, and if strands have been severed.

I am thinking of using carbon fiber on the bottom side of the stem to take the place of the severed strands.

Thanks for any comments.

 

[shin25]

I am not sure by reading your post if the strand is really severed of not.
Few questions will be -

1. Are the strands bonded or unbonded?
2. How many strands are damaged?
3. What is the location with regard to the span? Is it close to the center of the span or end of the span?
4. Without the damaged strand, can the design still work?

The effect of lost prestressing cannot reintroduced in the member without performing a second prestressing.

 

[lkjh345]

If it is determined that the strand(s) has been damaged, it is possible to post-tension the Double-T with external stands.

This is sometimes done to existing concrete structures to add capacity to members.

I have been to seminars where speakers describe this retro fit, but have never done this myself.

 

[1SEngineer]

Thanks shin25!

"1. Are the strands bonded or unbonded?"
Not Sure. It is an old, exsiting building.

"2. How many strands are damaged?"
Not Sure. It is unknown at this point.

"3. What is the location with regard to the span? Is it close to the center of the span or end of the span?"
It varies. Some are near the end and some are near the middle.

"4. Without the damaged strand, can the design still work?"
Not Sure. I might have to obtain as-builts and do calculations to determine this. However, I wouldn't feel comfortable leaving them with less strength than they originally had.

 

[1SEngineer]

Thanks LKJH345 for your input.

 

[Zambo]

Unless this is a more than averagely active plumbing contractor I would guess that he hasn't severed the strands. Even hitting rebar these guys will soon give up and drill a second hole - strands will take a lot of drilling power to sever.

The danger is that he has cut a few wires within the strand - but how to assess the liklihood and % damage. I think the only option is to plot the tendon profile and decide which are the most likely areas of damage, then carefully expose an area around a couple of dowels.

 

[shin25]

PCI has some tolerence on number of broken wires. If your findings show that you have none but very few severed wires then you may want to look into this PCI tolerence.

As zambo has mentioned, while drilling, drill bit usually deflects if it encounters strand or rebar. And, if the contractor is experienced, then he may stop and redrill. On the other hand, instead of drill, if he was coring then it will cut right thru.

 

[1SEngineer]

I am unaware of a text with calculations required for an external post tensioning repair.

Can anyone recommend one?

Thanks for all the help.

 

[Zambo]

I have negative thoughts about both external prestressing and carbon fibre.

Have you contacted your insurer?

What is the best way to see the damage that might have been done? Is GPR suitable? What about radiography? Ultrasonic I think not.

I still think no harm in a small local investigation by chipping the concrete away around a dowel where the tendon profile is nearest to the soffit of the beam.

 

[shin25]

1SEngineer,

First thing first, talk to the plumbing contractor and find out what they really have done? As, zambo has mentioned, it will be really difficult with a drill bit to cut thru an entire strand and not realizing it.

If the bad luck really prevails, try to find out the extent of damage. You can use NDT like GPR or any other suitable method.

If few broken wires, see if you are within PCI tolerence. If more than this, see if your design still works.

For the very last resort, you may need to consider external pt. This is not going to be easy, because this will require complicated anchorage design, hold down points etc. And more over will require speciality contractor to do the job.

 

[csd72]

shin25 made a good point,

With a regular percussion drill it is ussually very difficult to drill through the high strength steel strands. If it was diamond drilled then that is another matter.

I had this verified by a very experienced PT designer in Australia.

Also you may be able to verify the strand locations with a cover meter.

csd

 

[civilperson]

The average required embedment depth of a 3/8" insert is 2 5/8". This may miss the strands altogether. Ask the depth of the stop on the drill.