Post installed anchors in post tension concrete

[jplay2519]

How far do we need to stay away from the tendons? Is it the edge distance specified by the anchor manufacturer or is it specified by ACI?

 

[KootK]

I know of no formal guidelines. For the tendons, I think that you just need to avoid contact. For the anchors, I guess you'd need to give some thought as to what the concrete voids created by the tendons do to your presumed breakout cones.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Trenno]

100mm to be sure, to be sure.

If you're using post installed anchors, meaning the slab has been cast, the PT tendons will be hidden and their exact location only revealed by things like GPR, even then there may be some discrepancy.

 

[Teguci]

I usually approach with the question, how far away from the tendons can the design accommodate? I've always called for an inspector to x-ray and mark the surface. Then, I usually spec a 6" offset from the nearest tendon. I never design for anchors into the bottom of beams.

Consider, if anything touches a tendon, it's game over. The tendon is compromised, the slab will probably now be deficient and repairs to tendons are expensive and never leave the slab as good as it was. If we are talking about unbounded, you could even be looking at an explosive failure, requiring, if lucky, a change of pants and a long stay at the nearby bar.

 

[dik]

I usually use 6" min as a guide... don't want to get too close.

Dik

 

[jplay2519]

Thank for the input guys. They are definitely getting the tendons xrayed. I need to request some existing drawings to determine if they are bonded or unbonded, I believe they are unbonded due to previous site photographs but design documents tell the story I want to know.

 

[Ingenuity]

Consider, if anything touches a tendon, it's game over. The tendon is compromised, the slab will probably now be deficient and repairs to tendons are expensive and never leave the slab as good as it was. If we are talking about unbounded, you could even be looking at an explosive failure, requiring, if lucky, a change of pants and a long stay at the nearby bar.

The "explosive failure" is a bit of an urban legend, IMO. Granted, when a unbonded tendon is cut/severed it releases its elastic energy suddenly, but often with only a small exit of the strand tail from the stressing-end pocket, or nothing at all, so maybe a intermediate 'loop-failure' at top surface or soffit, with resulting concrete spall.

If an anchorage zone is being chipped out by a contractor, then an 'explosive failure' will likely occur, posing considerable safety issues to the operator and those in close proximity.

Here is a dead-end anchorage that was cored last month - drilled half-depth through the monostrand anchorage, one wedge, and the 7-wire strand released suddenly. Strand projected from the opposing live-end by about 3". Tendon was 50' long.

OP: X-ray is considerably more expensive than GPR scanning (about 5x in my area), and whilst it does give superior images, its cost and safety issues have resulted in that most concrete imaging today utilize GPR. however, GPR needs to undertaken by those experienced with it use and who have knowledge of the structural framing system too. There are considerable numbers of operators who drop $20k on a GPR system and 'sell' themselves as scanning experts, with little or no understanding of structural reinforcement systems.

Also, your 'design documents' may not be too helpful - they may give you how many kips of PT, and some high and low point drapes, and if unbonded - but to get a better appreciation of the PT layout you will need the PT shop drawings - and they can be very difficult to come by, especially for older structures.