Post-tensioning tendons - proximity to opening

[hope9010]

There is an existing opening 51" x 24" thru an existing post-tensioned slab. They want to increase this by 1.5" each of the 4 sides, at each of a number of floors, for a larger size duct, so that the opening will become 54" x 27" (see attached). We have had x-rays taken of the tendons. We have to allow some tolerance for the tendon location, when interpreting the x-ray.

Question:

How close to the opening can the tendon be, given the curvature of the tendon? Is there any industry standard for this?
(We realize that we should try to maintain at least the required fire protection, but our question is asked from the perspective of the tendon breaking out of the concrete due to its change in direction).

 

[BAretired]

Maintaining fire cover is the only consideration. The tendon you show in plan has no tendency to break out of the concrete. It exerts a lateral force on the concrete beyond the edges of the opening.

BA

 

[TehMightyEngineer]

Forgive my ignorance as I've only had limited experience with post-tensioned slabs but why doesn't the tendon exert a lateral force on the concrete? Is it only because it starts curving past the extents of the opening?

Maine EIT, Civil/Structural.

 

[hope9010]

That is what I was thinking, but bearing in mind that the opening is to be enlarged in both the north-south AND the east-west directions, the tendon may exert a lateral force on the concrete within the length of the enlarged opening (depending upon where the bend on the tendon is located relative to the opening corner). However since the enlargement is only 1.5" at each end, the effect may be small, so I think I can take it as you say, that it is ok. Thanks.

 

[hope9010]

I sent my last message (in response to BARetired before I read TehMightyEngineer's message. I think TehMightyEngineer's is saying essentially the same as I said.

 

[Ingenuity]

Do you have x-ray shots of the tendons, or are they GPR (sub-surface radar) scans?

If you have x-Ray shots at the 4 corners of the opening/s you should be able to determine the existing cover to the side of the closest tendon. If you have GPR scans you accuracy to determine side cover is NOT going to be good as x-Ray results.

We have undertaken numerous repairs of tendons to situations similar to what you are trying to achieve, primarily as a result of tendons being constructed with insufficient (zero?) side cover, and tendons sweeps in plan that 'cut the corners' of the opening.

Our preferred detail, most especially if the scan results are questionable to the magnitude of the side cover, is to jack hammer the corners of the openings at the top surface and determine the tendon locations directly, and from there proceed with making the opening wider based upon those "exploratory" probes.

Are you proposing that the 1.5" of cover concrete to be removed is going to be done via a concrete saw, or manually with a chipping/jack hammer.

 

[hope9010]

Ingenuity - we have x-rays. The hole will be enlarged by core drilling, not sawcutting. There are top and bottom rebars in the slab at the sides of the opening so jack hammering down from the top of the slab to locate the tendons would not work.

 

[Ingenuity]

hope9010

I just had another look at your marked up drawing attachment. There is a note on the original drawing that states "FIELD DEAD ENDS" and it possibly refers to the small tick marks representing dead ends for the uniform tendons that are adjacent to a solid line that is possibly a construction joint (running north-south). Anyway, if so, something else to watch out for in one corner of where you wish to increase the opening size.

If your X-ray photos are of decent resolution are you able to scale them and determine the side cover?

After enlarging the opening, 2" or greater is my preferred desired side cover for PT - may get away with less for interior condition etc.

Coring for the corners makes sense, but stitch/line coring for the 4 sides to remove 1-1/2" is going to be a real pain (will take some time and water/slurry control may be an issue) and you may still have to chip the 'dags' to square up the cut line anyway.

I would manually chip the lowest level slab where the opening enlargement is proposed (the first level where the original contractor was getting it all worked out/set up for sweeping tendons around openings) and work from the top at one corner, expose the top trimming rebar, then expose the PT - all using say a 15-20lm jack hammer. If the dimensional results correlate with X-day scaling then you are probably safe to proceed with the upper levels based upon the X-ray results only.

We have repaired many many tendons where X-ray, GPR etc were first used and given the all clear for core holes, openings etc only to sever multiple tendons and have to undertake repairs. Usually the tech using the NDT equipment got it wrong in these instances.

 

[hokie66]

That tendon which is displaced around the opening would not appear to be doing much. I would look at reassessing that section of the slab to see if the tendon can just be cut.

 

[Ingenuity]

Hokie66, agree, at the location of the opening where supporting walls appear to be, but this is UNBONDED PT, and a couple spans to the right there are no walls, and a cut tendon at ONE location is a lost tendon at ALL locations for UNBONDED PT. So it maybe possible to cut, just need to be checked out. Or tendon terminated either side of opening via new anchorages etc.

 

[hokie66]

Right, I see. How did you know they were unbonded? I suppose that is a good assumption, as most of the questions here about PT do seem to be from "unbonded" areas.

 

[Ingenuity]

How did you know they were unbonded?

Well the tell-tale sign for me that they are UNbonded tendons is the mid-70's practice of using graphic symbols to describe the # of UNbonded MONOstrand tendons, namely: half-circle = 1 tendon, full circle = 2 tendons, triangle = 3 tendons, square = 4 tendons, , and diamond = 5 tendons.

 

[jayrod12]

Thanks Ingenuity. That is the new thing I learned today.

 

[hokie66]

Me too. I have never done any structures using unbonded PT, so thus my ignorance of this practice. That's the thing about this site, there seems to always be someone around who knows.

 

[hope9010]

Yes they are un-bonded tendons. I am posting this all for another engineer in the office, so thank you all for your comments, which I will relay to him, including about how to make the opening. It is great to have comments that stay focused on the question at hand. I will convey all these comments. It is a interesting that Ingenuity has found that the x-rays are the most accurate, and that the high tech stuff can cause problems...that is about what I would expect from my experience as well. We sure as heck don't want to end up cutting any tendons that are required for the strength of the slab. (By the way, I am also known as ajk1 on this system).

 

[hokie66]

Why don't you ask management of the site to combine your memberships? Makes it simpler and more personal. People get to know each other, and it is difficult to deal with split personalities.

 

[Ingenuity]

...and if you use different computers to log on with (say home, work, and mobile) you are able to do so using a single username too, and actually stay logged on to Eng-Tips on multiple computers if so desired - all under one username.

 

[KootK]

In the interest of full disclosure, I was "ahpearson" and "KootenayKid" before I became KootK. Always the same account though. I've been tempted to change my name yet again. Sometimes I wonder if KootK might be a little too close to KKK in terms of looks and phonetics. I don't want to change my name again though as I would have to restart the whole "getting to know people" process.

If I had my druthers, I would also have two separate accounts. I would like one, primary account for getting to know people and a secondary account for when I would prefer to have utter anonymity. Some projects have confidential dimensions to them. Additionally, I occasionally have technical questions that are so silly that I'm hesitant to broach them even here.

Now that I think of it, I know the answer. Everybody should have only one account but you should have the option of posting / replying anonymously if your choose. How about that?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[BAretired]

You may think you have technical questions that are too silly to present on this forum, but if you don't understand something, chances are there are many others who don't either. You could always start out with "I know this sounds silly but something which has always bothered me is...". I doubt that anyone will really think it is silly when they try to respond intelligently to your concerns.

As for confidential aspects of projects, it is probably best not to reveal them whether posting under your forum name or anonymously. I have been on sites where anonymity was permitted, but I never used it. The administration would have to guard against offensive comments from anonymous posters which could use up a lot of their time.

BA

 

[KootK]

I dunno BA. Perhaps some examples are in order:

1) I don't understand rebar anchorage. As far as I can tell, neither does anybody else. Everyone just seems to treat a developed bar as though it is a bar that could not be pulled out from the concrete in which it is embedded. That's wrong. Rebar anchorage is rather important to the whole RC design thing. I design a lot of concrete buildings. This is a question that I've been hesitant to bring up semi-publicly for some time.

2) Many years ago, I was assigned the task of creating a whitepaper on the use of a concrete admixture that is produced in my (our) area of practice. There wasn't much information readily available so I thought to myself "hey, I should ask my engi-tips buddies!". I did exactly that and managed to find GOBS of relevant information in the process. However, several people on the forum pounced on me because I didn't instantly recognize the term SCM when it was mentioned. They carried on about how, if I didn't know that term, I had no business doing any work on concrete admixtures.

Of course, I'm quite familiar with the term supplementary cementitious material. I just had a brain fart when I first saw the acronym. If this wasn't bad enough, my client then stumbled across the thread and noticed where I'm located. He contacted my boss and said something to the effect of "why are we paying you to do this work for us if you're just trolling around the internet for information?". Then, of course, my boss paid me a visit. I defended my strategy successfully to both client and manager but it was not a particularly pleasant afternoon. In that instance, I wish that my profile had me listed as working in Belize.

3) In general, this is a community more interested in practical matters than theoretical ones. Usually this suits me but, sometimes, it doesn't. A review of the threads that I've started will reveal that I often drift into areas that others find overly pedantic. I worry about this because I fear that my forays into pedantry will make me seem like an impractical spaz -- which is only partly true -- within a community that I value highly. In the past, I've considered tracking down JAE, showing up on his doorstep, and seeing if he'll take me on as his protege. Will engineer for room and board. JAE probably wouldn't take me on because he's seen my ridiculous diatribe about composite beam theory.

As I understand it, the administrators of this forum are big fans of anonymity. I believe that we are discouraged from posting our email addresses or contacting one another directly for that reason. As an interesting aside, I'm fairly certain that you and I live in the same geographical area. And, through our mutual participation on the site, I know that you know stuff. Lots of stuff. I've often considered reaching out to you to see if you might be willing to serve as a sort of external mentor for me. But, alas, we are two semi-anonymous ships passing in the night...



The greatest trick that bond stress ever pulled was convincing the world it didn't exist.