PT Temorary Release Detail

[Lion06]

I've been looking for a good detail to temporarily release a PT slab from restraining wall elements that have multiple levels of wall and I'm having no luck. PTI has one detail they call a temporary release for this condition, but I see some issues with it and want something different. I specifically want to allow the slab to continue to shrink while the entire structure is going up and have the voids around the vertical wall bars grouted in at the end. The PTI detail, IMHO, only addresses the elastic shortening of the slab due to prestress and whatever shortening happens prior to the next wall lift being poured. That's not enough, to me, if you have very stiff elements at opposing ends and a pour strip is not an option.

This is specifically for shearwalls, because gravity walls can be a permanent release and then the detail gets a little easier.

With that in mind, an additional question I had is this. Is the bondbreaker critical to the release? If you get a hard trowel finish so there is no aggregate interlock, I would expect the prestress force to exceed the bond strength of the slab to wall interface and allow that movement. This may become problematic as additional floors are built and then the shrinkage may not be able to overcome all of the frictional force. My concern with a bondbreaker is that because of the cold joint, you're required to use shear friction for shear transfer across the joint for the in-plane shear forces. If you reduce the friction to essentially 0, you have no shear friction capacity. ACI 318 states that 0.6 was selected for the coefficient of friction of smooth concrete to smooth concrete to match test results, but that the primary force transfer is through dowel action. Is that reasonable to assume you can use 0.6 (or something close to it) for any interface, regardless of the coefficient of friction between the two surfaces?

I'm developing a detail, but it won't be an inexpensive detail, so I wanted to see if there is anything else out there.

 

[dcarr82775]

Maybe a corbel like ledge to support the slab with horizontal ducts through the wall to allow for strand/bar and grout solid later for continuity?

 

[KootK]

I think that sooner or later every PT designer feels compelled to try to reinvent this particular wheel. It's a right of passage. My attempt, from a decade ago, is shown below. I wouldn't do it again. The contractor didn't mind it at all but it made for a bunch of extra work for me:

1) Planning and communicating the locations of the "keys".
2) Checking the keys for bearing and shear.
3) Ensuring that the walls could span horizontally where there were openings on one side (at shafts, the slab is not attached to the walls for out of plane loads other than through friction).
4) Weird shear / punching shear conditions.

There were more issues, I just can't remember them off the cuff. I used this detail on a very long PT building with shafts at the ends. I saw little cracking but, also, little evidence of any real movement at my fancy joints. That's also been my experience with the PTI detail as well.

My impression is that it really doesn't take much clamping force on a wall/slab joint before slip becomes restrained. I have a hard time imagining any detail that would allow you to build up another dozen stories and still permit slip at the wall/slab joint. Maybe you can crack this nut however.

And yeah, no matter what you do, you need to wind up with a convincing wall/slab shear transfer mechanism. I don't think that bond breaker precludes the use of shear friction. You just lose the cohesion component. This is more easily accounted for in the Canadian code where the cohesion is broken out in to a separate term.

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.

 

[dcarr82775]

Actually, after thinking about it for more than 10 seconds, I decided I don't like my option. Nothing to see here, move along.

 

[dhengr]

KootK:
I like your sketches and details, they really help explain what you are talking about or trying to explain. But, the way you are adding or attaching them (not on just this thread either), immediately causes the entire thread to about triple in width on my computer screen. Am I the only one having this problem, or are others seeing this too. Maybe it’s just my problem on my end, on my computer, but I certainly don’t know how to fix it. And, when I have to scroll half way across the room to read the first line of the post, I quickly lose interest. Scrolling across the page is a real pain in the butt, in comparison to moving up and down the page. Whatever we attach to a post should not change the page width. But, there might be a need to be able to enlarge that block of the post, so we can read the print and details, size-wise, I would scroll right/left to see this. I couldn’t even print this thread without lopping off some of the right side of the page.

 

[KootK]

Yeah, you mentioned that once before dhengr. I'm happy to change my process but, at present, I don't know where it's going awry. I can't even experiment with it because it works fine on all my gadgets, even my iPhone.

Some diagnostics:

What browser are you using?
Home or work?
Is it my stuff and only my stuff?
Is it all of my stuff?

In the other post where you brought this up, somebody mentioned security settings on your browser. Did you try that?


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.

 

[dcarr82775]

I am using Chrome. I can't actually see your pics, just a 'image' placeholder. I have to right click, copy image URL, and then paste into a new window. That being said, this is the first time I have had to do that, normally they show up as an image in the post.

 

[dhengr]

KootK:
1. I’m using Firefox, but not sure of what version. They do seem to update it periodically, automatically. My service provider is Comcast and I believe they have Norton within their system. I also have Kaspersky as a security system on my machine. I am mostly computer illiterate, and don’t have anyone on this end to prevent me from throwing this damn thing out the window; I mean someone computer literate, an IT dept., who can show me how to fix things on the computer.
2. Home and work are one and the same computer. And, there are no special blocking or security features that I know of. I’m the only (not so capable) user of this computer.
3. It is not just your stuff. It seems to have to do with someone adding a sketch within a post. And, it seem to have something to do with the pixel width of that sketch. If you make that sketch/picture big enough (wide enough) so that it is readable, that causes the width of the entire post to explode in width. I believe I have seen pictures added which were small enough so they did not cause this width explosion. And, I believe you had posted such a small sketch, which gave me hope that the problem was fixed; then your next post (another thread) exploded and prompted my earlier comment. As I recall, this thread was normal width until you added your sketches/details, within your post.
4. It is not all of your stuff. If you just post a word reply, there is no problem with the width change. If you provided a PDF attachment or a link, I don’t believe that has ever caused the width problem.
5. This has all started since the forum admin. has moved the forums to the cloud, and they are apparently fiddling around with a bunch of other modifications in their system too.

Lion06:
I’m sorry for hijacking your thread like this. Please consider it akin to helping an old man across a busy street. Thanks.

 

[mike20793]

Post-Tensioning Concrete - Principles and Practice by Dirk Bondy and Bryan Allred has great practical detailing examples, specifically this one. I highly recommend this book as a reference for PT design. His father, Ken, basically "wrote the book" when it comes to PT design in the US and they are both extremely knowledgeable when it comes to constructability and engineering design.

 

[KootK]

@dhengr: I'll start playing with some variables. Please keep letting me know when you have problems. Without the feedback, I'll be flying blind.

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.

 

[asixth]

I too have been searching for the best temporary release detail. What I have found is that most contractors prefer to have PT ducts around the horizontal wall bars. This allows the bar to act as a dowel to transfer the loads to the wall in the construction case. 28 days after the slab is stressed the ducts get grouted up.

I have specified propriety items such as lockable dowels but contractors don't like to use them. Here's a detail I found.

 

[Trenno]

Seen that on a few of our jobs too, Asixth.

Similar detail for the case where slab is run over the top of a block wall, with PT ducts encasing the vertical starters/lap bars in the wall.

 

[Lion06]

Asixth -
The only issue with that detail is that if there is a wall above the slab for the next floor then either the contractor has to wait 28 days to pour the next wall or they get grouted in sooner. There's a good chance the latter will happen.

 

[Ron]

Lion06...sometimes you just have to go outside the box and do what YOU think is right. You're a smart guy with good experience. Modify the standards as you see fit and go for it.

 

[asixth]

Wait for what? The bars work as dowels in shear until they get grouted. It doesn't stop the construction of the structure above.

 

[Lion06]

Asixth- my apologies. I didn't look at your detail and the picture in my mind was different than your detail. That detail has a pinned connection and if you have the same condition on the opposing wall you won't be able to stress the PT.

Ron - thank you for your kind words. I'm going to develop a detail that makes sense to me for use on a current project. I'll see if I get any pushback from the contractor.

 

[KootK]

Somebody on here has a signature that reads "there's a reason that everybody does it that way". I always get a kick out of it because I've learned that lesson myself very slowly and with considerable discomfort. Whenever I think that I've invented a better mousetrap, four times out of five it turns out that I haven't.

Nowadays, whenever I come up with a "better mousetrap" detail, I run it by several senior colleagues and as many internet strangers as will listen before I put it into production. To that end, I hope that you'll post your improved mousetrap here for the consideration of the hive Lion06. I'm rooting for you.

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.

 

[Ron]

KootK....the equivalent to that is....I've got 30 years of experience so I know it works....when in actuality he has 1 year of experience 30 times....means nothing!

 

[KootK]

@Ron: yeah, I do feel a little guilty for not being more optimistic. However, since I posted a whack of high quality sketches showing my very own, personal, failed attempt at reinventing this particular wheel, I feel somewhat justified in what I consider to be a pragmatic view (rare for me).

I don't suspect that the conventional details are the best because I've used them many times. I suspect that the conventional details are the best because all of North America has used them thousands of times over the last few decades. Can you imagine how many hours Bijan Aalami must have spent so far, nodding off in the shower, trying to dream up a better release?

Still, when it comes to innovation, somebody's got to go first. I say bring it Lion06! Bring it! And, for goodness sake, please post it here.





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.

 

[KootK]

v2.0. Somehow, you gotta get the weight of the upper stories off of the slab. Maybe at the joint you could use larger vertical dowels capable of being unsupported over a (7-8") x 0.7 unbraced length.

Patent not pending.

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.