PT Tendon profile in slab overhang 3

[dcceecy]

we are designing a two-way PT slab for a parking deck.
The typical span of slab (distributed tendons direction) is 30 ft
at the pour strip, we have two conditions.
At first condition, there will be a 18 ft wide pour strip. So the overhang of slab from each side is about 6'. at this, the tendons can be straight in the slab overhang.

At 2nd condition, there will be a 4 ft wide pour strip. So the overhang of slab from each side is about 13'. at this, the tendons should be straight or reverse parabolic?

since the pour strip will be covered with closure panel, we are debating what profile should we use.

Thank you for your input.

 

[RHTPE]

Is this a single level parking deck?

Pour strips always create constructability challenges in multi-level structures when it comes to cycling formwork & shoring.

Other than that comment, I'll leave the tendon profile question to better minds than mine.



Ralph
Structures Consulting
Northeast USA

 

[dcceecy]

thank you for the input.

We use ADAPT to check the slab. the typical detail (from previous job) show if the overhang is short. The tendons could be straight in overhang. (long full span will control the PT force anyway)

If the overhang long, we are discussing if the reverse parabolic should stop at the inflection point (of the span with the pour strip), then the tendon should be straight from the inflection point to the free end (before we pour the closure panel)

if we use straight profile from the support to the free end, the PT force will be higher than normal conditions (I guess the big overhang will control the PT force)

any comments are appreciated.

 

[RHTPE]

If this is a multi-story building and if cycling of formwork is essential, then the ability to completely remove shoring & formwork BEFORE the closure placement is complete can be important to the contractor.

If the cantilevered slab must remain shored until the closure placement is complete and the concrete has reached an appropriate strength, then there can be the problem of "stacked" shoring. By stacked shoring I mean that the next level is placed before the level supporting it is fully complete. This can almost double (or triple) the load in the lowest level of shoring. Which, if not anticipated, can overload the shoring.

This can be a nightmare for the contractor.


Ralph
Structures Consulting
Northeast USA

 

[rapt]

dcceecy

Look at the overall condition after the pour strip is poured and everything is continuous. This will give critical negative moments at the support and the tendons will have to be high for this. The initial condition is not the critical one. The "cantilever" in the initial condition is a continuous span in the final condition and all of the loads from the infill have to be applied plus extra DL and LL on the whole span giving high negative moments at the supports and high positive moments in the bottom of the infill slab.

Otherwise, the infill is detailed as purely a simply supported infill with pin connections to the "cantilevers" in which case thee loadings on the pour strip area are point loads on the ends and there is no continuity.

In both cases the tendons need to be high at the supports for the high negative moments. But you need to analyse it based on the way you are detailing the connections and designing the infill slab.

Looking at the final condition would actually tell you where you should put the construction joints.

Secondly, a cantilever does not have a reverse parabola, it is half of a normal parabola. It should look the same as your normal bending moment diagram drawn with moments on the tension face.

 

[dcceecy]

Thank you for the input.

Right now the tendon is high at the support, then running straight to the CGS at free end.

I treated the pour closure panel as a simply supported slab. So in the ADAPT model, half of the load from closure panel goes to the tip of overhang.

If I want to check the span again as it is continuous, maybe I should assume we have the harped drapes in that span.

 

[dcceecy]

We are doing a two-story PT parking deck
in the original design, we make the pour strip 18 ft wide. So the overhang is only 6 ft from both sides.
But somehow, the contractor want to do a 4 ft pour strip at one location. So we have an issue.

right now, I checked both conditions for the span with pour strip: cantilever with concentrated load from the closure panel, continuous span as typical span

it seems ok the way we did now. But the PT will be higher than those if there is no pour strip or pour strip is 18 ft wide.

 

[rapt]

dcceecy


The tendon would be straight from the high point if the infill is being treated as pinned at the connections resulting in point loads at the connection points. If it is being designed as continuous in the final condition I would make it parabolic. But either way would probably work ok.

If the builder wants a small pour strip, is it at mid span or near one end?
- If mid span you will have problems with tendon profiling to get the tendons low enough at the connection points for the large +ve moments there and the "cantilevers" would have to be propped until the connection is made. So it eventually becomes continuous. It would cost a lot to make it work as cantilevers.
- normally you would put it at about .2 * span from one end. The long end would have to be propped until the connection is complete, but this way the reinforced pour strip is near the point of contra flexure so in a lower stressed area.

This should not affect the amount of prestress.

The main thing is that you detail it the way you are analysing it. I have seen people put minimum reinforcement in the bottom of the pour strip because it is "spanning a short distance to the ends of the cantilevers" but then design the "cantilevers" assuming that the whole slab is continuous and there is a need for full moment capacity for the span in the bottom of the pour strip and the moment in the bottom is for the much longer span.

 

[KootK]

I'm having a hard time imagining how having an 18' delay strip in the middle of a 30' foot span would be in any way efficient. Wouldn't that leave 60% of your slab span devoid of prestress in the uniform direction? Or are you prestressing the closure strip somehow?

Are you providing such a wide pour strip for some unusual reason such as vertical access for some equipment or operation?

If your contractor is proposing a narrow strip in the vicinity of the quarter span, that sounds spot on to me.

- it's presumably near a location of low moment.
- it's presumably at a location where the tendons are in the neighbourhood of slab mid-depth so that they can be conveniently terminated and stressed.
- it should result in a long cantilever that may require shoring but should not lift off of the formwork at all.
- it should result in a short cantilever that may not require shoring and should lift off from the formwork only an inconsequential amount if at all.


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.

 

[dcceecy]

Thank you for the input.

The slab is 8" and the 4' wide strip will be at the middle of span.
In this case, the high point at support will be 6.25", the low point at the free end will be 4".
I checked for two conditions, first the cantilever with point load at free end; 2nd condition, continuous span with the harped drapes tendon profile ...6.25" 4" 4" 6.25" ...
Then the PT force will be 30% more than the typical condition to make it work.

 

[KootK]

Unless I badly misunderstand the nature of your project, I still feel that you're on the wrong track here. In my opinion, the correct course would be as follows, assuming that you're sticking with the mid-span pour strip.

1) Plan to keep the cantilever spans shored until the pour strip is cast.

2) Don't bother checking the cantilever & point load case because, with the shores left in place, that condition will never be relevant.

3) Drape your tendons parabolically as you would would for any uniformly loaded span. Given the width of your pour strip and the need to ease the tendon profile in from the live/dead end, your cables probably wouldn't actually be straight until nearly the span third points anyhow.

4) Get your tendons as low as you can at midspan, keeping in mind stressing jack clearances. You should be able to get down to at least 2.5" or so I'd think.

5) Consider whether your "cantilevers" might lift off of the form work when initially stressed. It's unlikely given your geometry.

6) Design the pour strip reinforcement to deal with whatever moments and shears will occur at that location.

Approaching it this way, you ought to be able to get your PT force down in the delay strip span.

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.

 

[rapt]

Agree with you KootK (oh no!!!). I still cannot understand why the pour strip is at mid span. I would only do this when the span the pour strip is in is much smaller than the other spans so +ve moments and deflections are not as bad.

It is not a 2 cantilever condition. The "cantilevers" should be propped until the pour strip has reached full strength and the design treated as continuous. The pour strip will need a lot of bottom reinforcement in both directions (not just shrinkage and temperature minimum steel). Putting the 4' pour strip at mid span will just about triple your deflections in that span compared to a normal continuous PT slab.
Using the longer pour strip is much worse, and I cannot understand how you can justify it using the PT banded distributed logic as it is basically an RC slab when it is 2/3 of the span!

Are you using Adapt PT to check it? How does it handle the reinforced length of the frame? In the old days it could not do it properly and I doubt that its deflection checks would be right for it now.

I also cannot believe that you are using an 8" slab for 30'. I hope the end span is not 30', and you have a good solution for punching shear.

 

[KootK]

Agree with you KootK (oh no!!!).

What now? Full agreement with no caveats? No, wait! Don't even answer that. I'm just going to shut my trap and ride the high as long as it lasts...

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.

 

[hokie66]

I don't understand the objection to the pour strip being at midspan, as long as the slab remains shored until the infill is complete. That is where I normally like to have construction joints, PT or otherwise.

Agree that 8" is too thin for a 30' span.

 

[rapt]

Hokie,

The resulting RC section at mid span caused an increase in deflection of about 2.5 times in a quick example I ran (in a program that considers cracking and long term effects properly!!), from about 17mm to 45mm and requires a very large amount of reinforcement. A 8" RC slab in the middle of a 30' span just is not logical. Move it to the point of contra flexure where it does not have much effect.

The "cantilevers" cannot be made to work logically so it has to be a continuous span.

 

[dcceecy]

this is a two-way PT slab for parking deck, so It think 8" for 30' span is acceptable. I have seen several previous projects similar.

The shoring will be there all the time until the set of pour strip. So I agree the cantilever conditions will not be exist. I was conservative to check the cantilever conditions with point load.

As I said, I also checked the span with the pour strip as a typical continuous span with the harped tendon profile (low point is the CGS at both ends of the pour strip)

it seems still OK. Since the the low point is at CGS of 4" instead of 1.25 or 1.5" as other typical span, it is expected that we'll have more deflection at this span unless we add PT more.

Even the span with pour strip is continuous at final condition, the pour strip reinforcement in our detail seems enough for the maximum positive moment since we used the same detail as the 18 ft wide (span) strip in other locations.

 

[FoxSE14]

If I am remembering correctly, for our one-way conditions, we had added tendons for the cantilever condition and started them some distance into back span; tendon at jacking location was at slab centroid. We used Lenton form savers to lap bars across the pour strip when pour time came.

1a. If you are in a snow area - movement of the cantilever tip at the roof area can cause minor bumps at least and serious maintenance issues (future phone calls for you) at worst when snow plows drag their blade and begin chipping concrete at the pour strip. In a previous life we gave a general recommendation that the garage be maintained with rubber tipped plow blades to alleviate this concern.​

Check publications by PTI and within various structural engineer magazines -- there are some good guidelines out there on proper pour strip design that a google search should unearth.

 

[rapt]

As we do not know the details of the design you have done we can only comment on what could need to be looked at based on our experience with other people doing similar designs and point out the mistakes they have made!

In Australia, you would never do a 30' end span with an 8" flat plate slab (even if you converted it to sensible units)! The amount of prestress to make deflections work would not be economical. Punching would be a worry too and we would also be worried about vibrations with that span/depth in a parking garage.

Yes, you will get more deflection because of the terminating tendons at the centroid, but you will a lot more increase in deflection from the RC pour strip joining the PT slabs together. And positioning it at mid span will give the worst effect.