PT two way slab 2

[lisia]

Hi, I'm an italian structural engineer.
A question about PT two way slab: is it possible to design reinforcement layout with ordinary reinforcement and unbonded tendons in "column strip" for each direction and just ordinary reinforcement in "middle strip"?
(banded tendons in two way direction just in column strips)
Thank you.

 

[wildehond]

Rapt
Thanks for your detailed answer. But I'm unsure that I'm getting the my point accross.
If you consider the simplest of harped tendon profiles, then each change of angle produces a vertical load, some up some down. Across the length of the profile these forces total zero. So there is no nett vertical load on the structure. But if you model the positions of all these loads at the positions of the changes of angle (say in continuous beam analysis software), then the resulting moments and shears will be the values that must be reinforced for. In continuous spans, this will even account for the secondary moments.

The only effect that hasn't been modelled in the above is compression in the concrete. This compression in slabs must in any case be treated with some judgement in the end spans, but it is simple to calculate the moment capacity of an even compression load and then adjust the moments from the software results before choosing the bonded reinforcing.

If you accept the above logic as correct, then it is a short step from there to use FEM software to analyze a plate with multiple spans in both directions

 

[iyota]

RAPT,

So doing a column-middle strip configuration is too hard for unbonded tendons? -I was never aware of that. Is it more of a construction issue rather than design?

I guess i'm asking because last year we had a PT slab contracted out to a PT company. And it was designed by one of the authors of TR43 himself. He actually used unbonded tendons with a column-middle strip arrangement.

Do you think he wasn't actually aware, or maybe he found a way around the problem?

Thanks...

 

[rapt]

wildehond,

I understand you perfectly.

You are not understanding me. Yes, you can model the effects of the PT as applied forces to determine a PT moment/shear diagram. This produces MP. This can then be represented as 2 components at any cross-section, P*e (e = eccentricity of P at that cross-section) and Msec = Mp - P*e.

But in design, we calculate a section capacity based on the steel in the section and the concrete. The capacity of the cross-section includes the P*e component of the prestress effect on the concrete.

In design we have an applied moment M' and a concrete capacity Mu. The P*e component of the prestress effect is placed in the Mu side of the equation as it is part of the ultimate strength calculation, increasing the strength. It cannot also be used on the applied moment side reducing the applied moment, so the ap[plied moment is calculated as

M' = 1.2DL + 1.5LL + 1.0 Msec

Mu must then be greater than M'.

As hokie66 said, the prestress effects are treated as an interanl action, not an applied load. They cannot be treated as both.

What you are suggesting by ntreating the PT as a series of applied loads is simply a way of calculating the prestress effects on a slab.

You then have to understand how to use them. If you have been including it on both sides then you are grossly under-designing.

 

[rapt]

iyota,

Yes, it is a construction issue. It is possible but it is difficult. Unbonded tendons are made up full length before being delivered to site. So for a 50m square building, they would be 50m long.

For a column/middle strip layout, the tendons do not sit in specific layers, the tendons in the 2 directions interweave
throughout the slab to achieve their profiles. This is very hard to do with 50m long tendons.

All members of the TR43 committee understand this (I am one) as far as I know. One of the members is an unbonded proponent so I can guess who it was, I just cannot believe that he has done a proper column/middle strip layout with unbonded tendons. Did he actually have column and middle strip layout of tendons in both directions? Or was it banded/distributed with some extra intermediate tendons between the bands?

 

[wildehond]

Rapt, Thanks for your patient reply. No, I wasn't suggesting that you could use the PT on "both sides of the equation". But that by using it on the load side, it provided a simpler way of solving 2 way spanning slabs with odd support conditions. And Mu is simply provided with bonded "normal" reinforcing

 

[slickdeals]

Just out of curiosity:
I have never used the effects of the vertical component in shear strength Vp, as permitted under ACI 318.

Do you typically use the vertical component in shear calculations, at least on deep transfer beams? If so, what is the effect of the reversing of curvature on continuous PT beams? How far away from the face of the support are you allowed to account for Vp?

 

[iyota]

Rapt,

I'm pretty sure he used column-middle strip arrangement. It was a building in London we recently finished and the column spacing was 7.5x7.5m square bays. That's why I'm sure if I saw a banded-distributed arrangement, I would have commented on it.

Also, his firm advocates unbonded tendons because they can apparently zig-zag around obstructions better than bonded tendons. I agree in theory, but now that you mentioned a fixed length on-site, is that even possible??

However,, his firm maybe also fabricates its own tendons and does the layout on-site, just like a number of others in the UK. I'm not sure about that...

 

[rapt]

iyota,
Yes, the only benefit of unbonded tendons in my opinion is that they can move horizontally relatively easily.

But there are a lot of negatives, many oif which are more important to me, so I would never touch the stuff (I did many years ago when I was young and stupid).

If he is a PT supplier then he is not the person I was thinking of. The person I was thinking of runs a design business and does design for serveral of the suppliers.

 

[rapt]

Wildehond,

OK, it is just that several people previously have basically suggested the possibility.

That would be conservative as there is more capacity available from the tendons than you would get from the longterm tendon forces only, even with unbonded tendons.

Except that the P*e effect should then be reduced by the capacity reduction factor, or for South Africa, the material factor which I think is 1.15, so your load factor should have been .87 for the PT moments!

 

[rapt]

slickdeals,

Yes, you can allow for it in the Vc calculation (not the principal shear, only the flexural). But you must take into account the reverse curve regions properly (as some US software does not!!!).

 

[iyota]

Rapt,

No, this guy has a design firm. Whether or not his firm also supplies the strands and does the tensioning, I don't know. Retired now and apparently does PT design himself in projects whenever he feels like it.

But anyway, thanks for thanks for the info. Nice to hear things I never knew before. A number of people in my office like unbonded systems. They think the risks are over-rated. So I just wanted to get an opinion from people who discourage unbonded PT.

 

[rapt]

Iyota,

Same person then. They are not involved in the construction directly, only design. He has been pushing unbonded PT with frame contractors in lieu of bonded PT from PT companies because they do not want the extra subcontractor on site. A backward view in my opinion, similar to PT operations in USA. I would much prefer to have specialists doing it! We agree to disagree on this quite often!

An senior engineer in Hawaii was once heard to ask "why would you put reinforcing steel (unbonded tendons) into concrete and not bond it to the concrete?" Seems to defeat the purpose doesn't it?

 

[AUCE98]

Rapt,

Just out of curiosity, you had mentioned that VSL had issues with their bonded pt system. I had worked for them for about two years. They still try to sell the idea of a bonded system, which I am a proponent of, but indeed they are typically faced, with the issue of cost being about 150% of the unbonded systems. There were two different types of bonded systems; one was to be used in flat slabs, this had a flat duct with the ability to place up to (4) 0.6 strands. The other was a typical multi-strand system which was for transfer slabs, mats etc. How do these systems differ from systems outside the US?

Also, might the other PT software developer you seem to have differences with be Allen Bommer? He is in a very influential position. I have seen him post on this site once or twice.

Regards,

AUCE98

 

[slickdeals]

Is Rapt going to tell us who the mole is? []

 

[ash060]

Rapt

I am not sure I am following the discussion. I am a US engineer and have designed several unbonded PT slabs using bands/distributed and they have performed quite well. And many 100's or maybe 1000's of slabs other than mine have performed well also.

And they are somewhat partial prestressed because we are allowed to see some tension during service.

If they (unbonded slabs) perform well and are less expensive shouldn't we advocate there use?

 

[hokie66]

In Australia, we don't have the problem of deciding between bonded and unbonded, because unbonded is prohibited. But I am interested in the cost difference. A 50% premium seems a lot for an inferior system. I would have thought grease was more expensive than cement.

 

[rapt]

Auce98,

No, I have no problems with Allan Bommer. I was thinking of someone from the 70/80's and I am not naming names, but he has helped to make a mess of the PT section of the ACI code in my opinion.

The unbonded system VSL were pushing in the 90's in US was a plastic duct 2 strand system. It was very expensive compared to the 4/5 strand flat metal duct systems used elsewhere.

The difference in the costing is not just the grouting. Outside the US, PT companies provide a full site service from installation, supervision, checking, stressing and grouting. That does not tend to be the case in US.

Also, In comparing the costing, you have to look at everything, not just the PT price.

But this thread was not about unbonded vs bonded, it was about, tendon layouts in 2 way slabs. The problem with unbonded came into it because it is very difficult to lay unbonded tendons in a column/middle strip layout which has basically resulted in the ACi code having a design method for flat slabs that defies all engineering logic and places stupid limits on how flat slabs can be designed.

Partial prestress is not allowed by the code simply because of this design methodology and unbonded "forced" the design methodology.

It is a comvoluted arguement as the dog keeds chasing its tail!

 

[rapt]

ash060,

See my previous reply.

I have other problems with unbonded and yes, there are problems with bonded too if it is not conctructed properly, but unbonded is much more susceptible to problems simply becasue the whole strength of the structure long term is relying on the anchorages. This is not the case in bonded if it ius built properly.

My main arguement is against the design logic in ACI code, the design of flat slabs based on the full moment over the full width, the stresses used to decide when the concrete is cracked, the logic of Class U up to a stress equal to the tensile strength of the concrete.
The whole thing is very misleading. The designer does not know what he is really getting. Yes, the buildings stand up, but there is a lot of cracking and as I repeat, the designer does not know what is really happening in his slab. Maybe he deos not care, but I think he should. You are ending up with generations of designers who think that is how PT slabs actually work and think they understand what they are doing with design. Then you get people who start saying that, if the tendon distribution over the width does not matter, as the ACI code says, then you can use equally spaced tendons in both directions, or you can use the total moment on total width logic for slabs with drop panels or with concentrated loads. Because they do not understand how slabs really work. And this has been encouraged for years by those people I mentioned (No not AB).