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.

 

[slickdeals]

If you are following the ACI code, then it is not permitted. You are required to have cables placed no further than 5 ft or 8 * Slab thickness. This is in section 18.12.4 of ACI 318-02. This forces you to do a banded-distributed pattern.

 

[lisia]

Thank you for reply.
About structural research or technical paper, is it possible to design that layout about unbonded tendons?

 

[JAE]

I think you could design it that way but as slickdeals said, the ACI code apparently doesn't allow it (not sure about the Italian code). It would be interesting to know why ACI requires the banded approach.

By the way - I was in Italy two summers ago. What a great country!

 

[lisia]

Thank you for reply.

There is no italian code about two way slab. IMHO it is important to ask about it where there is knowledge.

I think it is possible to band tendonds just inside the column strips, because you could provide satisfactory sectional behaviour.
But I need knowledge and experience, so I ask to forum.

 

[ron9876]

A practical reason for bands/uniforms is placement. If bands in both directions there will be a conflict at columns.

 

[wildehond]

I agree that the banded in one direct with unbanded in the other is a practical way of avoiding the congestion you would have with banded in both directions. the section typically are so small, any loss in cable height leads to a big change in vertical component of the cable force.

But I can't think of a structural reason why you couldn't have banded in both directions

 

[rapt]

History

The banded/distributed system was basically created for unbonded sustems, because the column/middle strip system requires weaving of tendons. This is reasonably easy with bionded PT as the tendons are not normally pre-manufactured full length, they are made up on sode out of 6m lengths of duct. It is very hard to do with full length unbonded tendons. So the USA with unbonded tendons went this way and because of this they cannot do partial prestressed slabs.

As an aside, the ACI code is stupid on this as it was written by unbonded people for unbonded industries. Also, the PT software available from the US at the time was written to handle only single strips and the developers of those programs had a big influence on the direction the USA PT industry went and on the ACI design approach. Because of this it limits people from doing bonded patially prestressed slabs and achieving a better result in many cases.

In Australia and Asia with bonded tendons, column/middle strip arrangements are the norm as is partial prestressing for flat slabs.

Lisia
There is no reason why you cannot do it. It was done quite often in Netherlands in the 1970/80's (called support strip stresing I think).
As long as you put column strips of tendons in both directions, some top reinfroceemnt over the columns and and check for strength, crack control and cracked deflections properly, it will work properly as a partially prestressed slab for the column strips and a reinforced slab for the middle strips. Eurocode does not preclude this approach.

 

[hokie66]

Rapt has given a good synopsis of the situation of unbonded vs. bonded/partially prestressed. As a US trained engineer working in Australia, I agree that the ACI supported method is "stupid", and would encourage US consultants to do something about it.

 

[lisia]

Thank you for reply. I agree with Rapt.
Is there a link to technical paper about this "partially prestressed" two way slab?

 

[rapt]

Lisia,
The second edition of TR43 by the British Concrete Society covers it reasonably well, though the explanation in some areas leaves things up to engineering logic and a little reading between the lines as it covers both methods. It is not available online. You would need to purchase a copy.

Otherwise, there is nothing different to it than RC flat slab design to get the moments for column and middle strips in each direction and then designing each strip in each direction as a partially prestressed member, same as you would for an RC slab design.

Hohie66,
I have been suggesting that for 20 odd years. I am sure you know the response someone from Australia crtiicising a US methodology would get from a US expert or even a US design engineer! That is part of the reason that most US engineers who have stayed at home have missed out on the pleasure of using RAPT software for RC and PT design!

 

[slickdeals]

@rapt,
Do you really have a personal flame war with Bijan Alami? Just kidding.

Although I practice in the US, I see the merits of using bonded cables, although I am yet to use one in my practice.

 

[rapt]

Slickdeals,

Why would you think that? There was more than 1 PT software developer in the US in the 1980/90's. I would not limit the cause of the problems to Bijan, though he is a major contributor, but at least one other was/is just as bad or possibly worse in my opinion and he is in an influential position at the moment.

The benifits of bonded cables are many but it is also the design logic that needs to be modified and this is the big problem. You cannot go to partial prestressing until you go away from banded/distributed tendons. It is not unbonded that is the basic problem, though yoiu will never do column/middle strip tendons with unbonded and you need to to do partial prestress.
About 10 years ago VSL (now basically deceased in US building practice) tried to introduce a bonded slab system but, for some reason partly to do with PTI requirements and partly beacuse they did not know better, it was extremely expensive to use (far more expensive than the systems used in the rest of the world) and died a very quick death.

 

[wildehond]

Rapt
Help me out here. Why can't you use unbonded prestress for partially prestressed slabs. And why the need to do away with banded and unbanded?

I would see the design procedure as follows:
1. Determine a practical layout for the unbonded tendons (the banded will then produce high upward loads at concentrated areas while the unbanded will give a spread load)
2. Calculate the nett forces from this tendon layout
3. Model the slab plate with dead, live and upward prestress loads
4. design the bonded normal reinforcing for the nett remaining moments?

 

[lisia]

Thank you for reply.
It's very interesting to read your opinion.

Is there a link to an example (builded floor) about partially prestressed?

I tried by google, but I need key words.

 

[rapt]

Lisia,

There have been thousands of buildings designed this way in Australia over the last 40 years. Look at the prestress company websites for examples or at

http://www.ptia.org.au

there are links to the PT companies.

Also, the UK PT companies and many of the Singapore ones have been doing it for years.

In Italy, Arup in Milan have done some.

 

[rapt]

wildehond,

You can design partial prestress with unbonded tendons though it is not as efficient as bonded tendons. The problem is you cannot partial prestress flat slabs if you are not dividing the slab up into column/middle strip in each direction and allowing for the moment and stress concentrations where they occur.

The design method used to ACI/PTI bases the design on an average moment across the whole width of the slab. This is not real. There are moment and stress concentrations. You can justify ultimate strength the US way as long as you provide a load path with the reinforcement and the banded/distributed tendon pattern does this (that is why distributed tendons in both directions does not work, no load path). basically tyou get a one way yield line result. However, you cannot make th=ese assumptions at service. You have to look at the actual stresses and concentrations where they occur. You cannot take an average. This is where ACI318 is completely wrong (see hokie66's comment earlier).

You can even do a aprtial prestress design using banded/distributed tendons, but it is not efficient as the areas with no tendons have to be reinforced for the stresses in those areas, so you are doubling up on reinforceemntas the ultimate strength reinforcement is not in the right place compared to the moments/stresses so you need to put more reinforcement in.

The most efficient system is to lay out your tendons and reinforcement in the same pattern as the moments (as you do for RC flat slab design). This results in column/middle strip layout of tendons in each direction and extra reinforcement in the same pattern. Unfortunately, this is not easy to do with unbonded tendons. That is part of the reason why the PTI/ACI went the way they did. But unfortunately, that is not the way the slab wants to work so you pay a penalty.
Even for the current ACI design method, the service stress design is wrong and unconservative. Because the stress is not averaged over the full width, the real stresses are actually higher than those being calculated and the stress limit to decide on cracked/uncracked (a bad term but I will use it as ACI does) should be significantly less, especially at the support but also for positive moments.

 

[wildehond]

Rapt

Thanks for your reply. About 10 yrs ago I contributed to a course on PT run by a local polytech here in South Africa. In reading around the subject, I was astounded at the number of texts that were flawed in that they were prescribing calculations that made assumptions of section properties as if the sections were uncracked. This is all "wishfull" in partially prestressed slabs where one acknowledges that the slab will crack.

Is the flaw in the method I suggest the ultimate economy? Because if the moments and shears (and compression) in the slab are all going to be calculated at ultimate limit, then the PT load multiplier is up for discussion. I've been conservative and used 0.9. => 1.2DL+1.6LL+0.9PT

But I've got around the column/middle strip problem by modelling the slab (with the PT loads all correctly positioned upward and downward to suite the tendon profile) using a Finite Element Slab programme. Is this a structurally flawed method, or just conservative?

 

[hokie66]

I'll defer to rapt in answering your questions, but the PT should not be considered a load, it is part of the resistance.

 

[rapt]

wildehond,

The easiest example is alwaays single span, no end restraints uncracked. So that is what they put in books.

You can model the PT as equivalent loads to try to work out the Mp. Then subtract P.e from this to get the Secondary prestress moments so Msec = Mp - P.e.

In your ultimate load case, the combination would then be
1.2DL + 1.6LL + 1.0 Msec.
I hope you were not putting in Mp for this because as Hokie66 says, the main prestress moment is an internal action, not an external load. The only effect of the prestress that is considered to be an external load is the secondary moments.

Some designers agree that this should have a .9 factor (RAPT software allows this but it defaiults to the code value) but most codes use 1.0. There is a theory that it is actually zero at ultimate strength as after the plastic hinges form, there should be no secondary moments as there is no continuity!

As long as you consider the stress concentrations from the applied loads, then FEM is ok. It is not concervative, it is correct.

The ACI code method is unconservative. Many designers using FEM for PT design still take averages over the full slab width. This is useless. FEM is predicting the concentrations and the designer is ignoring them after going to the trouble of calculating them. Also, if the concentrations are ignored some funny things can start to happen, e.g.
1 when people average the moments in irregular buildings, often they are averaging across a zone that has both negative and positive moments. Theoretically, if the negatives equal the positives, the result is zero! Not very sensible as both moments exist.
2 If you are designing for a one way failure mechanism, as the US method does, you cannot analyse for it on FEM as it is considering 2way action. The torsions and effects of stiff elements from different areas of the slab affect the moments in the area you are designing and can lead to stupid results and gross underdesign (similer to 1 above).
3 You cannot average effects across a change in section (eg drop panels)