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RE: FW: 2- way post tension slab

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At 01:07 PM 17/05/2004, you wrote:
 thanks Gil,
 
I agree with you. That is the reason I was trying to see a correct tendon layout.
there are still 3 items that I like to know if you can help me on them.
 
1- in banded/distributed arrangements. Should I design my uniform tendons as one way slab?

USA/PTI practice is to do it this way.

As long as the slab is ductile and of constant thickness (no drop panels) and the loading is uniform there is probably no problem with this at ultimate strength.

In my opinion (and many others both inside and outside USA agree with me on this) the serviceability checks should not be done this way as the elastic stress distribution is critical for crack control design and it is still two way no matter what the tendon arrangement.

 
2- where should I put my additional nag. reinforcements? (equally spaced in width of strip or more at top of Col?)

ACI tells you where to place this reinforcement.

In my opinion if it is for crack control at service then it must be where the stresses require it to control the cracks caused by them.

 
3- Can you fax that sample tendon layout to me please?

Of what type? Some of the good books on PT design should have examples of typical layouts.

 
Thanks
 
Amir Miraftab
Structural Project Engineer
 
Flores Lund Consultants, Inc.
858-566-0626
858-566-0627
ww
w.floreslund.com
-----Original Message-----
From: Gil Brock [mailto:gil(--nospam--at)raptsoftware.com]
Sent: Sunday, May 16, 2004 5:51 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: FW: 2- way post tension slab
Importance: High

Amir

I assume you are saying that the tendons immediately over the column will be at 12.5" and the tendons between the columns will be at a lower level! (yes-between cols in other direction (direction of banded tendons)) And the un-tensioned reinforcement also! (additional negative reinforcements)(we should have more neg reinf at top of col in column strip than middle strip)
(what i use for design is "Adapt" software which does not assume any middle strip so my strip width is half of my col spacing at each side)
Why for both?  because if we do a finite element analysis of slab sitting on col. and look at moment diagram we will see max neg moment is immediately at top of cols. so I think I have to reinforce my concrete in proportion to the loads they have to carry.
 by the way do you agree with the idea of having different tendon heights in line of support? 
I have made my feelings about banded/distributed tendon arrangements before on this list so I will not go into them again.

Banded/distributed tendon layouts rely on a yield line solution to the slab capacity. They rely on the moments etc redistributing from the elastic two way slab pattern to a plastic one way pattern to match the tendon layout.  Whether or not you like this approach is up to you.

What you are doing in your design is trying to match your tendon strength in different areas of the slab to the  elastic moment pattern. This is also how I like to design so I do not use banded/distributed layouts unless absolutely necessary. But what you are trying to do does not fit with the logic of a banded distributed tendon layout. You cannot really mix the two.

If you want to match your moment capacity over the slab area to the elastic moment pattern you would need to do a proper two way tendon layout with column and middle strips in both directions with different tendon spacings in the strips to suit the moments. This will provide a more economical solution to the design than your approach  using tendons at different heights. This does not work well with unbonded tendons because they are very difficult weave into this type of pattern.. I do not think that Adapt can handle this but I could be wrong.

If you are willing to accept a yield line solution then you use the banded/distributed tendon pattern and lay it out like a one way system and assume the moments fit that system but you must accept the redistribution of the moments to the one way yield line failure pattern. There are limitations to using this system. It should not be used if the slab is going to crack under service loads or where the loads are not uniform or where the live load is large compared to dead load.

Remember that the service design should be based on the elastic moment pattern, not the yield line pattern, as this is the moment pattern that will crack the slab initially before the redistribution occurs.

Regards  Gil Brock
Prestressed Concrete Design Consultants Pty. Ltd. (ABN 84 003 163 586)
5 Cameron Street Beenleigh Qld 4207 Australia
Ph +61 7 3807 8022              Fax +61 7 3807 8422
email:          gil(--nospam--at)raptsoftware.com
email:          sales(--nospam--at)raptsoftware.com
email:          support(--nospam--at)raptsoftware.com
webpage:        http://www.raptsoftware.com

Regards  Gil Brock
Prestressed Concrete Design Consultants Pty. Ltd. (ABN 84 003 163 586)
5 Cameron Street Beenleigh Qld 4207 Australia
Ph +61 7 3807 8022              Fax +61 7 3807 8422
email:          gil(--nospam--at)raptsoftware.com
email:          sales(--nospam--at)raptsoftware.com
email:          support(--nospam--at)raptsoftware.com
webpage:        http://www.raptsoftware.com