Post-Tensioned Floors

[cecil123]

Hello All:

Just wondering with this question:

I have being going through the tutorial problems in "ADAPT"(software for design of PT slabs).

In one of the problems,the input specification states that, the minimum percentage of load to be balanced by post-tensioning is given as 25% of dead load and maximum percentage to be balanced by post-tensioning is given as 150% of dead load.

But I was wondering that if the percentage of load to be balanced by post-tensioning exceeds 100% of dead load then we would get (excessive) tensile stresses at transfer stage.Right?How could there be a specification in the input that maximum percentage of load to be balanced by post-tensioning as 150% of dead load?Why should one balance more than 100% of dead load?

Please help!!

 

[DaveAtkins]

I am no expert, but...

The concrete is cured prior to post tensioning, so the tension at transfer should be OK (I would think ADAPT should check this, and recommend adding mild steel if necessary).

If you balance more than 100% of the dead load, you are balancing some of the live load.

DaveAtkins

 

[AUCE98]

Depending on the type of member you are designing, the live load may be extremely high, would cause you to have a higher percentage of the dead load balanced. Also, in transfer girders the percentage of dead load may well exceed 200% due to the applied loading. In those situations it is important in ADAPT to run the post processor for the initial stresses, and possibly perform a stage stress analysis to avoid over cambering the girder and causing large tensile stresses in the top of the girder.

AUCE98

 

[cecil123]

Supposing if the live load is high, so we balance some of the live load too through post-tensioning.

We thus provide non-prestressed steel for transfer and allow the concrete to crack at transfer.Right?Lets us say it cracks by some "x" units

Now, live load comes.A part of it is balanced by prestressing.For the unbalanced portion of the live load concrete cracks by additional "y" units and we provide non-prestresssed steel for the purpose.

So, total cracking is "x+y" units.
Do you mean that had we not balanced the live load initially the cracking would have been more than "x+y"?If yes, can you justify?

Thanks a lot!!

 

[cecil123]

Awaiting a response?Anyone,AUCE 98??Please put in your views??

 

[Ingenuity]

cecil123,

At transfer, the tensile stresses are usually on the opposite face to the tensile stresses that may be produced in-service (under LL).

In your example the 'x' cracking would be on the top surface at midspan, assuming say a single span element with a parabolic tendon, maximum drape at midspan, but the your 'y' cracking under LL would be on the bottom surface - assuming the LL is acting downward.

Also, sometimes where you have to balance more than 100% of the dead load, you stage the prestress in two or more stages eg transfer girder picking up several floors - you may design the PT based upon staged stressing, accoording to the application of staged DL.

Do not get too hung up on 'balanced loads' - it is just a concept/tool to make PT design a little easier.

 

[cecil123]

Ingenuity,

Thanks a million for the response.Yes, as you said that in my example, the x crack would be on the top surface and y crack would be at the bottom.

What I want to know is:

1)Suppose if the live load is heavy, then we may balance more than 100% of the dead load by prestressing.Right?
Now, this MIGHT (I say "might" because the balancing means , "balancing" by the prestressing moment, the axial component of the prestress is still there, right?) create tensile stresses in the top fibre for which we provide non-presressed steel (say "p1%").Right?

2)Now, comes the live load.A part of the live load moment is already balanced but let us say we get some of tensile stresses in bottom fibre.We provide again non-prestressed steel there(say "p2%").The quantum of cracking is less than that it would have been if no live load was balanced.
Also the crack at top fibre may close now.Right?

3)Total non-prestressed steel provided over the whole section is 1%+p2%.Would there be a saving in total percentage of non prestressed steel percentage than in the case if we would have balanced only 100% dead load?

4)Or we balance more than 100% of dead load for minimizing the cracking at service loads?

 

[cecil123]

Ingenuity, awaiting your valuable response!!!!
Or, anyone to comment on my thread above!!

 

[cecil123]

Ingenuity,your expert suggestion on my questions above?
Anyone?Please help

 

[AUCE98]

cecil123,

What type of member are you designing?

 

[ron9876]

Remember it is p/a +/- m/s---balancing more than 100% doesn't necessarily mean tension in the top. If you have some type of heavy load you may need to balance more. One time had a planter on a pt slab and after stressing the shores fell from under the slab due to temporary negative deflection. Adapt %'s are the default and you can change to whatever. You should always do the initial stress check analysis and whatever you use for the concrete strength at stressing should be shown on the drawings (0.67 or 0.75 f'c).

 

[cecil123]

Am actually looking to develop a computer program/software for PT slab design

 

[ron9876]

It sounds like you don't have much experience with pt design. Seems to me it would be hard to write a user friendly program without experience with the items to design.