Post ensioned Floors-"ADAPT" Question

[cecil123]

I am comparatively new to the world of "Prestressed Concrete", though a bit experienced in Steel Design.I have been reading the "ADAPT" software manuals and have the following clearifications:

After structural modeling and data entry in ADAPT we run the program and then the "Recycle Window" appears wherein we can adjust the post- tension forces and the tendon drapes. I have read the ADAPT manual but have the following clearifications:

In the "Recycle Window" we have the following tabs:
a)Tendon Forces and Heights
b)PT Forces
c)Required Forces
d)Stresses Tension and Compression
Now, I have the following clearifications:

1) In the "Required Forces Tab" we have a section of , "Governing Forces","Based on Tensile stresses", "Other Considerations"

The manual says that under the "Governing Forces", the force selected is th e largest based on tensile stresses, minimum P/A, minimum percentage of dead load to balance.When we say based on tensile stresses does it mean the force selected is one such that the value of tensile stesses due to Dead Load +Live Load+Post Tensioned Force is not more than the permissable tensile stress value?When we say, based on minimum P/A does it signify the value of post-tensioning force is such that it should create minimum precompression in concrete specified by the user?The other consideration, is the percentage of dead load to balance, does this mean the force selected is one that
the percentage of dead load specified by the user should be balanced?
Am I right here?

Next question,

2) What is the basis/logic of specifying the minimum precompression?
What should be the value of the minimum precompression?

3)Considering, the "Tendon Forces and Heights" tab, here the user can vary the forces in any of the spans and the software checks for the adequacy of stresses,etc.
Now, if we have a different PT force in each span (mode selection is "Force Selection" , would this indicate that we have a different tendon of different strength in each span?

If the PT force in each span is different, then we cannot have the same tendon running through all the spans, right??

 

[rahuldby]

Design of Post-tensioned floors includes determining the required PT force and any additional rebar required to satisfy the strength and serviceability. The ACI requirements are different than what they are for Reinforced Concrete design. To answer your specific questions, I would like to start with the easiest one.

2) ACI requires a minimum average precompression of 125 psi. PTI recommends a higher value if water tightness and corrosion are a concern. The maximum average precompression recommended by ACI is different depending on if the member designed is slab or a beam. This upper limit on average precompression ensures economy in the design. If your design requires higher PT force than what is governed by this criteria, you might want to resize the members.

1) The answer for your first question is in the question itself. ADAPT calculates the required effective force for a span based on three criteria. a)Tensile Stress b) Minimum precompression required 3) Load balancing. So, its yes to all the questions. The forces it calculates is the minimum required to satisfy each criteria. To add more to this the load balancing criteria ensures in deflections and at the same time limits any excessive camber if the required prestress is too high.

3) ADAPT lets you select different tendon forces in different spans. But it is not practical and economial to change tendon forces in the spans. This feature is especially useful in those cases where you might need high PT forces in the end spans. In that case the force in the end spans can be higher. this is usually achieved by adding few extra tendons for that span. This might not be the case always...I have seen situation where tendon forces have been varies along the span significantly requiring special attention during construction.

Hope this helps.

 

[cecil123]

Thanks for the response.
Let us say , we have a two way slab which is to be post-tensioned in both directions.The slab is not a square one but is rectangular,so therefore,I have a short direction and a longer direction.

Recalling my engineering theory, the shorter direction contributes to carry more to carrying load.That is, more load is carried by shorter direction.And, less load by longer direction.My question is:

1)How does ADAPT decide the percentage of load balanced by the post-tensioned tendons in the shorter direction and longer direction?Does the software have any facility for deciding the load carried in shorter direction and longer direction?

2)What should be the general crteria for deciding the percentage of load to be balanced in shorter direction and longer direction by the post-tensioned tendons by the user?

 

[Ingenuity]

cecil123,

I advise you NOT to learn how to do PT design by first using PT software - you need to get a good text on PT, read it, then do a few hand calculations, before you use the "black box".

Unless your rectangular two-way slab is supported by walls (lots of walls are unusual in PT slabs due the restraint they provide) and assuming your slab IS supported by columns (ie flat slab, flat plate, banded or weaved tendons etc) then once you have selected the load to balance (maybe 75% of slab self weight), you have to balance this selected load 100% in BOTH directions. This is just an equilibrium requirement - in any two-way system the total load must be accounted for in each direction.

For the "special" condition of a rectangular two-way slab that is supported by walls (creating a regular rectangular panel) then you can select the % to balance such that wp = wpx + wpy. You could choose 100%/0%, 50%/50% or whatever, but you need to ensure the % you choose satisfies both strength and serviceability. Selecting 100%/0% (ie all tendons in short direction) will result in the least PT quantity, but you still need to satisfy crack control and strength in the long direction, so this would NOT be a wise choice.

HTH