uniform slab thickness post tension double pull cable length 1

[rlewistx]

Uniform Slab Thickness post tension slab on ground foundation:

At what cable length should I consider changing from a single pull post tension cable to a double end pull post tension cable?

At what length should I consider changing from a double pull cable to breaking the slab into multiple sections?

Thank you for your feedback.

Rich

 

[txstruc]

Length will affect the Final Effective Prestress in your design, so the longer the cables the more cables you will need to achieve the design prestress that you need. Our company standard is >100' double pull and >150' add a construction joint. But, in practice we see double pull 150' cables achieve their entire elongation on the first pull with negligible amount coming from the second live end. Elongations calculations (using (PL)/(AE) are accurate at these lengths and these length limits seem to be conservative. In the practice of elevated concrete (ACI-318 vs. PTI method) we rely on a software called FELT by Adapt which calculates losses for cables based on their actual lengths and profiles. In PTI method, of course it is suggested to use an estimated amount of losses of 15ksi to determine Final Effective Prestress but I have not studied at what lengths this actually will remain accurate.

 

[rlewistx]

Very Insightful. Thank you very much.

Rich

 

[rapt]

It is not just the stressing length that controls joint spacing. It is the amount of slab movement due to shrinkage, creep, temperature change and axial prestress shortening and the tension stresses due to restraint to that movement.