Prestress Losses for PT Column Rods

[Son of a Bridge]

Hello,

Which design code is typically used to calculate prestress losses for PT rods? From what ive seen, AASHTO (5.9) provides equations to calculate prestress losses for prestressing strands. However, I am unsure if these equations can be used for PT rods?

Or do designers typically assume an effective prestress level such as 0.60fpu to account for all losses?

Any help is appreciated.

Thanks

 

[bugbus]

The Australian standards cover losses in PT bars specifically, but a lot of the time we end up deferring to data from suppliers anyway (e.g. VSL, Freyssinet) since these seem to be less conservative than the code.

 

[BridgeSmith]

Post-tensioning isn't my area of expertise, but it seems to me that for post-tension rods, the only significant loss in tension after tensioning would come from long-term creep shortening of the concrete. That being the case, calculating the losses should be fairly straightforward, though likely not very accurate. You should be able to use the AASHTO equations for the creep portion of the losses. If the AASHTO equations don't include external (load induced) axial compression, you'll need to add that to axial compression from the post-tensioning. The modulus of elasticity may be somewhat different for the rods than for strands.

 

[Son of a Bridge]

Good morning,

Yes, i spoke to a supplier yesterday. They recommended using 0.60fpu as a working effective prestress but recommended doing a more in depth analysis using AASHTO or PCI equations to calculate losses accurately. Apparently relaxation losses vary drastically between suppliers. I just wanted to know if other references covered pt rods since aashto typically talks about pt strands.

Ill calculate losses for relaxation, creep, shrinkage, elastic shortening, anchor set and friction.

 

[rapt]

Actually steel relaxation would probably be the worst loss for prestress bar/rods. It is normally not Low Relaxation like strand!

 

[BridgeSmith]

According to this article from PTI, relaxation for bars is specified in BS 4486 as 1.5%, 3.5%, and 6%, at 60%, 70%, and 80% of breaking strength, respectively. JIS G3109 sets the relaxation at 4% at 70% of ultimate tensile strength.