Urgent - Prestressed Tendon Curvature Minimum Radius

2.5m is a generally accepted minimum radius for bonded PT 19mm ducts.

Section E of this PTI document deals with this issue and agrees with Trenno: PTI Tech Note.




I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

Normally this information is supplied by the prestressing company.

A basic rule for multi-strand tendons is 70 * duct diameter. This obviously assumes a logical duct diameter for the number of strands as the bearing stresses are related to the number of strands and the bearing area! It is not a good idea to use less than this.

Your question about hairpins is far too general to answer. I would not think that hairpins would overcome the concrete crushing and bursting problems that you get from too tight curvature. Or the duct wear or higher friction. So I would assume that they are there for another reason.

Thank you everyone. This really helps me.

@rapt: What do you mean? Isn't the hairpin used to bear the lateral force due to the curvature? For bursting, I think we use Bursting Reinforcement.

I doubt that the hairpin would develop much stress before the concrete was in trouble due to high compression stress. It may stop a failure but it will not reduce the compression stress significantly in my opinion.

Hairpins might work to stop breakout/spalling where an angle change is near a surface and towards the surface as happens at kinks in tendons near anchorages in large bridges etc. In a general area where it is concrete crushing that is the problem, I doubt that they would alleviate the problem as the concrete would have to crush before they were fully activated.