Tendon Bend Radius in Vertical Plane

[dengebre]

I am looking for the minimum bend radius in the VERTICAL plane of a 1/2" diameter unbonded tendon using a parabolic profile. The available literature discusses the minimum radius of sweeps in the horizontal plane, but I cannot find anything for the vertical plane. The following is what I have been able to find:

1. PTI Technical Notes (Issue 8) states that the radius of curvature must be at least 10 feet for horizon sweeps. It also cites CEP (1992) which recommends a minimum radius of twenty times the strand’s nominal diameter (ten feet for 1/2" tendons).
2. FDOT Table 1.11.4-2 lists a minimum bend radius of 6 feet. I believe the table is for grouted tendon ducts. Would this also be applicable to sheathed unbonded tendons?
3. VSL Report #3 states “bending strand to a radius smaller than about 0.6 m requires special techniques. Is two feet a reasonable value?

I am using RAM Concept to design a concrete building with post-tensioned flat slabs. It uses a default radius of 6 feet and will flag a warning if the drape profile yields a smaller radius. Concept allows you to change the radius value, and I’m trying to determine what a reasonable radius limit should be. I am curious as to what minimum radius others use.

Other PT design programs: I believe the RAPT manual says that the minimum curve radius is available from pre-stressing companies, but I cannot find any such information online. Another PT design program, ADAPT, does not check the radius.

 

[rapt]

In RAPT we set it for single strand unbonded and flat duct bonded (limit for duct crimping)to
- the absolute minimum 2.5m
- Preferred default is 5m as the strand tends to lay well at this radius without trying to lift off the chairs and friction change is more gradual.

Multistrand bonded and unbonded depends on tendon size to limit splitting of the concrete in the curve. Normally about 70 times duct diameter (not strand diameter). VSL used to have a formula based on tendon force but it relied on logical duct diameters being used, not related to strand diameter).

Why do you want a radius less than 2.5-3m? If the radius is too tight, the strand tries to lift off the chair supports so proper installation is more difficult and gives a sudden jump in friction over the reverse curve.

ACI-318 actually suggests .1 * span length which we consider to be far to big a radius in most cases.

We would never recommend less than 2.5m, and suggest only using something lower than 5m when absolutely necessary, not as a default.