PT slabs - Minimum revers radius in tendons profile 1

[dadomago75]

Hi mates,
i'm new to this forum, and i'm new too to pre-stressing and post-tensioningI, so please be patient
I'm designing PT slabs with RAPT and i can't find anywhere information about how to define the minimum reverse radius for tendons.
I mean, is there a thumb rule to decide the minimum value (maybe related to the slab span), and this value what does it affect?
I know that it affects the bearing in the concrete due to uplift uniform distribuited load within the curvature of the tendon and bursting forces, but is there anything else that has been affected?
Moreover, if someone knows RAPT, how can i check the ratio of the design bearing pressure on the concrete to the allowable stress?

Thanks to everyone.
(hope not to have typed too many grammar errors)

 

[dcarr82775]

Not sure if you mean vertical or horizontal. The tendon drape (vertical) is parabolic, so there is no radius. The distance from the support to the inflection point is normally taken as 1/10 or 1/12 of the span.

 

[slickdeals]

I am not sure of any codified requirements, but I have seen some crazy curves.

The PTI manual recommends using hairpins when the curves exceed 1:12. See page 125 (PTI Manual for Post-Tensioning)

 

[hokie66]

I think the OP is asking about the reverse vertical curvature over supports. We normally consider the the tendon profile to mimic the moment curve, but that would mean a sharp kink over columns, which we don't want. So there is a smoothing in that zone. I haven't looked into those provisions in some time, so maybe someone else will comment. A phone call or email to RAPT would be in order.

 

[rapt]

dcarr82775
Using a nominated inflection point location as is done in USA is dangerous. It gives the designer no idea of the actual curvature of the tendon over the reverse curve. For deeper members such as transfer beams and for multistrand tendons in particular, you can end up with severe problems!

Dadomago75,
RAPT defines the minimum radius of curvature for all defined tendon types in its materials data.
RAPT will use a default of the larger of 5m or the minimum for the tendon nominated.
The bonded monostrand limits are based on experience. The duct tends to split along its seam if the radius is less than the minimum of 2.5m defined in RAPT.
The multistrand limits are to limit possibility of failure due to bearing stressing in the concrete over the length of the curve and splitting stresses in the concrete as those bearing stresses spread our over the section. These limits vary for different prestressing companies so you need to look at prestress company brochures to get details. RAPT has its own limits built in which are in general agreement with the PT company brochures and gereral texts.

PS read the RAPT Help!

 

[dadomago75]

Thanks everybody,
the closest reply to my doubts is the one from rapt.
I've read the RAPT manual (not yet the theory chapter, I'll do as soon as a have soe free time), but it doesn't explain how to define the radius.
The default value is 5000mm, but I wonder if you know any reason other than the concrete pressure due to stressing of tendons, but reading your answers seams there's no other articular reason.
I'll take a look to a few brochures from prestress company too.
Thank very much.

PS it's strange for me to write in an english language forum after 8 years sharing knowledge in italian, thanks even for this.
C ya

 

[rapt]

Dadomago75,

There is no defined logic for the value to use.

The RAPT default values give manageable constructable tendon profiles for most building design. When I satrted in prestressing we used to use a larger radius for flat slabs, up to 10m, but we have found over time that the 5m radius is ok for all building members except where multistran tendons sizes require a larger minimum, and then the minimum for that tendon size would be used.

Most designers would use the default values unless there is a special reason to use a different radius to force a special curve, or for very long span members such as bridges.

 

[dcarr82775]

Rapt,

Do you really assume a circular radius? I have only ever seen parabolic drapes specified and provided by PT suppliers.

I will sometimes calculate a tendon profile but the way I do it requires an inflection point location be assumed to start and then the rest of the profile can be solved for.

 

[rapt]

dcarr82775,

Remember this is for the reverse curve over the support.

Normally, for parabolic profiled tendons, it is a parabolic reverse curve over the supports with a curvature equal to the curvature of a circle of the radius nominated.
We also limit the main parabola to this minimum curvature as well. This can be important in deeper members, those where nominating a transition point is completely nonsensical as the profile shape cannot actually be generated. This shows up immediately in the curvature approach to defining reverse curves as it gives no logical solution to the main parabola properties if there is no logical solution!

For harped tendon profiles, it is a circular reverse curve!

 

[dcarr82775]

I am thinking PT slabs as those are what I have done, not significantly deeper members. I think I understand what you are saying now however, the curve itself isn't circular but the curvature is limited to that of a certain circle. Makes sense.

It got me thinking, there is a pretty severe curvature in a pre-stressed strand at a harp point.

 

[rapt]

dcarr82775,

Yes, you could define it as a curvature limit if you wanted to, but tpeople think more in terms of a radius than a curvature!

I covered harped points above. You should provide a circular transition curve at them. RAPT does this automatically. You normally only have harped points in transfer members where L/D is small and the transition radius is critical. Unfortunately some commercial software ignores this and can give very misleading results to the unwary or unknowing!