Detailing tendon reverse curvature radii on engineering drawings 1

[asixth]

How are the reverse curvature radii of tendon profiles detailed on engineering drawings. I commonly design 70x19mm ducts with a reverse curvature radii of 2500mm but do not detail this with tendon profile points or cover it in the post-tensioning notes.

Is it that a steel duct cannot be physically bent at a radius less than 2500mm specified or is it the inspectors responsibility to ensure that the reverse curvature is not too sharp.

Thanks

 

[rowingengineer]

I normally provide this info in a detail.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[rapt]

Asixth,

The duct tends to split open along its seam at tighter radius than 2500mm. It is not a good idea to use 2500mm as the normal radius as it is the absolute lower limit, 5000 to 7000 is much better. One PT company in Australia uses 2500 but the rest are sensible and use the larger radii where possible.

You should check that the tendond profiles used on site conform to the requirements if you are the designer. The PT company might provide shop drawings but it is your responsibility to ensure that they conform with your design requirements. They should be submitted to you for approval.

 

[asixth]

I've been having a lot of trouble using the 5000mm limit and still getting tendon profiles to work. I found some reference's that suggest 2500mm. I've never detailed multi-stranders other than ground anchors but I know there 75 times the duct diameter.

 

[rapt]

Asixth,

You must have short spans with relatively deep members for 5000 not to work. For normal span/depth ratios for slabs and beams, we used to commonly use 10000 for slabs and 7000 for beams in the 1970/80's. This has been rationalised over the years to 5000 as a general figure for all members.

2500 is the absolute minimum for flat duct due to seam failure. The limits for multistrand tendons are with regard to the compression and bursting stresses within the curve. Because of the small number of strands in flat duct, this requirement would be significantly less than the 2500 limit so the seam splitting limit controls.

 

[asixth]

Thanks RAPT for your insights regarding post-tensioning design and detailing. I'm a bit green on the topic so I am going to go with the 2500mm. I have not seen it detailed anywhere on shop drawings to date and knowing the general lack of knowledge across the construction industry I am guessing that the steel fixers won't even know what a radius is!!

When trying to detail a tendon thru a step, does this absolute minimum curve radius of 2500mm drive the transition zone? For steps less than 100mm, this gives a minimum step length on 1000mm and when the vertical step is increased to 200mm, the step length becomes 1400mm. I have also seen a number of PT companies provide a hold-down detail thru steps and cranks, is this required when the minimum radius is adhered to?

Please see attachment.

I also noticed that the minimum radius default in RAM concept is 2000mm.

Regards

 

[rapt]

Asixth,

I do not understand what it has to do withy steel fixers (unless you are in USA where steel fixers install the PT). The PT company doing the shop drwaings should allow for the radius of curvature in calculating the profiles for the shop drawings. When I worked in PT, the site installers know the limits and the problems if they were ignored.

I do not know where RAM Concept got 2000mm from. Seeing there is very little flat duct prestress done in USA and most of their practice until they learnt better was to define transition points rather than curve radii, I can only assume they made the number up. 2500mm has been industry practive in Australia as a minimum for 36 years that I know of, and it was used before that.


The radius does not control hold downs. You are creating concentrated forces in the concrete that tend to result in spalling of concrete inside the curve, especially whe they are near the surface. Hold downs are necessary to stop this. Even if you used 5000mm radius. With a double curve like you are trying to achieve, I think you would have trouble at 2500mm. The duct is not elastic or flexible. It does not bend well through tight radii and tends to knik and crupple and the seams split open. To do a double curvature like you have will exaberate this. I would make the step longer and use a larger radius.

The step lengths you are talking about sound small for prestress.
I would tend to not try to kink the tendon through a step as you are doing. Allow for the varying height of the tendon from the centroid through the step instead and the steps tend to be fairly long to achieve this, depending where they are located along the profile.