Elongation for short self stressing bed

[etabs2002]

I am using formulas for elongation and evrithing is OK for long bad (400ft), but for short bad (50) I am getting the forces above the 80% of the specified minimum ultimeate tensile strength.
Any idea what is wrong.

Thanks

 

[hokie66]

When I first read this, I had no idea what you were talking about. Now I think it is an issue with some type of precasting arrangement, and you have a problem with tensile stresses on release. But it is hard to advise you when you give us so little information.

 

[etabs2002]

You right, I will try to provide more information. See attached document.
I hope the attached file will help to understand what I am tolking about.

Thanks

 

[hokie66]

Sorry, but that doesn't help me to figure out what your question is. Anybody else understand?

 

[rapt]

etabs2002,

You have added about 2.5% extra force, so if your initial force was within 2.5% of 80% then obviously it will be over. Is there a problem with this. After the force losses the final force will be what you started with which is less than 80%.

I do not understand why you are allowing for bed shortening. It happens as the stressing is done doesn't it, so you do not have to overstress to make up for it, same as the dead end slippage.

 

[Teguci]

If a strand is too short, the slippage losses will be detrimental to the stressing in the cable. ex. to stress a short strand you need to pull it an extra inch for every 20 ft of length. If the bed is only 10 feet and you lose 1/2" is slippage due to the terminations, you will not be able to compensate.

A good strategy to deal with this is to have long stressing beds. Large precasters have beds that are 200 feet long. Even if you are only producing product that is 20 feet long, you can put three pieces in one 60 feet long bed and distribute the slippage losses over the three peices instead of one.