Post tensioning in SAP 14 1

[AndreiAi]

irst of all, I need to mention I'm new to post tensioning.


I'm working on some 100m wind farm towers, made out of prefabricated concrete rings stacked one over each other. After being positioned, they will be tightened together using post-tensioned tendons.

My actual problem is creating a correct model in SAP using tendons, because I never used them before.

I started by performing some tests first, on a simple model: a 12m concrete column with a tendon inside (tendon split at every 0.3 m for greater accuracy). I use 2 load patterns: DEAD (selfweight) and PT (post-tensioning). Results look like these:

http://pic.civilea.com/viewer.php?file=15289040007990822747.jpg

These results didn't really solve any of my problems, but rather created new questions.

Mainly, the modal analysis is performed by using masses and rigidity factors. By common sense, it feels normal that the main period T1 will decrease if the post tensioning force increases. That should happen because the structure feels more rigid overall. However by looking at lines 2 and 3 in the above table, it seems obvious that SAP does not use any of the post tension force in the modal analysis by means of rigidity factor. Therefore, I tried talking it into account by means of mass, which I must say, doesn't smell right from the beginning.

I was starting to think that the very concept of this test was incorrect from the start, because the tendon is located at the neutral axis of my column. However, after trying again on a rectangular structure, results were the same.

Someone suggested to include P-Delta effects in an analysis, but modal analysis doesn't include PD in SAP14 from what I can see.

Any suggestions you might have are of great interest to me.

 

[hokie66]

The tower dynamic behaviour depends on the size, shape, and mass of the tower, not how it is reinforced.

 

[OCI]

Post tensioning might allow you to use the full (uncracked) section in your modal analysis, which will make it stiffer.

 

[AndreiAi]

That's what I'm thinking as well.

 

[ishvaaag]

As long that the earthquake forces don't open the joints, you are essentially driven for earthquake deformations and forces by the properties considered for the analysis (might be brute section or less); everything else is the correction imparted by the standing prestress. On the opening of the joints, or better, in its retard for the more highly prestressed towers, you would see greater difference in the behaviour (other than compounding the standing prestress) because upon opening the joints, the assymmetry of comppression and tension strains then grow higher, and being the member then proportionally more stressed, the displacements become bigger.