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FW: Post Tension (SAP Question)

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Although I have heard of PT being used for masonry as well as precast R/C walls, I (like Nicholas Blackburn) am wondering why you would put P/T in a CIP R/C wall, ESPECIALLY if it already has axial load due to gravity (and you stated that it is a cantilevered wall, so presumably there will be no net tension problems like those encountered with heavily coupled walls).
 
Be that as it may, I can see at least 3 approaches you could take if you are running an ELASTIC analysis:
 
1)  If you need the axial loads on the walls for design purposes, simply account for the P/T during the design, not the analysis, phase.  With an elastic analysis, it shouldn't matter if you don't have the P/T in there during your analysis.
 
2)  You could also put some horizontal dummy frame members at the level of the live ends of the P/T and load the wall uniformly (using frame distributed loads) to account for the P/T induced P/A.
 
3)  If you want to get fancy, model the P/T strands individually as vertical frame members integral with the shell elements.  Set the temperature dependent properties of all other elements (besides the strand frame members) to 0, then use a temperature load case to "shrink" the P/T strand frame members the desired amount to induce compression in the shell elements.  I have done this on RISA3D (but, I admit, not on SAP2K) succsessfully, and although it requires a fair amount of interation (since the "solution" is a certain P/A) it can be done.
 
All 3 of these answers are predicated on your performing an elastic analysis.  If you are performing a SAP2K pushover, let me know the specifics of your setup and I can give it some thought.
 
Hope this helps.
 
T. Eric Gillham PE 
-----Original Message-----
From: Wael Shabana [mailto:waelshabana(--nospam--at)yahoo.com]
Sent: Wednesday, January 31, 2001 5:07 AM
To: seaint(--nospam--at)seaint.org
Subject: Post Tension (SAP Question)

I am designing a structure whose system includes Walls which act as part of the gravity system and shear wall as well. The walls act like cantilever (cantilever span is 43' , worst span). I modeled the walls as shells on the SAP 2000 however I want to locate PT cables in those cantilevered parts of the wall. My question, is there a way to introduce the PT forces to the shells in the SAP or it is only available for Frames. If so, how do I model this wall as a frame? The wall height is 55' and rests on a fdn mat on a length of only 42'. The walls total length is 104'
 
Thanks in advance.
 
Wael Shabana, P.E.