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Re: Prestressing Transfer[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: Re: Prestressing Transfer
- From: GSKWY(--nospam--at)aol.com
- Date: Mon, 17 Mar 2003 08:53:01 EST
The point is to try and understand what is happening, not to get bogged down in the terminology.
ACI 318 - R18.2.1 and R18.2.2 refers to the "jacking stage, or prestress transfer stage". In post-tensioned construction, these are the same thing.
The point is also to try and understand the differences between pre- and post-tensioned construction. In particular, all tendons in a pre-tensioned member nominally have the same loss due to elastic shortening. In a post-tensioned member, elastic shortening does not occur until subsequent tendons are stressed. If you just have one tendon, you will have no elastic shortening loss. If you have a group of tendons, the last one tensioned will have no elastic shortening loss.
Most textbooks focus on pre-tensioned construction, because the concepts are easier to explain.
The various methods for calculating prestress losses are all approximations. The Zia approximations define initial stresses as before the reduction for elastic shortening. Other methods, such as the PCI method, have different definitions.
Some of these approximations, such as those involving time steps, are more complicated than others. In most post-tensioned construction, particularly building construction, the extra effort is usually not worth it.
Calculating prestress losses is a lot like calculating deflections; many of the same factors are involved. With both types of calculation, increasing the complexity of the equations does not necessarily increase the accuracy of the answers.
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