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Re: Prestressing Transfer

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Gail,

Transfer has always been the stage after initial losses (friction, elastic shortening and anchorage set) have occurred. This is true for pre-tensioned and post-tensioned members.


At 10:34 AM 16/03/2003 -0500, you wrote:
The terminology in the prestress industry is sometimes confusing, due to the differences between pre-tensioned (precast) and post-tensioned work.  In particular,  Chapter 18 of ACI 318 has been largely written by the precast industry,  it has been adapted to fit the post-tensioned industry with varying degrees of success.

With respect to the issue of  "transfer":

In precast work,  the tensile stress in the fully stressed strand is held by abutments.  When the strands is released (by flame-cutting or other means),   the compressive stress is transferred from the abutment to the concrete member.  This places the concrete in compression, the tensile stress in the steel is essentially unchanged.

In post-tensioning,  you are jacking against the concrete,  thus the compression is applied as the strand is being stressed.   What is "transferred" when the jack releases the strand is that the tensile stress in the strand is now held by the anchorage device rather than the jack.  The stress in the concrete and the steel is slightly less after this "transfer" due to the wedge seating loss but there is not the significant change in concrete stresses you have in precast.  

Rather than referring to  "transfer" in post-tensioning,  this stage is often referred to as the "stressing stage", since it more accurately reflects what is being done.  

The "inital stresses"  refer to the stage right after stressing. At this point, the stress in the strand has not yet been reduced by long-term losses,  the concrete is probably not up to its full design strength and there is less than full live load.

Gail Kelley

Regards  Gil Brock
Prestressed Concrete Design Consultants Pty. Ltd. (ABN 99003351504)
5 Cameron Street Beenleigh Qld 4207 Australia
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