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RE: How Can Force in "All Thread" be Det

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Bill,

I understand the method that you describe, but wonder how that would be 
applied to GLBs with cantilevers.  I can see how this could be applied in 
reducing the positive moments by lifting the center portion of the beam, but, 
in reducing negative moment, wouldn't the change in moment be reflected 
in movement at both the end of the cantilever *and* at the center of the span?

I have 4 different beam configurations that need to have moments reduced, 
some just in the positive moment area; another in both the positive moment 
area *and* at both cantilever supports; another at one cantilever support and 
I don't want to give the sub-contractor a lot of confusing information.

I have used prestressed strand before anchored to brackets with strand chucks 
(not proprietary wedges) and that has worked well, but this sub-contractor 
"can't find anyone who knows" what I want and he wants to use all thread but 
doesn't know how to determine the tension force in it.  (Other than using 
load cells, neither did I.  I did mention strain gauges for smooth rods, but 
that wouldn't work with a threaded bar.)

Roger Turk
Tucson, Arizona

You wrote:

. > Roger-
. > What Michael suggests brought back memories of a "prestressing"
. > reinforcement I did of a series of trusses about 25 years ago. The bottom
. > chords had failed on several adjacent trusses.  We shored the panel points
. > and replaced the bottom chords. Then we added prestressing bars on each 
. > side of the truss. To monitor when the dead load was carried by the 
. > trusses, we place a shore with a load cell under the center panel point.  
. > When the load went to zero, we knew the dead load was being carried. You 
. > could probably do something similar with the "all-thread."

. > As a side note, I was in the very early part of my career (about 5 years 
. > out of school).  The carpenter foreman was quite skeptical that by jacking
. > horizontally one could make the truss lift.  He protested vigorously but 
. > did what I asked. He kept muttering about "college boys."  When we 
. > stressed the first truss, the load on the load cell hardly moved as we 
. > stressed the bars. He kept saying, "This will never work. See, the bar 
. > load is getting higher and higher and the vertical load hasn't changed at 
. > all (not quite true but the change was a very small reduction." We 
. > reached nearly the design load on the bars (we had load cells on them 
. > too) when all of a sudden, the load on the load cell under the center 
. > panel point went rapidly to zero and the shore toppled. The foreman stood 
. > there silently for several minutes with his mouth wide open. Finally he 
. > said, "well, I'll be damned. The college boy does know what he's doing." 
. > From that point on, I had his complete cooperation!

. > Regards, 
. > Bill Cain, SE
. > Oakland CA

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