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Re: BOLT TORQUE/TENSION RELATIONSHIP

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Nels -

Our procedure for post-tensioning placed rods is as follows:

1) Tighten nut on threaded rod/fastener hand-tight with fastener
threads lubricated using copper anti-seize lubricant (I can track
down the manufacturer if needed).  Leave at least 22" of free
rod length exposed.  This length accomodates the length of our
center-pull jack, a "tensioning chair" (described below), a thick
plate washer, and an additional nut.

2) Place a "tensioning chair" on the excess rod length.
The chair is essentially a hollow steel box (0.25" thk) with one
side cut away to permit access to the nut which will be left
"in-place" after the connection is complete.

3) Place the center-pull jack, then the plate washer, then tighten
the nut to ~ 1/2 rod diameter above the plate washer.  Forget this
space, and you may find yourself cutting the jack off of the rod.
We haven't had problems *quite* this bad, but it's been close.
Make sure your jack has enough stroke to accomodate the fastener
elongation if you have a large active length.

The jack will now push against the plate washer, which is retained
on the threaded fastener by the additional nut.  This transfers
the load into the threaded fastener/rod.  So far, no surprises,
I'm sure.

4) Knowing the internal area of the jack, we apply a
specified hydraulic pressure to the jack, namely the desired
fastener pretension divided by the jack area.  This stretches
the fastener, allowing the in-place nut to be further tighened.
Again, no shock to anyone.

This is where some variability may come in.  *We* only care
about a *minimum* pretension, so we always boost our needed
jack pressure by 10% or so, and then tighen using a strike wrench
and a rubber-faced mallet.  Even for our smallest research staff
(H < 5'3", W < 150#), we can reliably develop *at least* our target
pretension using this technique.

5) Release the pressure on the jack, and remove the tensioning
assembly (nut, plate washer, jack & chair).  You now have ~ 22
inches of fastener sticking out.  This is fine for us, since we
want to be able to remove these particular fasteners.  For
someone else, I'm sure that a "coupling nut" arrangement can be
used instead of the extra length of fastener.  This should leave
only one or two fastener diameters protruding beyond the nut
after tensioning.

I can send a graphic by private email if the description of our
procedure isn't sufficient.

HTH,

Charley

--
Charles Hamilton, PhD EIT               Faculty Fellow
Department of Civil and                 Phone: 949.824.3752
    Environmental Engineering           FAX:   949.824.2117
University of California, Irvine        Email: chamilto(--nospam--at)uci.edu



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