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RE: Torque vs. Tension in Bolts

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Wrt torque vs. tension, you need to know something about the thread type and
pitch, lubrication, and general condition to get a good handle on this.  As
everyone else has said, it's pretty variable.  With respect to your problem,
it's good to keep in mind why you are torquing the anchor.  Anchors are
torqued for one or a combination of three reasons:  1) to set the anchor, 2)
to snug up the fastened parts, and 3) to prestress the anchor as protection
from vibratory loads.  Some mechanical expansion anchors (not all) require
torque to pull the cone into the expansion elements, i.e., to set the
anchor.  Bonded anchors, undercut anchors, and some expansion anchors do not
require torque to set the anchor.  Snugging up the connection to take out
any play is important for initial stiffness, however.  Finally, prestressing
for vibratory loads requires special measures to ensure retention of
prestress over time.

In the discussion of bonded vs. mechanical anchors for direct tension
earthquake loads, it is important to note that the tension performance of
any anchor (CIP or post-installed) is affected by concrete cracking.  In
your case, it's reasonable to assume that your foundation will crack in an
earthquake; if you reinforce it properly, the cracks should remain
relatively small and well distributed...  Bonded anchors suffer large
reductions in tension capacity when tested in cracks, as do some expansion
anchors.  If you've got a lot of concrete to anchor into and can reach deep
into the confined region of your concrete member, you should be ok.
Otherwise, you might want to consult with the manufacturer about the correct
product for your application.

J. Silva, SE
Hilti, Inc.

> ----------
> From: 	Peder and/or Cathy Golberg[SMTP:golberg(--nospam--at)e-z.net]
> Reply To: 	seaint(--nospam--at)seaint.org
> Sent: 	Tuesday, July 13, 1999 10:20 PM
> To: 	seaint(--nospam--at)seaint.org
> Subject: 	Torque vs. Tension in Bolts
> 
> Can someone tell me a rule-of-thumb formula or another method to convert
> the torque value placed on a bolt to the tension value.  I had a formula
> that a mechanical engineer gave about 10 years ago that I have now
> aparently misplaced and it is something I don't recall ever seeing in a
> text book.  I do read, however, in the AISC Connections book that any
> published method of conversion is not acurate.
> 
> My situation is for the use of expansion anchor bolts in concrete to
> connect a wood plate (sill or ledger).  The wood will crush before the
> required torque is achieved without a plate washer.  I want to determine
> just how big these plate washers need to be but need to convert torque
> value to do this.
> 
> Do others call out plate washer sizes in details, use more expensive epoxy
> anchors, or is this something that gets ingnored and the anchors are not
> always installed properly?
> Thanks in advance.
> 
> Peder Golberg, PE
> Portland, Oregon
> 
> 
>