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RE: Bolt retightening

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Kevin,

Torque wrenches are only supposed to be used in arbitration in the installation of A325 or A490 bolts according to the RCSC. Torque is a very poor indicator of tension in a bolt. The tension is what you are trying to achieve.

Getting a Skidmore on the project site (hydraulic tension indicating device) is not a big deal for most iron workers, and is fairly routine.

Because of the variability, I would NEVER allow calibrated wrench bolt tightening.

When you boil it all down, the best and probably cheapest solution is to replace all of the bolts with the tension indicating bolts using the splines. The installation wrench rotates the nut while holding the bolt. The spline snaps off when the appropriate tension is achieved. You still have to install the bolts to "snug tight" with the faying surfaces in full contact. And you still need the Skidmore on the site to verify the appropriate tension for the tightening procedure you select. The installation is a lot easier using the twist offs as opposed to using a pneumatic wrench or a spud wrench with a cheater (for turn of the nut). The labor is where you will save the money over other installation methods. And the QC is better.

If you want the ultimate in reliability, ease of installation, and 100% QC, use the DTI "Squirter" washers with "twist off" bolts. You get the ease of installation with the "twist off" wrench and you get the tension indication that the "squirter" washers provide.

If you decide to go against my incredibly sage advice and use turn of the nut, use match marking with a paint stick. And never tighten bolts without a Skidmore.

Regards,
Harold Sprague





From: Kevin Below <kevinbelow(--nospam--at)videotron.ca>
Reply-To: <seaint(--nospam--at)seaint.org>
To: seaint(--nospam--at)seaint.org
Subject: Bolt retightening
Date: Mon, 26 Sep 2005 22:48:26 -0400

I am in need of some guidance on the right technique for retightening
bolts on a 40-year-old steel structure.
One of my clients, suspecting that the bolts in one of their many
buildings were loose, asked me to check.  Surprisingly, we found that
his suspicions were founded.
The roof structure of this warehouse type building is the Gerber type
(cantilevered beams alternating with suspended spans), and the beams
that pass over the columns
have 4 bolts in the connection. These bolts are almost all loose.  You
can unscrew them by hand.
The original drawings call out hi-tensile bolts in non-slip connections,
which is obviously not what they got.
There is no bracing in the building, and no concentric masonry walls, so
I presume the lateral resistance comes from the non-slip joints of the
beams over the columns, forming rigid connections.  In this case, the
beam-column connections should rightly be non-slip, and not bearing
type.
So now I have to prepare plans and specs for tenders for the job of
tightening the bolts.
My search of the steel manuals and text books and web sites leads me to
conclude that it is not simply a case of applying a torque wrench to
these bolts.  In the old Canadian steel code (oh yeah, this job is in
Canada), the required torque was specified, but now it appears that a
predetremined torque does not always produce the required bolt
pretension.  So each shift, the guy using the torque wrench should
calibrate his torque wrench with the type of bolt to be installed, on a
machine that measures the pretension.

My first question is whether I should require new bolts, since I have no
control or knowledge of the existing bolts.  It seems like a good idea,
especially if I need to calibrate the torque wrench.

Also, the idea of a machine to calibrate the torque wrench each day
seems overkill to me, when this building doesn't seem to have suffered
in its loose state for so long, surviving mild earthquakes and strong
winds with no sign of distress, despite its lack of tightened bracing.
Would the turn of the nut be sufficient, or is this really only for
bearing type connections ?


What do the steel gurus think ?




Kevin Below, ing., Ph.D.





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