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Re: Bolt Tension

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If your intent is to attain full fixity using the full capacity of each
bolt, then you'll need to pre-tension the bolts to about 99 kips (Area=1.227
sq in  x  Fy = 81 ksi).  But, you may have extra bolts in your design, so
this calc may not be applicable.

Also, if you were to use a Skidmore to determine the installation torque,
you'd need to use short 1.25" dia bolts in the Skidmore, which wouldn't be a
representation of the actual installed condition.  You've probably already
found that out and that's why you have a testing lab looking at the problem.
I assume your testing lab is trying to establish a field installation
procedure that will pre-tension these particular bolts to your specs.

One of the problems with this scenario is that the length of each bolt that
is not embedded can't be established without testing in hardened concrete.
That is, as the bolt is tensioned the upper portions that are embedded will
break the bond between the concrete and the surface of the bolt and thus the
length of the bolt that is not embedded will change as the bolt is
tensioned.  If you're trying to find a torque to install the bolts to, the
testing lab will have to embed samples of the bolts in concrete. allow
sufficient time for the concrete to set, and then test the bolts.  I can see
this as being very unscientific and unreliable.

In the final analysis, I also tend to agree with the creepy theory.

My $.02 worth.

Lew Midlam, PE


-----Original Message-----
From: Paul McEntee <pmcentee(--nospam--at)>
To: seaoc(--nospam--at) <seaoc(--nospam--at)>
Date: Thursday, April 30, 1998 1:40 AM
Subject: RE: Bolt Tension

>While were on te subject of bolt tensioning...
>We were having this debate over whether or not to tension high strength
>threaded rods in a fixed base connection for a moment frame.  Some argue
>that you must tension the bolts, otherwise you'll get large rotations of
>base plate before it starts taking moment.  Others were of the opinion that
>pre-tensioning the rods loads the concrete, causes it to creep, and the
>tension will relax anyway.  I'm kind of with the creepy group myself
>(couldn't resist).  The issue of it being difficult and expensive to
>the rods came up.  In one particular case, our detail calls for 1.25"
>diameter A449 Rods tensioned to 48 kips, and we are now in the process of
>getting the inspection company to take samples of the rods into their lab
>(rods are too big and too strong to use a skidmore on site) to measure the
>torque required to get that tension using the contractors wrench.  I don't
>sympathize with the difficulties this causes the contractor too much if the
>pre-tensioning of the bolts truly increases the fixity of the connection,
>but if the tension is going to relax in a few months anyway, I'd hate to
>have the owner paying extra for this.  Not to mention the fact that there
>a risk of breaking the bolts while doing this.
>Appreciate any fresh (ie, not from my coworkers) opinions on this.
>A self proclaimed lurker