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RE: Bolt Tension

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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 the
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 bolt
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 tension
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 is
a risk of breaking the bolts while doing this.

Appreciate any fresh (ie, not from my coworkers) opinions on this.


A self proclaimed lurker

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