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RE: Bolt Tension

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We have used domed washers made of spring steel under pretensioned
anchor bolt nuts to help ensure that most of the pretension was
retained.  These washers are cupped and flatten out under a given load.
The ones we specified were made by the Rolex Company - a single washer
for a 1 1/2" diameter bolt would flatten fully at 20 kips, one for a 2"
bolt at 32 kips.  We used double washers to get double these pretension
loads.  One benefit of these washers is they act like load indicator
washers (you do not need to measure installation torque).  I would use
plate washer anchors at the bottom of such anchor bolts and coat the
upper shank so that they are free to elongate when pretensioned.

Ed Marshall, PE
Simons Engineering
Atlanta

> -----Original Message-----
> From:	Paul McEntee [SMTP:pmcentee(--nospam--at)home.com]
> Sent:	Thursday, April 30, 1998 1:30 AM
> To:	seaoc(--nospam--at)seaoc.org
> 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 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.
> 
> Paul
> 
> A self proclaimed lurker