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RE: Bolt Tension

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That's a good point.  If you turned the concave surfaces towards each
other (or away from each other) you load the washers in series and the
total force to flatten two is the same as one (the reaction from the top
washer is the load on the bottom washer).  The trick is to nest the
washers so they are loaded in parallel (the load required to flatten the
top washer can be thought of as reacting through the perimeter of the
lower washer into the foundation, with additional load required to
flatten the lower washer).  It's a little tricky.

Ed

> -----Original Message-----
> From:	Chance, Acie P. [SMTP:acie_chance(--nospam--at)wdi.disney.com]
> Sent:	Thursday, April 30, 1998 5:46 PM
> To:	'seaoc(--nospam--at)seaoc.org'
> Subject:	RE: Bolt Tension
> 
> Ed
> 
> Hope I am wrong but it seams to me the total force needed to flatten
> two
> domed washers would be the same as the force to flatten one domed
> washer.
> Two springs in a series have an equivalent stiffness of one half the
> stiffness of one spring. As you moved the nut through twice the
> distance you
> still only got the force of one domed washer. If you see this
> differently or
> the manufacture or testing lab indicated you can double up on the
> washers
> and get twice the force let me know.
> 
> Acie Chance
> 
> > ----------
> > From: 	Ed Marshall[SMTP:elmarshall(--nospam--at)HASimons.com]
> > Sent: 	Thursday, April 30, 1998 12:28 PM
> > To: 	'seaoc(--nospam--at)seaoc.org'
> > Subject: 	RE: Bolt Tension
> > Importance: 	High
> > 
> > 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
> > 
> > 
> > 
> 
>