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Re: Torque vs. Tension in Bolts

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Thanks to all for the formula that relates torque to tension based on the K
factor.  I believe that's the formula I remember using when I had to
pre-tension some anchor in the past.  The required torque on sleeve-type
anchors is an option I will look into tomorrow morning for my particular
application.  And yes, I do know that this formula can't be used for high
strength bolts which is probably why I couldn't find this equation in any
of my references.

As for my reason for needed this equation, we have a seismic upgrade
project of a school (1950's) with 8" concrete walls and wood roof.  Very
little connection cuttently exists between the two.  For both in and out of
plane support of these walls, we are adding (among other items) drilled
anchors at 48" o/c in the existing 2x8 top plate down into the concrete.
The existing condition is 1/2" cast in place bolts at at 6' o/c.  The
properties of the wood determines size and spacing of bolts, not the steel
or concrete strength.  We have been kind of vague on which type of drilled
anchor to use, giving the contractor some choices between wedge, sleeve,
and expansion anchors along with a couple different manufacturers.  The
contractor has bought hundereds of 3/4" diameter RedHead Trubolt wedge
anchors with standard washers (probably the least expensive product we
pre-approved).  A little card that comes with the box of bolts states the
torque shall be 3 to 5 turns of the nut or value in the table and also
lists ONLY the minimum embedment depth.

The special inspector and city inspector have both rejected the contractors
bid to use the 3 to 5 turns and are requiring the 175 ft lbs of torque per
the ICBO report.  As expected, the wood crushes before the torque is met.
After talking to the techincal rep, the 3 to 5 turns is misleading as this
relates to the tested torque achieved by bolting a steel plate to concrete,
not other materials such as wood.  The 175 ft lbs is also really required
to make the wedge anchor functional as per the manufactuer's requirements.

As the existing wood plate can't be removed and reinstalled, I have used
the provided formula and figured out that a 5" square plate is required to
distrbute this 12,000 lb clamping force as to not damage the wood- maybe.
The wood properties of this existing plate will surly not behave like new

It was looking earlier today like epoxy anchors are the only viable option
unless the sleeve anchors/torque will work with the wood.  Of course, the
contractor wants more money for the change although I don't see any change
per the design documents, only installation requirements different that
what they have done in the past. 

As to the card with the bolts listing only the minimum required embedment
lengths - not good.  Standard embedments are required for seismic
applications per the newer ICBO reports. Luckly I called out the embedment
depth instead of just per manufacturer's requirements.  This is probably
another reason why the contractors think that engineers overdesign

The contactor also has claimed that they have installed thousands of wedge
anchors and never been told to torque per the ICBO (and manufacturer's
values).  They always stop torquing when the wood starts to crush.  I have
always assumed that the bolts are typically installed and set before the
wood plates are installed and then the nut backed off and the wood put in
place.  We haven't paid too much attention to this in the past but I bet
there are many bolts not properly installed.

Well, I learned something new today.


Peder Golberg, PE
Portland, OR