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RE: Plate Washers

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Assuming the washers are going on the anchor rods... and the holes are not
typical anchor bolt/rod holes i.e. they are oversized, field modified, or

I would use a washer AT LEAST the thickness of the base plate.  This is to
prevent the anchor rod and nut from being pulled through the slot/hole.  I
have seen cases where the erector field adjusted the holes to accommodate
bad AB placement and placed a std "hardware store" washer under the nut.
That particular building was blown over during construction due to lack of
adequate temporary bracing.  Most of the rods, nuts, and washers were pulled
straight through the base plate without damaging the base plate at all.
Left a nice cupped washer over the anchor rod nuts though.  For that reason,
on oversized holes (above standard oversized AB holes) use a minimum
thickness equal to the base plate.

The size of the washer is up to engineering judgment.  If you have slots
that you are worried about then provide a rectangular washer that is +- 1/2"
past the slot edges and edge of the anchor rod nuts then field weld the
washer to the base plate with fillet welds.  Size the welds per shear
requirements.  I would assume the min. AISC/AWS welds would be adequate in
most of the cases.

For most cases I would not have a problem with 1" to 2" of non-shrinking
grout.  One thing to keep in mind is to have enough room for leveling nuts
or plates (if used).  If there is much force reversal in the bolts you might
consider "double nutting" the base plate... put a nut (typ. called a
leveling nut) under the base plate and another nut on top of the base plate
at each bolt/rod.  This will help eliminate any slop in the connection
caused by the shrinking of the grout.  Downside is this can also put
compression in the anchor rods... might be a problem if they were not
designed for that load... This compression force could also affect how much
grout is acceptable.  Another tip on the nuts is if the bolts see much
force/stress reversal than I would recommend doing something to upset the
threads on the anchor rods to prevent the nuts from backing off or loosening
up.  I would definitely upset the threads and consider the use of leveling
nuts/plates and grout on T-shaped frames like you are describing.

Greg Effland, P.E.

-----Original Message-----
From: Nicholas Blackburn [mailto:nblackburn(--nospam--at)]
Sent: Thursday, June 07, 2001 11:37 AM
To: 'seaint(--nospam--at)'
Subject: Plate Washers

I have a steel framed canopy with a single row of columns and an inverted
gull wing (think of the WW2 Corsair wing) roof.  The columns are anchored to
drilled piers with 3/4" threaded rods.  The threaded rods were cast into the
pier foundations at an earlier time than the design/construction of the
canopy and were designed by a different company.  There is no record of the
grade and type of rod used so I assumed A36 to be conservative.

AISC section J3.2.e indicates the use of washers that completely cover the
slot and that they should be 5/16" thick min when using high strength bolts.

I am looking for some suggestions as to the size and thickness of the

On a similar line, how much of a grout pad would you allow below the base


Nicholas Blackburn, PE

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