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RE: Eccentrically Loaded Baseplates

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It appears that you will be analyzing for the forces in the anchor bolts
assuming elastic behavior.  Theoretically, as I'm sure you are aware, the
behavior is non-linear.  One of the methodologies used by the nuclear
industry, and you know they have a lot of base plates in those plants, is to
assume the point of major compressive stress is a distance of 2t away from
the compression face of the attachment, where t is the thickness of the base
plate.  The distance between this point to the bolt location would be
considered the moment arm.  You would do this in both directions and add
absolutely the bolt tensions from each direction (here assuming you use the
axes of the plate).  I believe you could use the SRSS of the tension loads
for an individual bolt for the forces from each orthogonal seismic direction
but you may prefer CQC for the combination.  You would then add these
seismic forces algebraically to dead load forces.  To check the plate
stress, you would consider the plate moment arm as the distance between the
attachment face on the tension side to the bolts in tension.  This
methodology was found to work quite well for a uniform pattern of bolts.

The question of prying action on the the bolt forces and the plate stresses
always was an issue for these applications.  As I'm sure you are aware, the
bolt stiffness (including the stiffness of the substrate to which it's
attached) greatly affect the final bolt tension load in the ultimate.  For
instance, kwik bolts would displace thus relieving the prying action.
Cast-in-place anchors would not, therefore resulting in some prying forces.
I believe that you would have some displacement in the adhevise anchors
under load.

Another factor affecting the prying force is the base plate thickness, but
it appears to me that 7/8" is a mighty thick plate and you will not have a
large moment arm between the face of the attachment to the anchor bolts.

I hope this helps; it may appear conservative, but the results were
documented by Teledyne Engineering Services using the then current form of
ANSYS.

John Russ
Diablo Canyon Power Plant

-----Original Message-----
From: Lutz, James [mailto:JLUTZ(--nospam--at)earthtech.com]
Sent: Friday, October 04, 2002 11:14 AM
To: seaint(--nospam--at)seaint.org
Subject: Eccentrically Loaded Baseplates


Anybody know of a good reference for estimating anchor bolt loads on column
base plates with bending in two directions? 

I am evaluating some existing pipe rack frame connections where this is an
issue. The base plates are 18 inches square by 7/8", the column is
HSS8x8x.25, and the plate is anchored to a thick concrete slab with 8 each 1
inch diameter Hilti HY-150 anchors in 4 ksi concrete, 8.25 inch embedment.
The rack is subject to seismic loads and we intend to modify it to support
an additional 30 inch pipe. The frame can handle the loads just fine, but I
am concerned about the connection. 

Jim Lutz, P.E., S.E.
Earth Tech, Inc.

10800 NE 8th Street - 7th Floor
Bellevue, WA 98004
(425) 455-9494
(425) 453-9470 FAX



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