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RE: Anchor Chairs Question

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1.  Anchor bolt chairs for most tanks and vessels are usually required
when the base plate is less than 1 inch thick.  Thin base plates bend
excessively under the load and cause a very concentrated load on the
corner weld (the plate to the bottom shell ring)
2.  The anchor bolt chair serves two purposes- to transfer the load into
the shell, and to provide a "stretching length".  A short bolt thru the
base plate only should be debonded over 10 to 12 inches.
3.  Chair height should be a minimum of 6 inches for skirt supported
vessels and 12 inches for flat bottom tanks.
4.  Chair design procedures can be found in Part VII of Steel Plate
Fabricators Association publication "Steel Plate Engineering Data"
5.  It is possible to use gussets in some application if the base plate
is thick.  For thin base plates or typical flat bottom tanks inverted
chairs or gussets should not be used for two reasons. First,  look at
the stresses in the attaching welds (assume a triangular load from the
eccentric moment).  A gusset will give a very high stress concentration
at the tip of the gusset.  A normal chair has the width of the chair (4
inches or so) to distribute this load.  Second, the AB chair design
procedure in the SPFA book is based on the Bijlaard work on loadings on
shells and assumes the base plate / tank bottom is the neutral axis for
a stiff rectangular fitting ( i.e. the chair is a rigid box).  Using
gussets does not create the rigid box and the current design procedures
will under-estimate the peak stresses in the shell for inverted
chairs/gussets.  An inverted chair on a flat bottom tank can also create
a maintenance problem since the bottom of the chair is not accessible
for cleaning and painting
6.  The chair height and width are a direct function of the applied
eccentric moment.  Chair heights of up to 36 inches are found in high
seismic applications for tanks with thin shells.  Sometimes a solid
stiffener ring replaces the chair tops to reduce the chair height.
Occasionally, the lower shell ring must be thickened to reduce the shell
bending stresses.
7.  Anchor and chairs should be not further than 10 ft on center and if
less than 2 ft 6, a ring should be considered.
8.  The chair should also be designed to develop the yield of the
anchor.  We want the bolt to stretch in an overload
9.  The contact between the bolt/nut and top of chair is also important.
If the bolt is out of plumb, or the chair is not flat, a lopsided
bearing on the nut may cause the bolt to fracture prematurely (prying).



Stephen W. Meier, PE, SE
Vice President
Tank Industry Consultants
7740 West New York Street
Indianapolis, IN 46214

Voice:    317-271-3100     Fax:      317-271-3300
Mobile:  630-728-5986    Pager :  800-539-7102
email:  meier(--nospam--at)tankindustry.com
web page:  www.tankindustry.com


-----Original Message-----
From: Cornaby, Sam - EIMCO [mailto:Sam.Cornaby(--nospam--at)eimcoprocess.com] 
Sent: Thursday, March 14, 2002 9:38 AM
To: 'seaint(--nospam--at)seaint.org'
Subject: Anchor Chairs Question

List,

In the design of anchorage for tanks, I understand that anchor chairs
are
necessary to minimize secondary bending in the shell with anchor
tension.
My question is: Why can't a pair of gussets of the same length as the
chair
be used instead, with the chair "seat" at the bottom of the tank instead
of
being elevated, utilizing much shorter anchor bolts?  Doesn't the tank
shell
react the same way to either configuration?  Is the length of the anchor
itself important, independent of the anchor chair?

> Sam Cornaby
> EIMCO Process Equipment Co.
Salt Lake City, UT
801-526-2542

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