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RE: Anchor Chairs Question

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I can think of several reasons why it's better to have the chair seat at the
top of the gussets.

First of all, it seems like a much easier fabrication. If you put the seat
on the bottom, how do you attach it to the tank? There is always an inch or
two of projecting floor plate at the base of the shell, so you would have to
either mill the chair seat, trim the floor plate, or live with the chair
being mounted further from the shell, which just increases the eccentricity
of the bolt load and leads to longer gussets.

Then you've got the issue of taking out the moment caused by the
eccentricity of the bolt tension relative to the shell. With the chair on
top, you've got in effect a couple of flanges (the bolt chair and the floor
plate) to provide a resisting couple, and the chair distributes the force
across some length of shell. With the chair on the bottom, the moment has to
be provided by tension of the gusset weld at the shell, which isn't very
efficient and will require longer gussets and bigger welds.

With the chair on top, the transfer of bolt load from the chair to the
gussets is by compression, so the capacity of the weld between these two
pieces becomes much less critical.

Finally, by keeping the chair off the ground, you have less potential for
corrosion and easier access for maintenance and recoating.

-----Original Message-----
From: Cornaby, Sam - EIMCO [mailto:Sam.Cornaby(--nospam--at)eimcoprocess.com]
Sent: Thursday, March 14, 2002 6:38 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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