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RE: Bracing of the compression flange for steel beams

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As some recent discussions have indicated, to qualify as a brace stiffness
and strength are both important.  Welding channels to the walls of tanks and
hoppers to provide stiffness and strength has a long history.

The bracing aspect of the compression flange is an interesting problem.  As
in a previous discussion on purlins, the metal building industry considers
the bottom of purlins "braced" for uplift with floating standing seem deck
and clips.  You can't make the numbers work, but they have conducted tests
that show that somehow it works.  I don't like it, but those are part of
systems that are performance specified.

You can look at the stiffness of the plate and run numbers to get an
indication of the stiffening effect of welding the channels to the plate.
Obviously translation is not a problem, so rotation and rotational stiffness
is what requires checking.  Blodgett's Design of Welded Structures may give
you a bit of input.  There is also a book called "Simplified Design of
Structural Steel", by Marcus that has some good stuff on bracing.  Joseph
Yura has also written extensively on bracing.

You will have to use mechanics principles to find out the effectiveness of
bracing.  AISC will give you the basics, but they will not tell you what
exactly comprises a brace.

Harold Sprague
The Neenan Company


-----Original Message-----
From: Andy Costello [mailto:acost(--nospam--at)cwfab.com]
Sent: Friday, August 20, 1999 2:50 PM
To: seaint(--nospam--at)seaint.org
Subject: Bracing of the compression flange for steel beams


Dear learned friend,
A discussion has arose in our organisation on this topic,
I would like to hear some other opinions, if you please.

When stiffening an rect empty storage tank of steel plate,
with the stiffeners placed on the outside of the tank,
using C8 channels as primary and FB8 flat bar as secondary
stiffeners, both in opposite directions.
The C8 compression flange (side not welded to tank skin),
under wind loading, is fully braced torsionally and laterally,
by the coped FB8 welded into the mouth of the C8.

Is it acceptable to either AISC or any other organisation,
to use a secondary stiffener that does not attach to the
compression flange, like a FB4 in the mouth of the C8?
There is probably some torsional restraint but lateral is
questionable, which affects the effective length used.
Any comments welcomed.

Thanking you in anticipation,
Andy Costello