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Re: Compression flange bracing

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This may be applicable when the item supported (plate, grating, etc.) is
continuous.  For simply supported members bearing on the beam, however, the
restoring moment will not be created.

David Finley, P.E.



-----Original Message-----
From: Charles Greenlaw <cgreenlaw(--nospam--at)speedlink.com>
To: seaint(--nospam--at)seaint.org <seaint(--nospam--at)seaint.org>
Date: Friday, October 29, 1999 4:31 PM
Subject: Re: Compression flange bracing


>Might it be that it is not friction after all, but the width of the beam's
>top flange, which has the relatively rigid load-bearing grating or tank
roof
>bearing down on it, that is the source of supporting beam's stability?
>
>The bottom, tension flange wants to stay in a straight line. The top
flange,
>to go sideways, has to twist the whole cross-section. To do so it lifts one
>top flange edge. Then the load bears on the lifted flange edge and puts an
>eccentric moment in the beam that twists it back where it belongs.
Stability
>results. No friction is required.
>
>I learned this principle from a Navy Senior Chief Bos'n's Mate named
>Zurybida, who had gold hash marks the whole length of his sleeve. Only he
>was talking about upward restoring moment that comes from water pressure on
>a ship's hull that is listing to one side due to an overturning moment. The
>relationship between the ship's center of gravity ("grabity" he called it)
>and its metacentric height (that results from its hull form in the water,
as
>heeled over) is what counts. Friction from the water wasn't in the game at
all.
>
>I can see an analogous "metacentric height" phenomenon in a beam's
>width/depth ratio vs the stiffness and form of the contact that is loading
>the beam down.
>
>Charles O. Greenlaw  SE   Sacramento CA
>
>At 12:45 PM 10/29/99 -0500, you wrote:
>>From over 30 years ago...  In design of power plants (non-seismic area), I
>>asked how we could consider bar grating as lateral support for the top
>>flange of grating-support members.  The answer was that the beams would
not
>>be loaded if it weren't for the load on the grating.  If the beams are
>>loaded, we can count on a 20 percent coefficient of friction for lateral
>>support.  And 20 percent of the vertical load is gracious plenty for
>>lateral support.
>>
>>So far as I know, the law of gravity has not been repealed.
>>
>>Now, you "seismic dudes" may want to come up with some other restrictive
>>rules, but it remains that beams which aren't loaded don't buckle.  And if
>>they're loaded on the top flange, they *might* have a bit of lateral
>>support.
>>
>>Fountain E. Conner, P.E.
>>Gulf Breeze, Fl. 32561
>>
>>----------
>>> From: Lutz,James <JLUTZ(--nospam--at)earthtech.com>
>>> To: seaint(--nospam--at)seaint.org
>>> Subject: Compression flange bracing
>>> Date: Friday, October 29, 1999 10:33 AM
>>>
>>> The issue of compression flange bracing has come up in my practice on
>>> numerous occasions with respect to rafters supporting the roofs of steel
>>> water storage tanks. Typical practice is to not weld the rafters to the
>>roof
>>> plates, so compression flange bracing is only provided by friction of
the
>>> roof diaphragm. This is specifically permitted by AWWA D100, which
allows
>>> rafters to be designed as if continuously braced.
>>>
>>> The argument I get into when reviewing the work of other designers is
>>that I
>>> think UBC (AISC Specifications) should overrule the AWWA procedure. The
>>UBC
>>> permits the use of "approved national standards" (e.g. AWWA) for the
>>design
>>> of ground storage tanks (UBC 1634.4), but I read this as applicable to
>>> seismic design only. In all other aspects, the UBC should govern as is.
>>>
>>> Anybody have any thoughts on the validity of counting friction for
>>lateral
>>> support? I can't imagine AWWA recommending this design approach without
>>some
>>> sort of basis.
>
>