Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Re: Compression flange bracing

[Subject Prev][Subject Next][Thread Prev][Thread Next]
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So far as I know, the law of gravity has not been repealed.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.