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Re: Compression flange bracing

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API Storage Tanks

Some years ago I was faced with the same problem. Purlins and the roof plate
of a storage tank , designed exactly according to the API Code, erected in
one day buckled immediately in the following day.

A consulting engineer was hired to study the problem and produced quite a
massive report but could not give a single credible reason why this was
happening. He thought the cause was the roof plate  applying a lateral force
to the purlin!!!!

The problem was given to me to solve. It was easy! In one page I
demonstrated that the designer forgot to check the purlin deflection!
Excessive purling deflection was causing the roof plate to detach from the
purlin upper flange allowing it to buckle laterally.

Without changing the purlin size, I recommended some stoppers be welded to
the roof plate confining the purlin top flange lateral movement maintaining
at the same time the rule prescribed by the API Code that the roof plate
should not be welded to the purlin.
Ivan Lippi
Structural Engineer

-----Mensagem original-----
De: Fountain Conner <fconner(--nospam--at)>
Para: seaint(--nospam--at) <seaint(--nospam--at)>
Data: Sexta-feira, 29 de Outubro de 1999 15:51
Assunto: Re: Compression flange bracing

>>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So far as I know, the law of gravity has not been repealed.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
>Fountain E. Conner, P.E.
>Gulf Breeze, Fl. 32561
>> From: Lutz,James <JLUTZ(--nospam--at)>
>> To: seaint(--nospam--at)
>> 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
>> 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
>> permits the use of "approved national standards" (e.g. AWWA) for the
>> 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
>> support? I can't imagine AWWA recommending this design approach without
>> sort of basis.