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Re: Long-Span Beam

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Donald,

As I recall, and some smarter people on the list (like Charlie Carter, for example) can correct me if I'm wrong, it was actually a requirement of earlier codes to limit the depth of a steel beam to not less than L/24 as a means of limiting deflection (hence controlling serviceability). This was fine for statically determinate structures (simple span beams, etc.) and low strength steel, however, with the advances to continuous structures and high strength steel, this requirement was eliminated in favor of other methods of serviceability such as limiting deflections to values such as L/360.

I think if you were to do a full analysis using a deflection limit you are comfortable with and properly stabilizing the beam you should be able to reduce the depth to say 30" without any serious problems.

I happen to like L/24 as a depth guideline, however, when it seems to be a complete waste of material I don't mind using L/30. I don't think I'd like to use anything less than L/36 without giving it a lot of thought though.

These are my thoughts, however, I must point out that I'm not very familiar with your codes.

Regards,

H. Daryl Richardson

----- Original Message ----- From: "Donald R. Bryant, PE" <dbryant61(--nospam--at)cox.net>
To: <seaint(--nospam--at)seaint.org>
Sent: Wednesday, March 08, 2006 2:34 PM
Subject: Long-Span Beam


I have a project that requires a clear-span of about 70 feet. To complicate matters, the beam must be a continuous wide-flange with three kinks, one at
the center for the peak, either side of which is 4:12 pitch, and then two
kinks 10' from the ends where the roof slope reduces from 4:12 to 3:12.

That said, I almost religiously follow the beam depth rule of thumb of 1/2
the span in feet equals the beam depth in inches.  This rule requires the
beam to be a W36x135, but in my RISA analysis, the beam is only 30%
stressed. I would like to reduce this beam size safely, but I am afraid to
violate the rule for stability concerns.

Could anyone enlighten me on the origins of that rule of thumb, and a solid
rationale for breaking it.

Please email me directly as I am in digest mode.

Sincerely,

Donald R. Bryant, PE
Structural Integrity
518 Bushnell Drive
Virginia Beach, VA  23451
757-428-6471 (Office)
757-428-6473 (Fax)
757-407-6471 (Mobile)



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