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Long-Span Beam

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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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