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RE: Long-Span Beam

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The 1/2 the span rule is an old rule of thumb that can be derived by equating a full stress condition (of 0.66 x 36 = 24 ksi) for uniform load to a maximum deflection of L/240 to derive the beam depth in terms of span length. 

With different stress levels, different load conditions or different deflection limits, the rule of thumb changes.

Make sure that you adequately brace the kink points of the beam. If you have uplift, you may need additional bottom flange braces. I would also think that you could reduce the depth of your beam as long as you do not exceed allowable stresses and deflection limits. 

I hope this helps!

Jim K.
Green Bay, Wi

-----Original Message-----
From: Donald R. Bryant, PE [mailto:dbryant61(--nospam--at)cox.net]
Sent: Wednesday, March 08, 2006 3:35 PM
To: seaint(--nospam--at)seaint.org
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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