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RE: U. L. Steel Beam Substitution

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In the 1997 Std. Building Code (Tbl 709.5.2) W/D ratios are tabulated for
many common W shapes down to W4x13.
Email me directly and I will be glad to fax a copy.

Hal Riddle

> -----Original Message-----
> From:	rlewis(--nospam--at) [SMTP:rlewis(--nospam--at)]
> Sent:	Monday, March 29, 1999 4:49 PM
> To:	seaint(--nospam--at)
> Subject:	U. L. Steel Beam Substitution
> I am confused about a U.L. floor assembly.  I have a floor assembly of say
> U.L. D902  which states the minimum beam size is a W8x28.  Now if you scan
> through the book you find that most beams are a minimum of W8x28.  In the
> front of the manual it states:
> "For fire resistance purpose, the minimum beam size is expressed in terms
> of
> a W/D ratio, where W is the weight of the beam per lineal foot and D is
> the
> perimeter of protection material at the interface between the steel
> section
> and the protection material.  Accordingly, beams of the same configuration
> and having a greater W/D ratio than the beam size specified in the fire
> resistive design are considered larger than the specified minimum size
> beam
> and may be used in that design."
> So, I interpret this to take the weight of the beam and divide it by the
> perimeter of the beam section, minus the top flange width which is against
> the slab/deck.  If I want to substitute a deeper beam in this W/D ratio, I
> must use a heavier beam than the 28 PLF because a deeper beam will
> increase
> the 'D' value which reduces the W/D ratio.  Now I know I have used beams
> smaller than 28 PLF in previous designs and nobody has ever questioned it.
> Have I always been wrong using beams lighter than 28 PLF that are spray
> fireproofed?  Am I interpreting this wrong?  Is there a table somewhere
> that
> already has all of the W/D ratios figured out?
> Thanks for your help.
> __________________________________________________
> Richard Lewis, P.E.
> Missionary TECH Team
> rlewis(--nospam--at)
> The service mission like-minded Christian organizations
> may turn to for technical assistance and know-how.