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U. L. Steel Beam Substitution

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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)techteam.org

The service mission like-minded Christian organizations
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