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RE: Seismic Zone 1 - U. L. Steel Beam Substitution

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Good way to get attention!!!

We routinely use UL D902 with beams lighter than 28 plf based on an
increased thickness of fireproofing. As I understand, the front of the UL
book indicates that lighter sizes can be used if the fireproofing thickness
is increased.  A formula is provided in UL.  Hopefully, your architect is
taking care of this.

Tom  Bouffard

		-----Original Message-----
		From:	rlewis(--nospam--at) [mailto:rlewis(--nospam--at)]
		Sent:	Monday, March 29, 1999 4:49 PM
		To:	seaint(--nospam--at)
		Subject:	Seismic Zone 1  - U. L. Steel Beam

		I am sending this out again with the Seismic heading because
nobody responded
		to the first message.  It seems everbody responds to a
Seismic question so I
		thought I would give it a try.  By the way, it really is in
Zone 1!

		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
		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

		The service mission like-minded Christian organizations
		may turn to for technical assistance and know-how.