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RE: 97 UBC

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As it was pointed out earlier the delta s is based on strength design level
forces. Now I'll try to take a crack at what I believe the source of
confusion is. 

When checking drift, you only need to verify that lateral displacement of
your vertical lateral force resisting system (in your case the tilt-up
walls) meets the drift limitations. This is not cumulative with the
diaphragm deflection.

However, the cumulative displacement should be used to check for deformation
compatibility of the gravity members, such as the interior columns and walls
perpendicular to the direction of load.

Hope this helps

Ben Yousefi, SE
San Jose, CA

	-----Original Message-----
	From:	Gerard Madden [SMTP:GerardM(--nospam--at)crjarch.com]
	Sent:	Friday, July 02, 1999 11:40 AM
	To:	'seaint(--nospam--at)seaint.org'
	Subject:	97 UBC

	Question For all the Engineers interpreting the 97 UBC.

	I am designing Tilt-ups to the 1997 UBC and I find it extremely
	confusioning mainly due to the factored or strength level seismic
forces
	obtained in the base shear equations and the usual guessing game of
what
	the code is really asking me to check.

	At first glance, I decided to design the bldg using the Simplified
	Method. This would mean about a 20% increase in force compared to
the
	non-simplified method. What this would allow me to do is ignore the
	messy calculation of determining the drift of the structure which
must
	include foundation rotation, cracked sections & diaphragm
deflection.

	However, being that I am 4.3km from a type A fault in Zone 4, My
	diaphragm design force is 0.3W (working stress) compared to the
.183W of
	the 1994 UBC. My structure design force is .267W for everything else
but
	the diaphragms. This makes for extremely heavily nailed diaphragms
and
	my collector forces are ridiculus even when assuming the diaphragm
	yields at 2X the allowable capacity. Overturning of the interior
	shearwall on my building controls the collector force fortunately
but
	with the force level about 45% higher (using the simplified method)
	versus the 94 UBC, many connections simply can't work in a slender
wall.

	I began to re-examine the need to do the simplified method. The
	diaphragm deflection calc is not difficult. The wall deflection
in-plane
	is insignificant. Leaving me to figure out the foundation rotation
and
	its contribution to the drift during rocking. For the time being,
I'm
	assuming that the foundation, as long as it meets the bearing
pressure
	requirements under seismic loads, will have a mininal contribution
to
	the drift.

	Now, I calcuate my diaphragm deflection which turned out to be about
3.5
	inches. I have a 21ft tall building with the walls designed for the
	p-delta effect per 1914.8 alternate design method for slender walls.
The
	walls are designed for a displacement greater than the diaphragm
	deflection and I could say that they are adequate for this
deflection.

	But when I read the code, it says Delta S shall be calculated using
the
	design seismic forces. I am confused, does this mean STRENGTH DESIGN
	SEISMIC FORCES only or either the WORKING STRESS LEVEL FORCES or
	Strength Design Forces. To get the DELTA M drift the 0.7RDelta S is
	compared with the 0.025H limit which I would think is based on the
	Strength design force. This is all destroyed if I am forced to
compare
	the wall deflection capacity to a factored force diaphragm
deflection.
	The wall deflection is based on the Nominal Moment and Cracked
Section
	properties of the wall. The reinforcing and adequacy of the wall is
	based on the 1.1x1.4xFp. Fp being the working stress seismic
	coefficient.

	If anyone has any insight on the simplified method and drift I would
	appreciate there opinion. By the way, when are the seismic design
	manuals II & III available? I'm tired of these examples that have
given
	forces, displacements, etc... that all figure to a clean answer or
some
	assumption that is pulled out of nowhere.

	Gerard Madden
	CRJ Associates
	Design Engineer, Structural Department
	650-324-0691x129
	650-324-0927-fax
	gerardm(--nospam--at)crjarch.com