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97 UBC

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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