] [Thread] [Date] [Author]

["Dennis Wish" <dennis.wish(--nospam--at)verizon.net>]

This brings up a couple of interesting issues. First, if all the walls
stack and the diaphragms are at the same level (no irregularities) then
it sounds like you are on the right track. The bracing of the open front
is the most important. However, you don't describe the number of
openings in the longitudinal direction where there are likely to be many
windows doors and other openings. Are these in the same plane or do
cantilevers exits. You have the weak direction covered and I doubt that
there will be much additional shear from rotation to add into your
shearwalls. 

This is where the additional questions come in. If the building is long
and narrow, you could conceivably place your shearwalls between living
units far enough apart to force the analysis to show the diaphragm as
flexible. The closer you get to a 4:1 ratio, the more flexible the
diaphragm deflection analysis will show. However, is this a safe and
practical approach to take? 

Next issue - the code allows the building to be designed by Conventional
Framing or prescriptive methods (without engineering) if it complies
with the tests for structural [ir]regularities. If the walls stack and
the there are no cantilevered diaphragms, this building might comply. If
you were the developer, would you push to save money by going to an
architect or designer and allowing them to follow the prescriptive
methods in the 97 UBC chapter 2320?  
As an engineering community, we  might agree that this would put more
profit in the pocket of the developer or owner of the project, but would
create a structure that does not meet the minimum standard of
performance which would be discovered by comparing analysis to what 2320
prescribes.

Next, issue - Because of the regularity you describe in the structure, I
would be interested in discovering if you designed the shearwalls and
have checked to see if there was considerable capacity left in them to
take the add ional shear from torsion and more. How would this compare
to a simplified static design using V=3.0(CaW)/(1.4R) rather than
V=2.5(CaW)/(1.4R). 

In the strong direction, seismic will probably govern (especially for a
230-foot deep diaphragm). However, in the weak direction, you will most
likely find that wind governs. Still, you must design to seismic
standards using the worst case applied load (wind in this case). 

Final Issue - Most of us in California understand the problem with
living units above garages or soft-story and "tuck-under" parking. You
resolved this the way most of us would, but I've seen questions from
others who live in the mid-west and who question whether the code would
allow an open front since it allows design by rotational methods. In
other words, if the diaphragm can distribute shear to the other three
walls, theoretically, the open front should be stable. 
Now I don't believe this for a second. It doesn't work well with my
intuition and knowledge of how a wood building performs - yet the code
does not specifically warn the engineer about potential hazards of
soft-stories due to rigid diaphragm analysis.

Any comments?

Dennis S. Wish, PE

-----Original Message-----
From: G M [mailto:newabhaju(--nospam--at)hotmail.com] 
Sent: Monday, June 30, 2003 1:43 AM
To: seaint(--nospam--at)seaint.org
Subject: Advice on a 230 ft long bldb


To all:

I am starting to work on a 2 story 10 unit apartment complex (wood
building) 
in seismic zone 4 area.  The builiding footprint is 37 x 230 feet.  The 
second floor and roof is continuous for the whole length (230 feet).
Party 
walls accross the building width will act as shear walls thereby
reducing 
the diaphragm aspect ratio to less than 2:1.

The first story has an open end for garage.  Steel moment frame will be 
provided at the open front.  In order to reduce the effect of torsion,
shear 
walls will be provided throughout the length of the building at about
the 
mid widht of the bulilding ( will be designing this shear wall to fully 
resist the seismic forces - this is in addition to the moment frame and 
shear wall on the opposite side).

I would appreciate if you could let me know what other pitfalls I need
to 
worry about.

Gautam Manandhar, SE

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