Return to index: [Subject] [Thread] [Date] [Author]

RE: Advice on a 230 ft long bldb

[Subject Prev][Subject Next][Thread Prev][Thread Next] 
Dennis:

Thank you for your input.
The project consists of 10 units each 23' by 37. The walls on the back is about 50 percent open with at least 4'0" wide shear wall at each unit. The second story wall above the open front is also about 50% soild. There is garage under each of the unit at the first floor - i.e. every unit has an open front. The shear walls are generally stack up. 

You raised several issues:
1. conventional framing: The building meets the conventional framing requirement; however, I do not feel comfortable using the conventional method and try to stay away from the prescriotive method.
2. Simplifed base shear 3 Ca W/1.4R versus 2.5 Ca W/1.4R: Several San Francisco Bay area cities waive the rigid diaphragm analysis requirement if one uses the simplified base shear. However, I do not use the simplified base shear method because I do not find it conservative. It is true that the total base shear is increased; however, because the code does not require a vertical triangular distribution of base shear (first mode of vibrartion), I find that the use of simplified base shear can grossly underestimate the overturning, and therefore the holddown, requirements. I use the 2.5 Ca W/1.4R base shear and use the envelope method (flexible and rigid diaphragm analysis) in my design. Once a spreadsheet is set up, the rigid diaphragm analysis and shear wall deflection calc is only a click away.
3. Open front: Seeing the damage from Northridge, I neglect the fact that the code allows shear trasfer by rotation (section 2315 of the code). 

I have not started the design yet - I am still in the contract phase.

Gautam



From: "Dennis Wish" <dennis.wish(--nospam--at)verizon.net>
Reply-To: <seaint(--nospam--at)seaint.org>
To: <seaint(--nospam--at)seaint.org>
Subject: RE: Advice on a 230 ft long bldb
Date: Mon, 30 Jun 2003 02:30:35 -0700

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

_________________________________________________________________
Add photos to your messages with MSN 8. Get 2 months FREE*.
http://join.msn.com/?page=features/featuredemail


******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
*
*   This email was sent to you via Structural Engineers
*   Association of Southern California (SEAOSC) server. To
*   subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
*   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you
*   send to the list is public domain and may be re-posted
*   without your permission. Make sure you visit our web
*   site at: http://www.seaint.org
******* ****** ****** ****** ******* ****** ****** ********


******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
*
*   This email was sent to you via Structural Engineers
*   Association of Southern California (SEAOSC) server. To
*   subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
*   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you
*   send to the list is public domain and may be re-posted
*   without your permission. Make sure you visit our web
*   site at: http://www.seaint.org
******* ****** ****** ****** ******* ****** ****** ********


_________________________________________________________________
Protect your PC - get McAfee.com VirusScan Online http://clinic.mcafee.com/clinic/ibuy/campaign.asp?cid=3963 


******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
* * This email was sent to you via Structural Engineers * Association of Southern California (SEAOSC) server. To * subscribe (no fee) or UnSubscribe, please go to: 
*
*   http://www.seaint.org/sealist1.asp
*
* Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you * send to the list is public domain and may be re-posted * without your permission. Make sure you visit our web * site at: http://www.seaint.org ******* ****** ****** ****** ******* ****** ****** ********