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RE: Offset Shearwalls

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I started this yesterday and see that some of you have already answered.
There have been some large debates on the use of the Over-strength
factor. As I recall, the use of Omega was waived some time after the
code (97 UBC) was published for single and two family residential
light-framing. I did some searching on the Internet and came up with a
draft of the 2005 IBC Chapter 4 prepared by the BSSC and they are
suggesting exempting the use of the over-strength factor for
light-framed residential.

I personally have not used Omega factor as most of the homes I have done
are single story. There are notes related to use of the Simplified
Static design that requires increasing the demand on the Holddowns -
which may be similar to the use of Omega. 

Although I am checking on the 1997 UBC, it was my understanding that the
Over-strength factor (Omega) was waived for single and two family
residences in light-framing. This is confirmed in the BSSC Draft for the
2005 Chapter 4 section 4.3.1.2 provides an exception for the
Over-strength faction for one- and two-family dwellings of
light-framing.

I believe that this was the intention of SEAOC subsequent to the
publication of the 1997 UBC and if I recall, they published a position
statement on this - however legal this is if challenged in court.

Sorry, I didn't post this sooner, but to be quite honest, I have
forgotten about the use of Omega and use formula 30-1 (I think that is
the equation) in the Multi-Lat software.

I think I also mentioned that the capacity of the diaphragm must be
sufficient on both sides of the wall to transfer the shear to the
nearest walls below - this is something that needs to be checked - not
just the diaphragms capacity and nailing, but the plate connection to
the beam or joists below. So be cautious about this and please make sure
you have a good detail showing these transfer connections - the analysis
and notes alone don't mean a darn thing to the framer - it's what he can
interpret from your drawings. 



Dennis S. Wish, PE


California Professional Engineer

Structural Engineering Consultant

dennis.wish(--nospam--at)verizon.net

http://www.structuralist.net

 


-----Original Message-----
From: sea [mailto:sea(--nospam--at)builderspost.com] 
Sent: Sunday, April 25, 2004 8:35 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: Offset Shearwalls

Mr Wish;
Is there an omega factor 2.8 involved here and if so; how is it applied?

----- Original Message ----- 
From: "Dennis Wish" <dennis.wish(--nospam--at)verizon.net>
To: <seaint(--nospam--at)seaint.org>
Sent: Sunday, April 25, 2004 2:13 PM
Subject: RE: Offset Shearwalls


Irv,
I must be out of the loop as I have not followed the NDS High Winds
Provisions. With that said shear can be transferred through walls that
don't stack according to the 97 UBC but you must be able to resolve any
uplift in the support members below - including taking the reactions in
compression and tension down to the foundation.

Theoretically, the wall at the upper level must resolve the tension and
compression (and any loads it might carry if there is a level above
this. But the diaphragm below the wall should be able to transfer the
lateral demand on the wall attached above the diaphragm to the nearest
resisting walls at the level below and there must be a positive
connection to these two walls (unless there is a cantilever to
consider).

So let's assume you have 1000 pounds of shear in the upper level wall
from a roof above and the wall is 10-feet long. The diaphragm shear at
the base of the wall (forgetting for the moment the uplift and
compression on the wall itself) is 1000 pounds/10 feet or 100-plf. If
the wall is 10-feet from a parallel shear wall in the level below and
5-feet from the parallel shear wall to the right side, then the reaction
of 1000-pounds on the upper wall should be able to contribute
(10/(10+5))*1000= 666.67 lbs to the wall on the right and 333.33 lbs to
the wall on the left.

Remember to add this to the story shear that is distributed to the
demand on the two walls in the lower level.

I know this is very basic stuff but I was not sure how elementary you
wanted to get this - just make sure that each wall can resist tension
and compression as well as shear through the structural members
supporting it. There will be a couple on the beam or joists below
roughly 10-feet apart that results in designing the supporting beam for
a moment placed at the center of the wall it supports.

Again, I am giving you the UBC directions on this rather than the NDS
wind loads that I admittedly have not paid any attention to.

Hope this helps,
Dennis

P.S. The purpose of the diaphragm linear demand was to assure that the
diaphragm on each side of the wall had the capacity to transfer the
shear down as you might have to add nailing to the joists to make sure
that the diaphragm can transfer the load to the walls. For the load we
assumed, the minimum unblocked nailing of the diaphragm is sufficient.


Dennis S. Wish, PE


California Professional Engineer

Structural Engineering Consultant

dennis.wish(--nospam--at)verizon.net

http://www.structuralist.net




-----Original Message-----
From: IRV FRUCHTMAN [mailto:ifaeng(--nospam--at)yahoo.com]
Sent: Saturday, April 24, 2004 1:53 PM
To: seaint(--nospam--at)seaint.org
Subject: Offset Shearwalls

Fellow Engineers:
I'd appreciate design advice re shearwalls vertically
offset between floors, with floor joists parallel to
the walls. According to the NDS (1995 High Wind
Addition) the max offset is 4d (d=joist depth) when
floor joists are normal to the shearwalls. I can't
reconcile this simple rule with my load model so I've
likely missed something.

Thanks,
Irv





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