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# RE: Calculation of drift in Wood Frame Buildings

• To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
• Subject: RE: Calculation of drift in Wood Frame Buildings
• From: "George Richards P.E." <george(--nospam--at)borm.com>
• Date: Thu, 16 Jan 2003 17:00:14 -0800

```Sarcasm ON>

Who said one should actually READ the code and try to comply with it's
intent?

>OFF with the Sarcasm

-----Original Message-----
Sent: Thursday, January 16, 2003 4:17 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Calculation of drift in Wood Frame Buildings

Sarcasm ON>

What school did you go to? Didn't they teach you this stuff in your
Gypsum Engineering Course? This is why a masters degree should be
mandatory for structural engineering !!! What do you mean a diaphragm
could be flexible, prove it ... I DARE YOU.

>OFF with the Sarcasm

Al,

This is half the complaining I and others have been doing on this list
in the past. Your situation is even more of an example of light frame
construction code gone awry.

Trying to prove a flexible diaphragm is a waste of time and will not be
true in your case. The bigger problem you have is determining the
rigidity of the GWB shear panels. I supposed you could try to use the
same equations for plywood shearwalls with the different properties for
GYP instead of OSB or Ply, but I don't know where you would find that.

Here is how everyone else has handle the drift calculations in
residential buildings ( YAAAWWWWWWWNNNNNNN ) and that is still how it is
done for the most part outside of seismic areas. In seismic areas -
simplified analysis and you don't have to do the drift calc - about 80%
of engineers are doing this if I had to guess.

I strongly suggest that you avoid gyp shearwalls if there is a seismic
concern. Wind is probably okay. By the way, if you use wood sheathing,
most of your problems and questions are still valid. The whole shearwall
deflection/rigidity thing is not valid due to the fact that no special
inspection is required to make sure the wall is built as designed.

HTH,
-gerard

-----Original Message-----
From: Al Meyer [mailto:ameyer(--nospam--at)cagleyharman.com]
Sent: Thursday, January 16, 2003 3:25 PM
To: SEAINT(--nospam--at)SEAINT.ORG
Subject: Calculation of drift in Wood Frame Buildings

List -

I know that in recent times we are trending away from the use of gypsum
sheathed shear walls based on their performance in recent seismic
events.
The IBC 2000 does, however, provide values for shear resistance for
gypsum
sheathed shear walls.

Essentially if your building is an SDC A you don't have to do a formal
lateral analysis of the building, although a minimum force level and
connection requirements still must be considered.  When a wood frame
building is such that it is in an SDC of B or higher and it does not
meet
the requirements for a simplified analysis (which allows story drift to
be
be computed as 1% of the floor to floor height) or the requirements of
section 2308, Conventional Light Frame Construction, it must be analyzed
as
specified by the code.

In the past, wood sheathed diaphragms were generally considered
flexible,
and Breyer's "Design of Wood Structures" textbook even states this.
Now, a
wood diaphragm may be flexible or rigid depending on its deflection
relative
to its supporting shear walls.  Equations are available to calculate the
deflection of wood panel shear walls, but there is no rational basis
(per
information I have read in NEHRP documents) for calculating deflection
of
gypsum sheathed shearwalls.

While it seems logical that in many cases a wood diaphragm would seem to
be
rigid relative to gypsum sheathed shear walls, I'm certain that this
isn't
always the case.  How has anyone handled this issue other than the
obvious
route of not using gypsum sheathed shearwalls?  I kn	ow that
buildings have
been constructed all over the country which have used gypsum shear walls
but
don't know how anyone else has handled drift calculations.  Thanks in
advance for any input you can provide.

Albert J. Meyer, Jr., P.E.
Cagley, Harman and Associates
Structural Engineers / Parking Consultants
1015 West Ninth Avenue
King of Prussia PA 19406-1222
ameyer(--nospam--at)cagleyharman.com
(610) 337-3360
(610) 337-3359 Fax
www.cagleyharman.com

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