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# Calculation of drift in Wood Frame Buildings

• To: <SEAINT(--nospam--at)SEAINT.ORG>
• Subject: Calculation of drift in Wood Frame Buildings
• From: "Al Meyer" <ameyer(--nospam--at)cagleyharman.com>
• Date: Thu, 16 Jan 2003 18:25:11 -0500

```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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