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RE: rigid diaphragm chord forces?

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If you are considering a wood diaphragm (more specifically residential
construction) it is not uncommon for a discontinuity to occur by a
setback or irregularity in the plan between lines of shear. This same
issue was raised during the 1999 SEAOC Convention Panel Discussion by
John Coil. Coil admitted that the condition exists often (a raised Porte
cohere at the entry of a home is one example where the discontinutity in
the chord occurs). The problem is that there is not practical way to tie
the chord at this condition other than to step back into the diaphragm
where the chord force increases proportionally (in flexible analysis) as
the depth of the diaphragm decreases. However, if the chord is only the
width of the discontinuity then you have to assume that you can tie the
chord back into the diaphragm on each side of the chord and this is not
always possible.

Another problem with the design manuals is that the examples are rather
pristine. The wood examples end up going through the difficult gyrations
only to discover that the reserve capacity exists in the shearwalls to
offset the rotational (torsion) shear distributed in the diaphragm.

I have not reviewed the examples or papers that Nels refers to but I
have a less enthusiastic opinion of the rigid analysis solution for wood
for two reasons. It encourages soft story design whereas flexible
analysis will identify the open-front. Second, rigid analysis
distributes shear into the interior walls by stiffness, yet flexible
analysis prohibits wood shearwalls from being designed by rigidity - you
can't have walls in the same line of shear that have different sheathing
and nailing (different rigidities).

It's an imperfect system out there :>)

Dennis


-----Original Message-----
From: Nels Roselund, SE [mailto:njineer(--nospam--at)att.net] 
Sent: Wednesday, August 27, 2003 8:59 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: rigid diaphragm chord forces?


Moments and chord forces in rigid diaphragms:  A SEAOSC Seminar on Feb.
15, 2003 presented by the Seismology Committee included a presentation
by Martin Johnson and Jeremy Welton titled Analysis of Collector/Drag
Elements in Rigid Diaphragms.  The  presentation included a method for
calculating moments in a rigid diaphragm.  It is basically a mater of
statics, but it may be helpful to review the paper from the seminar.

The syllabus from that Seminar can be purchased from SEAOSC: contact Don
Gilbert:
seaosc(--nospam--at)aol.com, or 562-908-6131.

Nels Roselund
Structural Engineer
South San Gabriel, CA
njineer(--nospam--at)att.net




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