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# Rigid wood diaphragms, more to know

• To: "seaint(--nospam--at)seaint.org" <seaint(--nospam--at)seaint.org>
• Subject: Rigid wood diaphragms, more to know
• From: merrick group <merrickgroup(--nospam--at)compuserve.com>
• Date: Tue, 23 Mar 1999 21:31:52 -0500

```UBC earthquake rigid wood diaphragm

A plywood official reported that to analytically test for a rigid
diaphragm (94UBC, 1628.5 third paragraph) there needs to be an
equation for unblocked diaphragms. Therer is no equation for such. He
believes it is not then  intended for unblocked diaphragms to be
considered rigid. He then hints of existing fudge factors for blocked
diaphragm equations to represent the unblocked.

Possible factors are.
******Increase the deflection six times for loads parallel to unblocked
edges.
***Increase the deflection three times for loads parallel to the joists.

Most wood floors are glued and are most likely rigid. No fudge factors
For this one.

The method of tributary area reduces the likely result of highly
eccentric shearwall systems. Tributary area method (most of the time)
will control the seismic design. But does allow larger cantilevered
diaphragms to be justified.

The code only suggests a rigid diaphragm system for seismic and not
wind forces. But it sure creates a great back up to justify reduced
deflections to extend  cantivered diaphragms! (do not assume rigid

This might free the need for the cantilevered columns.

Wood diaphragm force distributions by tributary area, are modeled as
being flexible in shear and rigid in flexure. Following is a summary of
models that I have heard of.

1. Flexible shear, rigid flexure: This is the tributary area
method
2. Rigid shear, flexible flexure: This is where interior shear
walls are increased per a continuous beam model
3. Rigid shear, rigid flexure: By demand of the code section
of 1628.5.  This is the method used for simple concrete
structures, and seems to be the recommended approach
per code.
4. All of the above enveloped has yet to be recommended as
enveloped.
5. The enveloping of case 1 and case 2: is found in ATC-4

I have yet to find a program intended for 1994 UBC wood shear walls,
with the capability of rigid diaphragm checks. I know of one
programmer who is now trying to implement the issue as a secondary
check. Evidently he got enough calls.

The above is mearly a group of expressed ideas and not my opinions or
recommendations as to how to design. I hope that the concepts can be
intertained and responed to. Of course with the same limit of liability.

```