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# Re: More questions about rigid plywood Diaphragms

• To: seaint(--nospam--at)seaint.org
• Subject: Re: More questions about rigid plywood Diaphragms
• From: Seaintonln(--nospam--at)aol.com
• Date: Sat, 3 Jul 1999 14:13:35 EDT

```I'm a bit confused by your explanation. The rigidity of a diaprhagm is
calculated by UBC formula for a blocked diaprhagm. Since most residential
diaphragms are not blocked, the American Plywood Association has recommended
multipying the rigid diaphragm results by a factor of 2.5 to determine the
deflection for an unblocked diaphragm. This result is compared against two
times the story drift which is calculated as 0.005h where h is the height of
the story from plate to plate.
The only time a residential diaprhagm will be considered flexible is if the
aspect ratio (distance between shear walls) compared to the depth of the
diaprhgam is large (say approaching 3:1 or more). The closer the shearwalls
occur, the stiffer the diaphragm is and the less deflection is expected.

I would guess the rationale comes from the design of retail stores which
historically used very little shear restraint in the open front. As the depth
of the diaphragm increased, the rigidity of the diaphagm became more
appearent. Historically, these were designed to calculate the rotation and
design the three "closed" sides to resist the open side.
The problem was that after earthquakes like Loma Prieta and Whittier it
became quickly noted that the front of the building collapsed like an open
front. If my memory serves me correctly, Los Angles City B&S stopped allowing
design of openfront structures by rotation.
After Northridge, Los Angeles and nearby cities created Open Front ordinances
to stiffen the open front of any commercial structure and residential
structure (such as a garage or parking area) with a living unit above.

The '97 code takes a slightly different approach. It makes you determine the
worst resulting shear in each line based on Rigid and Flexible analysis (and
Wind). Therefore, even though the diaphragm works out to be rigid, It is my
opinion from the design examples, that you can not simply design the whole
structure as rigid if the resulting shears are controled by the results
obtained from a flexible analysis.

Dennis S. Wish PE

In a message dated 7/2/99 4:25:15 PM Pacific Daylight Time, byggtak(--nospam--at)itn.is
writes:

<< The defination for rigid is one
that has less than half the overage story drift. Both stif and flexible
diaphragms shall be calculated asuming flexible diaphragms. The result of
this for wood Structural Panel Sheathed diaphragms and shear walls is that,
if allowable shear is used in full for the nailing of the panels, then it is
very likly that the diaphragms shall be calculated asuming flexible
diaphragms. >>

```