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Re: More questions about rigid plywood Diaphragms

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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.

Any comments?????

Dennis S. Wish PE

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

<< 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. >>