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Re: Unblocked Plywood Diaphragm Deflection Calculation

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Niaz A. Nazir,

You wrote:
"I am trying to compare the FEMA 273 equations for the deflection computation 
of blocked and unblocked plywood diaphragms"

The unblocked diaphragm deflection is given in FEMA 273, Section, 
page 8-27, bottom of right hand column, "The deflection for unblocked 
diaphragms may be calculated using Equation 8-5, with diaphragm shear 
stiffness, Gd as follows:
Unblocked, chorded diaphragms:  Gd = 800,000 lb/in., Unblocked, unchorded 
disphragms: Gd = 400,000 lb/in." 

Note: Equation 8-5 is presented on page 8-25, FEMA 273, for calculating the 
deflection of straight sheathed diaphragms.  Apparently, the authors of FEMA 
273 felt that Equation 8-5 could be used for wood structural panel unblocked 
sheathed diaphragms with the proper Gd values greater than Gd value = 200,000 
lb/in. for straight sheatheed diaphragms. 

You wrote:
" The blocked diphragm stiffness is given by equation 8-6 & 8-7 (FEMA 273).
Using these equations will yield unblocked diaphragm stiffer than blocked 

Since Equation (8-5) is so different from Equations (8-6) and (8-7)  which 
have many more and different terms than Equation (8-5), it  is difficult for 
me, without seeing your detailed calculations, using these three equation,  
to agree with your conclusion,  
"Using these equations will yield unblocked diaphragm stiffer than the 
blocked one."

You wrote:
"Using ATC-7-1 one could compute unblocked diaphragm stiffness which calcs 
out to be much less than FEMA 273."

I did not pull out ATC-7-1 to review what you wrote, but could you be a bit 
more specific, using mathematical examples to illustrate your point.

I am not agreeing or disagreeing with your conclusions.  I need more 
information using mathematical substitutions in the above three equations to 
better understand what you are saying.

There are other problems with Equations (8-6) and (8-7) which will be the 
subject of another email message by me to the SEAOSC List Server.

Keep up the good work and an open mind in questioning the provisions of FEMA 

Frank E. McClure      FEMCCLURE(--nospam--at)   May 19,1999