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Re: Rigid vs. Flexible Diaphragm

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Bill.

Good try.  I like your explanation.  Here's another:

Concrete floor slabs are usually considered rigid because they are relatively 
stiff, compared to the vert elements.  Lateral loads (wind and/or seismic) 
are distributed to the vertical elements based on relative rigidities of the 
vertical elements.

On the other hand, metal roof decks and wood sheathing are generally 
considered flexible diaphragms.  Lateral loads are distributed to the 
vertical elements by tributary area of the roof (and floor if wood).

Rigid diaphragms are real nice when preparing three-dimensional computer 
analyses, because you can use the rigid diaphragm feature of ETABS or SAP or 
RISA or whatever to avoid modeling the diaphragm.  Flexible diaphragms are 
even better, because you don't even need to do a computer model to figure out 
the load distribution.

Having said that, in reality there is no such thing as a perfectly rigid or 
perfectly flexible diaphram.  Occasionally, one gets challenged by a plan 
checker to justify his assumption.  In fact, the topic of rigid versus 
flexible for residential construction has gotten quite a bit of debate 
recently here in California.  Engineers always have based design of wood-roof 
structures on flexible diaphragms, but that is being questioned.  But save 
that topic for another day.  Okay???

Take care.

Carl S.  

<< My understanding of "rigid" vs. "flexible" diaphragm design is that the 
former
 is appropriate for seismic design, where the dynamic nature of the 
force--it's
 sudden introduction and just-as-sudden change in magnitude and direction 
(or, in
 short, it's impulsive characteristics)--means that the actual flexibility of 
the
 diaphragm itself and its connection to other elements is meaningless. The 
forces
 are introduced solely on the basis of the relative stiffness of the 
shearwalls. >>

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