# Re: Rigid vs. Flexible Diaphragm

• To: seaint(--nospam--at)seaint.org
• Subject: Re: Rigid vs. Flexible Diaphragm
• From: CarlS95(--nospam--at)aol.com
• Date: Fri, 1 Sep 2000 20:22:58 EDT
```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

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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