# Re: Rigid vs. Flexible Diaphragm

• To: seaint(--nospam--at)seaint.org
• Subject: Re: Rigid vs. Flexible Diaphragm
• From: Scott E Maxwell <smaxwell(--nospam--at)engin.umich.edu>
• Date: Thu, 31 Aug 2000 22:41:19 -0400 (EDT)
```Bill,

I won't claim to be any smarter than you, but I will offer that
following...

As I understand it, a rigid diaphragm is such that the diaphragm is
stiffer in relative terms than the vertical elements of lateral system.
Therefore, say a load is applied to just one of the vertical elements
(i.e. a VERY eccentric load, but we are mainly talking theoretical "what
ifs"), the diaphragm effectively ties the response of that vertical
element to the other vertical elements.  Thus, the other vertical elements
will "go along for the ride", which will result in some force/load being
imposed on those other vertical elements.  To distill it down, the
relative movements between different vertical elements is very minimal, so
what one "does" all will do.

A flexible diaphragm is such that vertical elements are stiffer in
relative terms than the diaphragm.  In the same above example, a load
applied to one vertical elements will cause that one element to move, but
the diaphragm is not stiff enough to bring the other vertical elements
"along for the ride".  Again to distill it down, the relative movements
between different vertical elements is significant, so one vetical element
doesn't really effect the others.

Obviously in the "real" world all diaphragms fall into that large gray
area in between "rigid" and "flexible".

Rigid vs flexible diaphragm should have nothing really to do with wind
loads (semi-steady state) vs earthquake loads (transient state).  It has
to do with the sections and material properties of the structural
elements.

Hope that helps, (and I am sure other will/can correct me if required)
<grin>

Scott Maxwell, PE, SE

On Thu, 31 Aug 2000, Bill Polhemus wrote:

> Okay, I'm going to have to finally come clean, and admit my abstruse ignorance.
> It pains me to have to do so, but unless I ask stupid questions--and thereby
> expose myself as stupid--I know I'm not going to get wise to the answer. So here
> goes.
>
> 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.
>
> The latter, however, is used, I might say "exclusively", for lateral loads such
> as wind, which are characteristically more a "steady state" force whose
> influence due to impulse is much less. In that case, the "flexible" diaphragm
> imparts its force equally to the active shearwalls.
>
> Now, did I get that right, or did I just display irrevocable ignorance? OR...is
> it ignorance, but their is hope?
>
> Please, enlighten me.
>
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