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# Re: Plwd: Rigid Diaphragm Analysis - Opinions Wanted

• To: seaint(--nospam--at)seaint.org
• Subject: Re: Plwd: Rigid Diaphragm Analysis - Opinions Wanted
• From: Jake Watson <jwatson(--nospam--at)inconnect.com>
• Date: Mon, 12 Jul 1999 21:26:35 -0600

```<Dennis>

> The evidence is that we consider,
> nail slippage, and elongation of the tension anchor in the calculation for
> deflection. These are acknowledgments that the connection is not rigid and
> that there will be rotation caused by bending and elongation of the
> restraints.

Two points here: first, by strapping the diaphragms cords we are not
attempting the emulated moment connection locally, but rather globally.
The chord itself may not be fixed, but the couple created by the
opposite (compression) member emulates a fixed connection. Second: all
diaphragms rotate no matter how stiff.  Even concrete deflects, so lets
talk about magnitudes.  Steel is roughly 10X stiffer than concrete, and
concrete it 10X stiffer than wood. So do we inherently consider concrete
flexible because it is so much softer than steel?  The key point is
this, as someone has already said. We can't simply paint the black and
white picture we would like to regarding rigidity.  Sometimes wood will
behave rigidly, other times it will not.  How we determine which model
to use is a much more important discussion.

> Okay, consider this. The new code converts the base shear to allowable stress
> design and reduces the base shear by 40% (1.4). Historically, I have always
> designed conservativly with an R=6 rather than 8 (therefore a base shear of
> .187Wd rather than the new code which gives 0.14Wd.). It seems that we took a
> simple approach and made it complicated. The argument in favor of the new
> code is valid if and only if you use the results for torsion only and design
> the wall stiffenss based on whichever shear controls over the full area of
> the diaphragm.

What does deflection have to do with strength vs. working stress?  The
new code is just a 1.4 factor, no new methodology was introduced
regarding diaphragms. (again someone already said this).  This is not
any more complicated than before, now it simply being enforced.

Let me pose this question a different way.  If you original model is
rigid, then the deflection will be smaller, justifying the rigid
assumption. However, if your model is flexible, the deflection will be
greater justifying your assumption again (possibly).  A better way to
ask the question other than calculation relative movements may be to
compare wall to diaphragms stiffness.  Food for thought.

Jake Watson E.I.T.
Salt Lake City, UT

```