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Re: Seismic Upgrade prob. W/97 UBC

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Lynn H wrote:
> Michelle
> If you run the numbers, in one direction the plywood
> diaphragm does in fact calculate out to be rigid.
> In the other direction, it calc's out as flexible.
> This is another thing engineers have never had to
> deal with before, a diaphragm flexible in one
> direction, rigid in the other.
> Answer me this, if the diaphragm is rigid, then it
> will distribute loads by rotation to walls
> perpendicular to the forces if the center of gravity
> and center of rotation are not coincidental.
> However, the as in the other direction the diaphragm
> is flexible, it is not not capable of transmitting
> the loads by rotation.  The design theory breaks
> down at this point.
> How in the world to you figure this one?  One can or
> worms after another!!
> Lynn
You answered your own question.  Take a package of plastic drinking
straws down to the building dept and demonstrate what will happen to the
non-isotropic diaphragm by laying a dozen or so alongside each other. 
Now push horizontally across the straws, but off-center and see if the
mass rotates.  Rigid one way, flexible the other (OK, I'm using
primarily shear deformation rather than flexural to make a point).

I'm open to better demonstrations.