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RE: Master and slave joints

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>Have every point in a rigid diaphragm the same translation and
No. A rigid object undergoes both translation and rotation. The rotations 
will be the same everywhere, but the angular movement also causes 
translations over and above the imposed translation. Imagine a rigid 
rectangle rotating in plane about its one corner. The translation of the 
fixed corner is zero, but the the other three corners move a distance 
equal to the angular displacement times the distance to the center of 

Combined rotation and displacement gets very tricky if the the rotation 
isn't a very small anlge because superposition no longer holds. Also the 
use of 'rigid' plate element s in a finite element model is tricky 
because in-plane rotational degrees of freedom give trouble. ANSYS plate 
elements for example are stable but soft. Rotational degrees of freedom 
in COSMOS/M plate elements had weird instabilities, that I could never 
get a handle on. 

And yes, techniclaly any point could be the master and the rest slaves 
(at least in ANSYS and COSMOS/M), except excessive creativity might give 
you a weird mass distribution.

Christopher Wright P.E.    |"They couldn't hit an elephant from
chrisw(--nospam--at)        | this distance"   (last words of Gen.
___________________________| John Sedgwick, Spotsylvania 1864)