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Re: SAP, RISA3D, STAAD - Master/Slave Relationships

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Aleksander Urbanski wrote:

<< ---------------------------------------------------------------
<< "RIGID LINKS" 
<< Designed to model rigid body inclusion within the deformable system...
[clip]
<<
<<This enables us to model diaphragms (floors in building skeletons) in a
<<following simple way (assuming Z as vertical direction, consider that nodes
<<1,2,3,4 are the nodes by which 4 columns are hinged to the in-plane
<<perfectly stiff floor) it is enough to put:
<< 
<<RIGID
<< 2 3 4  LINK  1 UZ  RX RY 

What if my entry were (either intentionlly or erroneously) :

RIGID 2 3 4 LINK 1  UZ RX RY RZ

or this:

RIGID 2 3 4 LINK 1  UX UZ RX RY 


Would you perform the transformation anyway, would you flag this as an error,
or would you do something else?
 
<< I have to admit that programming this selective DOF rigid link option
<< was one of the most complicated parts of the whole development. 

That depends on what you do with situations such as I've described above. If
you require the rotational transformation, it's pretty straightforward. If you
try to handle any user request, it starts to get interesting.

<< "COMPATIBLE NODES" to handle very common situation when different nodes
<< share user-specified DOFs. Example - pinned connection of crossing X -bars.
<< The corresponding nodes should however occupy the same position in the
<< space. 
<< 
<< Attachment of the same DoFs to the group of nodes occupying DIFFERENT
<< positions in the space, but without "rigid link" type transformation is not
<< physically based, and as such, would never lead to correct results...[clip]

Consider this example: I'm analyzing a simple 2D frame (XY plane, 2 column
lines) for lateral (X dir) modes.I lock out Z translations and X and Y
rotations, giving me a 3 dof model. Now I slave the X translations to reduce
the size of the matrix. I'm now using slaving of physically separated nodes
without transformations to get the results I want. 

<< The only known physically based and sensible application of such an option
<< is imposition of periodic boundary condition in the case of FEM analysis of
<< the representative cell of the periodic composite, but I do not think that
<< this was the reason of the appearance of this option in early FE codes.
  
Huh? 


The original intention of the master/slave relationship was to eliminate dof's
in models where simplifying assumptions can be made. The extension of
master/slave to diaphragm modeling requires that dof be slaved in groups so
that the rotational transformations can be performed, i.e. X translation can
not be slaved without also slaving Y translation and Z rotation. This
requirement does now make master/slave applicable to diaphragms, but you've
lost the ability to slave dof's in a wholly independent way. I can see ROBOT
addresses this by offering the RIGID LINK option AND the COMPATIBLE NODES
option, giving the ability to do it either way.

What ROBOT describes as "compatible nodes" is termed the master/slave
relationship in RISA-3D. What ROBOT calls the "rigid link" option is termed
the rigid diaphragm option in RISA-3D.

Bruce Bates
RISA Technologies