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# Re: Master/Slave (Was STAAD or RISA?)

• To: "?" <seaint(--nospam--at)seaint.org>
• Subject: Re: Master/Slave (Was STAAD or RISA?)
• From: Christopher Wright <chrisw(--nospam--at)skypoint.com>
• Date: Fri, 6 Sep 02 23:45:36 -0500

```>As I see it making a group of nodes slave to one other node makes ALL of
>the degrees of freedom for the group the same.
It doesn't necessarily mean that. 'Coupling' is one form of slaving where two or more otherwise unconnected nodes are forced to have identical displacements or rotations. Typically you'd simulate a pinned connection between two members by coupling the displacements of the common node but not the rotation. If the coupled nodes aren't coincident, you can get unequilibrated moments in reaction loadings so that the calculated reactions aren't correct.

A more general form of slaving nodes is the so-called rigid region. In this type of constraint (It's also called a kinematic constraint) the displacements of the slave nodes equal the displacement of the master plus a displacement due to rotation. The resulting group of nodes effectively undergo rigid body rotation and translation. Such constraints can be used to model finite sized joints in frameworks or connections where the members don't lie in the same place. Usually the rotations are assumed small (tanØ = Ø) so the constaints only apply to linear elastic problems.

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

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