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

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I've gotten several private emails asking my opinion on the following
questions, as well as the postings to the list, so please indulge my
(hopefully) final contribution to this thread:

Q) Why worry about out of plane actions when modeling a diaphragm, since the
diaphragm is intended to model in-plane stiffness only?

A) If you are, in fact, modeling the beams in the plane of the diaphragm, you
probably don't need to worry about out of plane effects, because the beams
will handle them. 

If you are ONLY modeling the diaphragms (and not the beams), you definitely
need to consider out of plane effects (such as axial shortening in a tall
structure, as pointed out by Greg Luth). I've had many models pass over my
desk where the program user was primarily interested in column design and so
had put together the model using columns and diaphragms only (no beams). In
such a model, ignoring out of plane effects in the diaphragms could be

I don't want to promote RISA-3D here (well, OK, maybe I do), but the rigid
diaphragm option in RISA-3D can be configured either way, membrane with only
in plane stiffness, or planar where out of plane stiffness is also provided.

Q) What is a "master/slave"?

A) My definition can be summarized by the following excerpt from "Finite
Element Procedures" by K.J.Bathe (1996, page 740): "The basic assumption in
the master-slave analysis is the use of the constraint equations U1 = U2 = U3"
(where Ui are the master/slave node displacements). 

I hasten to add that no one is "wrong" here, the master/slave definition that
incorporates transposition of forces and displacements is different but
equally valid, so the answer to this question is "read your program's
documentation to understand how master/slave is defined for your particular

Also, I hope Rudra is able to keep his cubicle ;-)

Bruce Bates
RISA Technologies