Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Rigid Diaphragms

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Bruce

I understand what you are saying about the out of plane degrees of freedom 
and I agree.  Without those out of plane degrees of freedom you do not 
capture the effect of column axial deformations.  In tall frames that would 
result in significant errors.  It is possible to use condensation (Gaussian 
elimination) to selectively remove degrees of freedom from the global 
stiffness matrix.  I have used this technique to condense out the dependent 
in-plane degrees of freedom at the column lines to a rotation, x 
translation, and y translation at the center of mass leaving the 
independent out of plane degrees of freedom, two rotations and a 
translation, at the columns. I thought that selectively slaving degrees of 
freedom to other degrees of freedom accomplished the same thing.  In the 
example the two in plane translations and the rotation about a vertical 
axis are "slaved" to the center of mass while the others are allowed to 
remain independent.  This would accomplish the same thing as your "rigid 
links."  Having said that, I am glad you provided the diaphragm feature in 
RISA3D to eliminate all the drudge work.

STAAD did have the same definition of master-slave that you do, at least 
their programmers did.  Unfortunately, the users manual was written by 
someone who had an "engineer's" definition, which is what you call a rigid 
link.
The following is a direct quote from the user's manual (pg. 2-28, circa 
1992 edition) :

"The master/slave option is provided to enable the user to model rigid 
links in the structural system.  This facility can be use (sic) to model 
special structural elements like a rigid floor diaphragm."

So far, so good.  However, in the very next sentence it becomes clear that 
the programmers have used your definition, Bruce.:

"Several slave joints may be provided which will be assigned same (sic) 
displacements as master joint."

This last sentence negates the first two, since the program literally does 
exactly as the last sentence states.  No transformations, no static 
condensation, Gaussian elimination, and no mathematical justification for 
"assigning" the answer to an analysis problem.

I It is the last aspect of this definition for master/slave that I find 
extremely objectionable.  There is too much at stake when a structural 
engineer is performing an analysis for the answer to be held hostage to a 
programmer's trick that has no scientific justification.


***********************************************************
Gregory P. Luth, Ph.D., S.E.
KL&A of California
160 Jefferson Drive
Menlo Park, CA  94025
PH:	650 688 4710
FAX:	650 688 4712