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

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As much as I hate myself for continuing this discussion when apparently
Bruce 
has provided a nice way of terminating it, I have to respond to this from
an engineering
standpoint.

So my apologies in advance.

----------
> From: BRBATES <BRBATES(--nospam--at)aol.com>
> To: seaoc(--nospam--at)seaoc.org
> Subject: Re: SAP, RISA3D, STAAD - Master/Slave Relationships
> Date: Friday, January 02, 1998 1:25 PM
> 
> So, in summary, I agree that your definition of master/slave will model
> diaphragm behavior (sort of), with these caveats:
> 
> --You don't care about out of plane displacements and rotations
> --You don't consider the diaphragm to provide bending restraint to the
columns

Instead of calling the above two issues "caveats" I would term them sound
engineering
assumptions. Diaphragm action is predominantly in-plane (membrane action).
The 
out-of-plane stiffness of a diaphragm in a typical engineering structure is
much smaller
than the stiffness of the MF beams/ BFbeams&braces and hence the diaphragm
is 
pretty much ineffective in providing bending restraint to the columns. For
example, a 
typical high-rise hotel bldg. could have a 200mm thick (I proportional to
0.2^3 = 0.008) 
flat slab providing the diaphragm action whereas the MF beams could be of
the order of 
700-900mm (I proportional to 0.9^3 = 0.729). The ratio of the stiffnesses
is about 91.


> --You do the front and back end transpositions (described by Rudra)
> --You account for the master/slave and non-slave/slave COUPLED stiffness
> matrix terms (not described by Rudra)
> --You don't have any out of plane loads/effects applied to your diaphragm

The out-of-plane loads applied to the diaphragm are distributed to the
columns based
on the floor-framing (I am relating to buildings specifically). The axial
shortening in the
columns is definitely not ignored, the flexural stresses produced in the MF
beams 
due to this differential vertical displacements in columns, is also
modeled. The only thing
that will not be modeled is the resistance of the diaphragm to the axial
shortening which
in engineering terms is quite small as explained above.

So unless, you are working to the accuracy of the third decimal point, it
is reasonable
to ignore the out-of-plane displacements.
Having said that, I would have to agree with Bruce that RISA being a
general analysis
 program, should stick to its definition so that the out-of-plane
displacements are not 
ignored and the program is accurate to the nth degree, and it should be up
to the
Engineer to appropriately interpret the program methodology.

Its been a long time since we had such an "analysis" discussion on this
server.

-Swaminathan Krishnan

> 
> As for me, I'll stick with *real* rigid links.
> 
> As pointed out in a previous post, anyone using master/slave should
review
> their program's documentation to understand what exactly is being
formulated.
> 
> Bruce Bates
> RISA Technologies
> 
>