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RE: Partial Rigidity (was ALGOR, EAGLE, ETABS, STAAD etc.)

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At 07:18 AM 6/9/1998 -0400, you wrote:
>> connections used in wind or seismic applications.  How rigid should I
>> assume
>> it, 100%?  90%?  Please comment.
>> Y. Henry Huang

As for the reference " Stability Design of Semi-Rigid Frames" by W.F. Chen. 
et. al. It was published by JOHN WILEY in 1996. 

J.Luo and, specially, Christopher Wright have interesting comments. I think
the design considering a very realistic model for semi-rigid connections is
not easy at all. It is not too bad if we are dealing with design of frames
with gravity loads, as monotonic behavior of connections (even including
non-linear moment-rotation relationship) can be modeled by curve fitting on
data points from actual tests on the connections.

However, it becomes more complex to incorporate nonlinear, inelastic model
that would match very closely the actual cyclic behavior of connections
subject to reverse sesismc forces. It is control of drift and ductility
limit on the connections that dictates how you should change your connection
size. In fact, you can choose connections with any degree of flexibility you
want as long as you respect limits of drift and ductility capacity of the
connections. So, you need to perform non-linear dynamic time history
anaylsis for various earthquake records, and watch the drift and the
connection rotational ductility. That means you already must have chosen
your connection to model and incorporate it in the analysis. Thus, such
analysese must be done after you completed your normal design based on
elastic and linear analysis. 

I think we have to try, as much as we can and as realistically as
possible,to model and analyse our structures. It is true that model will not
be perfectly representing the real structure, but has to be "close enough".
for the same reason that we model a simple connectoion as pin and not as
fixed, we should somehow recognize that a partially restraint joint would
have to allow some degree of rotation, in a elastic or ductile manner. 

As for the rigid connections, it may make some difference in elastic design
if you consider them having 90% or 100% degree of restraint. But we know for
the fact that they yield and deform inelastically, and what is important
then is how much distortion they can sustain before they fail.
I hope my humble explanations help.


>(1) "100%" means the connector has a rigidity=infinity. In real
>structures (i.e, steel, wood), connectors are deformable.
>(2) If the connector is deformable, it is an example of partial-rigid
>analysis. However, without an analysis, there is no design. We cannot
>have any info about the connector. Simply assume a connector strength,
>i.e., 90%, and have an analysis. Partial-rigid analysis may output the
>"connector stiffness" that is corresponding to the assumed strength
>(i.e., 90%). Based on the analysis result, we can design member
>sections, and then design the connector. It is necessary to calculate
>the connector stiffness and check if the connector stiffness is
>satisfied with analysis output. If not, adjust the connector strength
>and re-start the process again.

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                   \               Majid Sarraf             /
                    \             Ph.D Candidate           /
                     \   Department of Civil Engineering  /
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