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Re: ETABS Modelling-Flexible Floor

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John rightly pointed out that my answer was incomplete earlier.  So here
goes.....(I took the following info from the FAQs in CSI's web site, without
any wrong intentions of violating any copyrights)

Question 3:  How do I model floor flexibility in ETABS?
Answer:  Previous ETABS versions modeled the entire floor plate as a rigid
diaphragm. This is still the default in ETABS V6. The following steps should
be followed when modeling the flexibility of a floor using the floor
1. Define the floor bays and elements to model the stiffness of the floor. A
refined mesh is usually not necessary;
2. Disconnect all joints from the default rigid diaphragm number 1;
3. Choose suitable mass lumping points on the floor. These are the rigid
diaphragms required in the analysis;
4. Specify the joints which contribute their mass to each of these rigid
5. Finally, connect the rigid diaphragms to two or more column lines closest
to the center of mass.

Question 4:  Why do we need to connect two column lines to the rigid
diaphragm when floor elements are modeled?

Answer: Since diaphragms will typically have both translational and
rotational mass and floor elements do not have rotational stiffness, we need
to connect the diaphragm to two joints in order to provide rotational
stiffness in the form of a couple.

Incidentally, the support staff at CSI is truly supportive (no punintended
:-)) and I call them all the time to discuss specific program related
modelling issues.

Hope that helps.

-Swaminathan Krishnan, P.E.
Over Arup & Partners, Los Angeles