Difference between tow types of definition of rigid diaphragm

[Albertcnzheng]

I want to define floor in rigid diphragm.
First, I define the area element as rigid diphragm.Period is 2.86s
Secondly, I define the joint(point element) as rigid diphragm,period is 1.95.
I don't know why? Are there some differences between these tow types of definition?

 

[amadeus]

Albertcnzheng:

In ETABS there is no need to define a master joint since it does the job of converting loads and self weights to masses automatically. If you wish to use rigid diaphragms, wich by the way is no longer necessary, all you have to do is select all the different areas in a story and assign them rigid diaphragm property.