Hi all,

I'm exploring the steel construction manual 13th edition. But I'm unable to find how I should calculate the rotational stiffness of a connection. Please enlighten me.

Thanks

Are you referring to a "partially restrained" moment connection?  

I'm not aware of any standard guides that have this. I've always told people to look up the test results data (moment rotation curve) and use that as the basis of their connection stiffness approximation.  

For fully restrained moment connections you can assume that the beams are rigidly connected to the columns.

Well in Eurocode (EN1993-1-8 chapter 6.3) there is a formula to calculate the rotational stiffness of a connection. They also give an upper value for a connection to be nominally pinned and a lower value for a connection to be rigid.

Now I have found those boundaries in AISC specification for struct steel buildings (Ks >= 20EI/L fully restrained, Ks <= 2EI/L simple). But I can't seem to find a way to calculate Ks, which is the secant stiffness of a connection.

I need to calculate the rotational stiffness for moment connections in order to use them in my global analysis. In fact, what I'm trying to do is theorically predict the moment rotation curve you would get from tests, without actually having to perform these tests.

If you are looking for information on non-linear moment rotation curves, you might try the following:
FEMA 356 Prestandard and Commentary for Seismic Rehabilitation of Existing Buildings
UFC 4 23 03 Design of Buildings to Resist Progressive Collapse
ASCE 41-06 Seismic Rehabilitation of Existing Buildings
 

I agree with wannabeEIT (what kind of a name is that?!).  Those Push-Over and Progressive collapse documents have a good amount of information about moment rotation curves or such for connections.  So, they're probably a good place to start. Unless someone else replies back with a better reference.   

One reference that even gives old DOS software to the purpose is

Stability Design of Semi-Rigid Frames
Chen, Goto, Liew
Wiley Interscience

This isn't referenced in AISC anywhere but I have always treated a connection which able to transfer the minimum of the design action and/or 30% of the member flexural capacity as being a rigid connection for the analysis.