Stiffness classification
The classification of joints with regard to the stiffness is beneficial for modelling the stuctures, as rigid connections does not affect the distribution of internal forces any more. These limits depend on the structure type and type of analysis. The estimation is described in EN 1993-1-1.
Two types of structure have to be distinguish when classifying the anchoring joints: braced and unbraced structures. The accuracy is 3% for ultimate limit state and 20% for serviceability limit state (Wald, Jaspart; 1998). For common cases 
, the stiffness of braced structures may be calculated for
using formula
, for interval
using formula
and for
using formula
where 
is the relative slenderness of the column with hinged ends. This expression may be used for any slenderness. For relative slenderness limited by 
the limiting value may be considered as 12EIc/Lc.
Unbraced frames are more sensitive with regard to the stiffness of bases. For these structures, horizontal deformation is a decisive one (Wald, Jaspart; 1998), limiting value is
Stiffness classification
References
Wald F., Jaspart J. P.: Stiffness Design of Column Bases, v 2nd World Steel Conference, San Sebastian No.: 135, Journal of Constructional Steel Research Vol. 46, Nos. 1-3, 1998, ISSN 0143-974X.