Not sure about cidect, but typically EC3 for ultimate limit states are governed by equations giving say N1.Rd (or Nc,Rd for axial force in beams or Mc,Rd for bending and so on) which is the maximum force or moments (they are not based on VM stresses).
Thus to kind of compare, one could look at the ultimate load of these welded joint. So including plasticity, push the joint and obtain the load displacement curve.
See here for an example (codes tend to be more conservative than the full NL FEA):
http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1679-78252015001102143
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