Zero Length Section Problem

[mmb]

I modeled a simply supported steel beam with a crack at bottom flange and I used zero length section element to model crack in beam (ENT material for bottom flange in fiber section modeling of section at damage location). In mode shapes of this beam i see a jump at location of zero length section element. why does it occur?

[fmk]

has to do with differeing tangents from the two elements .. simply think about the eigenvalue equation [K - lambda M] phi = 0

[mmb]

set Es 2.1e6;
set matundamaged 1;
set matdamaged 2;

uniaxialMaterial Elastic $matundamaged $Es -----for top flange and web and for undamaged elements of beam
uniaxialMaterial ENT $matdamaged $Es -----for bottom flange

???

[mmb]

I think zero length section acts as shear hinge. what is is the best solution?