Seismosoft | Earthquake Engineering Solutions

Hello,
When a plastic hinge forms at step n, at step n+1 the expected bending moment should be reduced to a maximum of 20% of the moment from step n. But in SeismoBuild, I see that the bending moment stays the same. Why does this happen?

Also, what happens if the plastic hinge forms earlier due to shear failure instead of bending moment failure?

Upon the formation of a plastic hinge the expected bending moment does not necessarily decrease to 20% of the capacity. This is a matter of convention. In particular, with fiber modelling which is the default in SeismoBuild, the decrease in the bending moment capacity is determined by the stress-strain curves of the monitoring points of each member cross-section.
You might want to try plastic hinge modelling: Go to Menu>Analysis Parameters>Advanced Settings>Advanced Building Modelling and select the infrmDBPH model for walls, columns and beams.
Seismosoft Support

I understand now — since SeismoBuild uses fiber modelling by default, the reduction of the bending moment capacity after hinge formation doesn’t follow the conventional M-θ curves defined in codes such as KAN.EPE., but is instead controlled directly by the stress–strain relationships of the fibers (concrete and steel) at the monitoring points of the cross-section.
When switching to plastic hinge modelling (e.g. the infrmDBPH option), the software applies backbone curves consistent with code provisions, so in that case it’s expected to observe the conventional reduction to around 20% of the peak flexural capacity?
And finally, when a shear failure is appeared, what Seismobuild does? Form a plastic hindge? or not take into account this member in the next step of analysis?