DBPH and FBPH

[draven]

Hello Seismosoft team.

I ran into a problem regarding the elastic dynamic analysis. I used DBPH and FBPH and realized that the values of the eigenperiods and so the response of the structure is slightly different. I know these type of elements are only used for inelastic analyses, but i saw these results and just wondered why they are not the same. Thanks in advance!

[seismosoft]

These differences are related to small differences in the initial stiffness between the two types of elements, DBPH and FBPH. Whereas in FBPH the stiffness is updated at each step of the analysis, in DBPH the moment-curvature curve is bilinearized and initial stiffness is kept until yield. The differences in the stiffness degradation is related to how inelasticity is modelled in each case, lumped plasticity vs. distributed plasticity.
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