period of MDOFs

[n_siahpolo]

Dear all
H
I have modeled a series of MDOF 2D frames with nonlinearbeamcolumn and elasticbeamcolumn to do inelastic and elastic time history analysis. But as I checked the first period of each models, there is an approximate 25% difference between one which is resulted from using nonlinear element and another from elastic element. I checked all frames. In some cases this difference is about 3% and in some cases it is up to 25%. For instant in 20st frame with nonlinear element the period is 3.063sec but in elastic case it is about 2.433sec.

[fmk]

the periods before you apply load should be the same if the initial properties of the material and section result in same E and I values of the elastic beam. if different, then i suggest you check the nonlinear or linear model as something in your models is amiss. usually students go from eleastic to nonlinear through a number of steps. first step is to use nonlinear element with elastic section, then nonlinear element with fiber section contatining elastic materials and finally full nonliner model with fiber and nonlinear materials.