problem with periods

[behrangv]

Hello Dear Freinds
I have 6story-3bay RC frame modeled in opensees ...
my periods
for model with elasticBeamColumn is 0.82.
for model with elasticBeamColumn including crackin effects (cc=0.7 ,cb=0.35 ) is 1.26 .
for model with nonlinearBeamColmun is 0.76 .
Iam suspicious of period, result from nonlinearBeamColmun model .

have you any Idea about my periods?

[Jhno]

I assume you are talking about your undamped natural period. This period is given by the root of k on m. So, if you undergo plasticity, your K value will change. Obviously, your period will also change. So the answer to your question is it depend on the level of plasticity you went to.I would try to use a load that does not produce any significant plasticity (stay in the elastic range) and compare the period. They should be the same then.

[fmk]

is the 0.76 at the start while the structure is still essentially elastic?