Different natural period building

Prayag Sayani · Post by **Prayag Sayani** » Sat Mar 01, 2008 4:53 pm

Hello Dr. silvia,

I am trying to find natural period of a 3 story steel braced frame building. ( 2d model and just 1 bay). I am considering 2 types of element modeling cases.

1. All beams and columns are elasticbeamcolumn elements and braces are truss elements.
with this modeling, I am getting 0.55 s fundamental period which matches with my expected Tn.

2. All beams and columns are nonlinearbeamcolumn elements with Wsection and braces are nonlinearbeamcolumn with circular patch section. In this case, I am not getting same Tn which is 0.55 sec.

I should mention that I am using same seismic mass for both the cases.

Why am I getting different Tn for both cases?

Thanks in advance,

silvia · Post by **silvia** » Sun Mar 02, 2008 12:00 pm

because a fiber section only samples at the fibers, hence you should get a softer system.

Prayag Sayani · Post by **Prayag Sayani** » Sun Mar 02, 2008 7:32 pm

Okay, So if I want to get same Tn from both the modeling ( i.e. with elastic beamcolumn elements and inelastic beam column elements with fibers), what I should do?

silvia · Post by **silvia** » Mon Mar 03, 2008 6:27 am

try putting in lots and lots of fibers......