Building Period

[gzdee]

I am designing a base-isolated building in ETABS and my target period is 2.5 sn according to ASCE 7-10 equations. For my calculations I am using mass which is given by ETABS. In program I am defining ısolators as link elements and assigning to point springs.
My problem is I know effective stiffness values of link elements and I know building mass but I can't get my target period. According to my hand calculations based on link stiffness and building mass, period is supposed to be 2.52 sn. But ETABS is giving me 2.61 sn. What could be the reason?

 

[structSU10]

within 3.5% is pretty good. either force your ETABS model to match the ASCE 7-10 equation, or run with what you get from ETABS.