Element Mass

mehdi12345 · Post by **mehdi12345** » Mon Sep 07, 2009 9:53 am

hi Dr. Silvia
in element definition
element nonlinearBeamColumn $eleTag ...... -mass $massDens
when i want to define mass for an element do i have put a difference between horizontal and vertical elements , i mean something like this
-mass 100 and -mass 0 100 ???
Plz help me
Thanx

silvia · Post by **silvia** » Tue Sep 08, 2009 5:46 am

no because it just looks at inertial mass, which is independent of gravity.

mehdi12345 · Post by **mehdi12345** » Tue Sep 08, 2009 12:15 pm

I ask this because when I calculate total mass and use mass command my periods are very diffrent than when i use -mass in element.
by mass comannd T=0.35
and by using -mass in element T=1.24

and both are diffrent from T calculated by sap2000
sap T=0.86

Which on is correct ? How can I find real T ?
Thanx Dr Silvia
Regards

silvia · Post by **silvia** » Tue Sep 08, 2009 8:59 pm

sap and opensees models may be of different stifnesses.
i never use element mass because OpenSees still has a lumped-mass model, and not all elements take mass density, so i use the nodal masses.

likely your periods are different because of your stiffnesses, not just masses -- check units.

try this out on a simple and elastic model to check first.