Bidirectional Earthquake groundmotion

[hoseinrezaei]

Hi every body,

I want to apply bidirectional ground motion to a portal 2D frame.
are the masses assigned correct?
# ^Y
# |
# 3____________________4
# | |
# | |
# | |
# | |
# | |
# | |
# | |
# =1= =2= -------->X
#
#
# calculated parameters
set Weight [expr 2000.]; # beam weight
set PCol [expr $Weight/2]; # nodal dead-load weight per column
set Mass [expr $PCol/$g]; # nodal mass
set rotationalInertia [expr 1./12.*($Weight/$g)*pow($LBeam,2)];

# nodal masses:
mass 3 $Mass $Mass $rotationalInertia;
mass 4 $Mass $Mass $rotationalInertia;

Thanks in advance,

[fmk]

you do have masses in all the degree of freedom at the two free nodes if that is the question.

[hoseinrezaei]

Thanks for your reply.
In bi-direction analysis the masses should be assigned to X and Y direction?
Another question is about rotational inertia. Is it need to be considered?
How does OpenSees consider the mass in transient analysis?
Thanks.

[fmk]

most everybody ignores the rotational term. the mass terms goes into a mass matrix. How the mass matrix is used depends on the integration scheme. look at theory for Newmark or Cenral Difference to see what each does with the mass matix.