mass moment of inertia

[eftekhar]

hello everyone,

i'm trying to model a 3D one story buiding with mass IIrregularity, the model has 4 elastic columns and rigid diaphragm. the model has no beam, so i used rigid diaphragm to equal dof of top nodes of columns. i use a model from a reasercher's article, he note that the modal frequences of first three mode is 1.16 , 1.96 , 2.37 HZ respectively. i builded the model in etabs and can verify my model with 0.33% error, but i couldn't verify model in opensees. i trying almost everythings to verify it but Unfortunately all my work was unsuccsesfull. in my reaserch i found that mass moment of inertia is principle parameter in model with mass IIrregularity, so i try figure out how modal analysis run in etabs and opensees and what difference between them. i found that etabs centeralized all masses in center of mass and then make mass matrix with 3 parameters like that
[mass in x direction 0 0
0 mass in y direction 0
0 0 mass moment of inertia about hight axis ]
but i wondering opensees generate mass matix for each node(also we can use center of mass for assigne mass) and also mass matrix for 6 dof model with 3 limited dofs (dofs limited in z direction and rotation about x ,y axis) is 6*6, so i'm confused to understad how opensees derrive mass matix and calculate modal frequences. i try both assigning mass to nodes and center of mass. in first condition i assigned mass moment of inertia like that
mass n "mass in x direction" "mass in y direction" 0. 0. 0. "mass moment of inertia about height axis"
etabs calculate mass moment of inertia with multiple each mass of nodes by distance of node from center of mass then sum these valus and attain mass moment of inertia abuot height axis. also we can use another formula for it. i try all patterns i found but i can't verify my model because i can't understand how opensees run modal analysis and make eigen values. for best understanding i upload my model in opensees and etabs.
https://drive.google.com/file/d/1MEJ_zE ... sp=sharing
I'm so thankful if you can help me.
with best regard

[mhscott]

This post might be helpful: https://portwooddigital.com/2021/07/11/ ... aint-easy/

[eftekhar]

dear prof scott
thank you so much for offering that post i'm afraid it didn't helpful.
wish you best

[selimgunay]

You should use a RigidDiaphragm constraint in OpenSees and assign the masses and mass moment of inertia to the center of mass.

[selimgunay]

Rigid Diaphragm command https://opensees.berkeley.edu/wiki/inde ... gm_command

[eftekhar]

dear selimgunay
i used this command for my model but it didn't work.
also when i assign mass to center of mass, i get more far result.
wish you best

[eftekhar]

dear all
i think make a progress with this question. i used two change wich think make better result. first i used rigidLink beam between cm and each node insted od rigid diaphragm command. second i used Elastic TimoshenkoBeam insted of elastic element or other type of element and it also made me beetter result. but i steel have big problem in understanding rotational mass in opensees.
any help will so appreciated.