Problems in moment releases in 3D model

[Prayag Sayani]

I am doing eigen value analysis of a 3 story building with a 3 D model. I have defined rigid diaphragm with all nodes constrained to master node at each floor level. The building has 6 bays in X and 4 bays in Y direction with 10 moment frames in each direction. I am getting time period of 0.68 seconds that matches with my SAP model. All the connections are rigid in this model.

Now I want to put moment releases in gravity frames and I have defined dummy nodes at appropriate nodes and used equalDOF command to slave it to the master node. To make sure my dummy nodes are correct, I am using equalDOF command like this,

equalDOF masternode dummynode 1 2 3 4 5 6

Basically, I am making rigid connection again instead of moment releases. This means that I should get the same time period as I get from my other model without dummy nodes, Right?

Instead, I am getting different Tn which is 0.44 seconds instead of 0.68 seconds. I have 76 dummy nodes per floor (total of 228 nodes). I am writing equalDOF 228 times to write all the dummy nodes.

Am I doing anything wrong? Why am I unable to get same Tn = 0.68 s for basically same model?

Please help,

Thanks

[fmk]

how are you doing the moment releases? .. equalDOF is not doing it.

[Prayag Sayani]

Okay.. I want to put moment releases in my 3D model.

All I knew was to create a dummy node where I want to generate moment release and slave it to the original node at that location using equalDOF command.

Can you suggest a better way to make moment releases in my 3D model?

Thanks,

[fmk]

you have no moment release if you slave all dof .. you might as well have no dummy node .. you can try slaving translational and put rotational springs with very low stiff for rotations.

[Prayag Sayani]

Thanks for your reply.

I wanted to get same time period as the model with rigid connections by slaving all the DOFs, but I could not.

So to get moment releases, I can slave translational dofs and put zerolength spring with elastic material having very low stiffness. Something like this,

set kspring 1e-5
uniaxialMaterial Elastic 1 $kspring

and connect it by
element zeroLength elementno masternode dummynode -mat 1 -dir 6;

Is it right? Thank a lot for your help.

[fmk]

why don't you tell us when you run it .. also play with the stiffness of the springs and post some conclusions for others to see. thanks.