mass-loads

[jovo]

hello,

When I use the mass command and the load command I was said to put in the load command the same kind and amount of loads that I have put in the mass command (like a source of loads:dead loads).Is it correct like this or it will be counted two times?

thank you very much.

[vesna]

Mass that you assign to the nodes of the structure will be used to form mass matrix for your structure. This mass matrix is used either in eigen or in dynamic analysis. The dead load that you specify to act on the structure is a static load. Thus, there is no correlation between the mass command and the load command.

[nitadorakis]

Dear Vesna

Does it mean that the vertical loads applied on nodes or distributed on the beams should not affect the eigen periods i.e. not calculated as ''extra mass''??
In a plane frame that I'm working on (with displBeamColumn elements+fiber sections) there is a change in the periods when loads are applied.

Thank you in advance

[vesna]

Dear Nitadorakis,

When you apply load on your frame the stiffness matrix may change if some of your elements enter into nonlinear range of behavior. This might be the reason for getting different periods before and after loading the structure.

[nitadorakis]

Dear Vesna

Thank you for your reply.
I just wanted to make clear whether or not OpenSees accounts for gravity loads as "extra mass" apart from the fact that stiffness of the members (K matrix) could be reduced in high loading.

As I understand from your answer, there should be no difference in T in the case of linear members, irrespective of loading...

[vesna]

Dear Nitadorakis

OpenSees does not account for gravity laods as "extra mass".

[nitadorakis]

Thank you for your answer Vesna.

It seems that when using distributed plasticity elements with fiber sections, it's better to use distributed mass along the elements than lumped mass at nodes....
It's about the consistent mass matrix vs the lumped mass matrix.

With distributed mass I got eigen periods similar with the ones in the model with concentrated plasticity members.

[tsteele]

Hello,

I know this is an older forum, but I've been thinking that it would be nice for OpenSees to have the option to apply gravity loads to the model based on the mass matrix and specified gravity. This would be particularly helpful when using the massDens option with beam-column elements, and would prevent the user from having to calculate each nodal load for gravity analysis. That being said, is there a better forum for this post? Or better yet, does this option actually exist in the current OpenSees release?

Best,
Taylor

[fmk]

the options does not exist .. for most structural analysis unless it is a full 3d model incluidng the gravity columns (and few do that) the mass and gravity loads are not simply related by G as frames only take trib area in loads but large portion of building mass.