Seismic analysis problem

[adrian_ricinschi]

Hello,

I have a problem regarding the seismic analysis.

I have some beams with loads on them. If I have some joints (the joints do not have loads applied) on them they will load from lateral earthquake.

If the joints are missing they will not load.

How can I fix this?

Thank you

[Artur.Kosakowski]

Hi @adrian_ricinschi

Could you attach a picture illustrating what you mean?

[adrian_ricinschi]

Hello,

The load from case 2 is used as mass source for the seismic load. The seismic load is perpendicular on the beams.

It is the same load for all 3 beams.

The difference is that the upper beam has 3 joints added, the middle beam has one joint and the lower beam has no joint.

One problem is the different values and diagram shape for the upper and middle beam.

And another problem is that the lower beam has no efforts. In the structure model the maximum lenght for a bar for automatic division is set to 1 m.

Thank you!

[Artur.Kosakowski]

Hi @adrian_ricinschi

Set the consistent mass matrix type in the parameters of the modal analysis. The option you refereed to is not used for the dynamics.

If I managed to answer your question(s) press the Accept as Solution button please. This will help other users to find solution(s) much faster. Thank you.

[adrian_ricinschi]

I had the option of consistent mass matrix activated.

But the results are the same for all 3 mass distributions options.

Can you please test in the model that I send to you?

Thank you!

[Artur.Kosakowski]

Hi @adrian_ricinschi

As far as I can see you attached only the picture.

[adrian_ricinschi]

In the first post is the Robot file.

[Artur.Kosakowski]

Hi @adrian_ricinschi

I'm sorry - my bad. I overlooked the fact that you have fixed supports.  For pinned there would be no need to have nodes to have bending but of course having them would increase the precision of the solution.

 

If I managed to answer your question(s) press the Accept as Solution button please. This will help other users to find solution(s) much faster. Thank you.

[adrian_ricinschi]

Hello,

I tried the same thing and I am not so satisfied by the results. 

In all 3 casses the result should have been almost the same.

I noticed this "bug" because in a structure that I modeled I have some seccondary beams.

On some of them I have applied a vertical load in a point that was not converted as seismic mass.

And all thease beams were loaded on the weak direction from seism.

Maybe you should fix this.

[Artur.Kosakowski]

Hi @adrian_ricinschi

By the formulation of bar element type as used in Robot they cannot be similar when there is only a single element between the supports and it is typical for all FEM programs with such bar element types. For the precision of the solution you need to 'replace' it with a chain of smaller elements which can either be done automatically (which is not what happens in Robot) or manually (e.g. by adding nodes along the bar length).

If I managed to answer your question(s) press the Accept as Solution button please. This will help other users to find solution(s) much faster. Thank you.