Explicit dynamic analysis of strut with lumped mass

[JK7070]

Hi all,

I am trying to perform an Explicit direct-integration dynamic analysis.
I am simulating a single degree of freedom (SDOF) structure initially to test my method and the results.

My structure is a simple strut (cantilever column, see attached) which is fixed at its base and free at the other end. I apply a lateral acceleration (in the form of an acceleration time history using an amplitude curve) to the fixed end and I want to observe the behaviour of the strut, especially the free end as it deforms laterally.

My problem comes when I try to apply a lumped mass to the free end. So far I have included a density in the material which I have tested and does have an effect on the results. But for all my efforts I have been unable to get the lumped mass to influence the behaviour at all.

I have applied the lumped mass as a point mass inertia in a number of ways to no effect. Firstly was to apply the point mass directly to the vertices at the free end of the strut and more recently to a reference point which I coupled to the end of the strut. Every scenario has no effect.

At present I am fresh out of ideas and could really do with some advice.

Regards,
Jack

 

[Drej]

Try a modal analysis with and without the mass attached to firstly ensure you've attached the mass and its properties correctly.


------------
See faq569-1083 for details on how to make best use of Eng-Tips.com

 

[rtalageri]

Modal analysis is a nice, quick and dirty way to check if the model is ok. To attach a nodal mass element did you try using MPC beam connection between the reference node and the free point mass, i think that should solve the problem.

 

[JK7070]

Thanks Drej and rtalageri for your response.

I have taken your advise and completed a modal analysis to check whether the mass is attached properly and it is. I have used the approach of applying a point mass inertia to the vertices at the end of the strut and the results are correct. I have confirmed this with some hand calcs!

When I use this exact same procedure in an Explicit direct-integration dynamic analysis I get the following warnings:

Output request evf is not available for element type s4r

Output request evf is not available for element type mass

Output request pe is not available for element type mass

Output request peeq is not available for element type mass

Output request misesmax is not available for element type mass

The option *boundary,type=displacement has been used; check status file between steps for warnings on any jumps prescribed across the steps in displacement values of translational dof. For rotational dof make sure that there are no such jumps. All jumps in displacements across steps are ignored.

Basically I want to plot the base shear against the lateral displacement of the free end of the strut. But I am not getting any realistic results for deformation or stresses ect...

Regards,
JK7070

 

[JK7070]

Does anybody have any ideas what could be the issue???

I have left the problem for so time and have now come back to it with fresh eyes.
But I still cannot overcome this issue.

Regards,
JK7070

Regards,
JK7070