How to do modal transient with force as a base excitation

I was trying to do truck frame modal transient analysis. Full vehicle (truck ) model with base force excitation at wheel center. since it is force a input i dont understand where to apply the constraint. if spc not applied then model travels with large displacment which is illogical.

Please suggest some solution on this

0 Don.Graff over 6 years ago

Try removing the rigid body modes... jut make a small, positive, non-zero lower bound for the eigensolution so the rigid body modes aren't obtained. The 'relative displacement' based results shouldn't change, but the displacements will be without any rigid body motion.
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0 more.rajendra3 over 6 years ago

Thanks Don.Graff for suggestion, we tried doing with the updates you suggested, additional we used LFREQ card for defining lower bound of frequencies. Its working, but still having one doubt that, dont we require rigid body modes for transient calculations? does not it effect the results?

Please give some inputs on this too.

Thanks

Rajendra
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0 Don.Graff over 6 years ago

Hi Rajendra,

Rigid body modes would not really influence the elemental responses (FORCE, STRESS, etc) as these responses are based on the relative motion anyway.

When you do NOT include any rigid body modes, the displacements, velocities, and accelerations obtained are relative to the overall motion of the structure and are not absolute response quantities. When you DO include rigid body modes, you then additionally have the overall motion of the structure acting as a rigid body, which can influence the absolute displacement, velocity and acceleration responses.

In my experience, people rarely look at the displacement results when rigid body motion is included, but sometimes will look at accelerations. It is not uncommon to run the analysis twice, once with rigid body modes included, and once without for the data recovery of accelerations and element results, respectively.

Regards,

Don
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