Smart questions
Smart answers
Smart people
Join Eng-Tips Forums

Member Login

Remember Me
Forgot Password?
Join Us!

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips now!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

Join Eng-Tips
*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.
Jobs from Indeed

Link To This Forum!

Partner Button
Add Stickiness To Your Site By Linking To This Professionally Managed Technical Forum.
Just copy and paste the
code below into your site.

Tutorial 17 Femap Frequency response of towerHelpful Member! 

naeng (Industrial) (OP)
4 Jul 10 5:01
I´ve been working in this tutorial of femap, and my question is if should it be possible to perform a time history analysis applying an acceleration, on the node base, dependent of time like an accelerogram?, if it should be possible, wich will be the proccedure?

Thanks in advance.
Helpful Member!  BlasMolero (Mechanical)
4 Jul 10 14:22
Oh yes, you can perform a time history analysis using an enforced base motion (SEMTRAN SOL112) analysis. But if you have an earthquake ground motion on a building or an explosive shock on a small component in a ship I suggest to perform a response spectrum analysis, the advantage over a conventional transient analysis is economy and simplicity.
Best regards,

Blas Molero Hidalgo
Ingeniero Industrial

naeng (Industrial) (OP)
4 Jul 10 16:26
Hello Blas,
first af all a lot of thanks for your help, with a enforced base motion, do you mean perform a dynamic analysis with the large mass method (wich is explain in the tutorial)?, an then apply to the element mass the acceleration dependent of time, thats right??, if i´m right, wich must the value of the mass be in order to perfom this analysis?, i think, the value of the mass, must be greater than the mass of the fem model, but how many?

ok, thanks in advance.
BlasMolero (Mechanical)
5 Jul 10 4:13
I you perform the command "Model > Load > Enforced Motion ..." this command directly will creates a base mass (0-D) CONM2 element, links it to a set of "base nodes" in your model with a rigid element (RBE2), and applies an equivalent base force.

The command will ask you the mass and the acceleration scale factor. They are utilized to generate a nodal force (force = base mass * specified acceleration) at the independent node of the newly created rigid body. The values are automatically computed based on your current model and the acceleration that you chose.

The default for the mass value is several orders of magnitude larger than the mass of the current model so the large mass will drive the rest of the model. You can either simply press OK to accept them, change them here, or edit the force later with the Modify, Edit commands.

The Large Mass/Spring approach is actually a modeling technique in which the user places an element with a large mass or stiffness at the points of known acceleration or displacement. In effect, this large element acts as a constraint on the connected point. The user then supplies a corresponding large force via RLOADi or TLOADi inputs to produce the desired motion. If the added element is sufficiently stiff or heavy, the reaction forces from the actual structure will not affect the input motions.

Generally, the value of Large Mass should be approximately 1e6 times the mass of the entire structure for an enforced translational degree-of-freedom and 1e6 times the mass moment of inertia of the entire structure for a rotational DOF. The factor 1e6 is a safe limit that should produce approximately six digits of numerical accuracy, this is the default value used in FEMAP.

Best regards,

Blas Molero Hidalgo
Ingeniero Industrial

naeng (Industrial) (OP)
6 Jul 10 2:48

ok Blas, perfect explanation, i´ve understood now

thanks again.


Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!

Back To Forum

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close