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# Tutorial 17 Femap Frequency response of tower

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## Tutorial 17 Femap Frequency response of tower

(OP)
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?

### RE: Tutorial 17 Femap Frequency response of tower

Hello!,
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.

~~~~~~~~~~~~~~~~~~~~~~
Blas Molero Hidalgo
Ingeniero Industrial
Director

IBERISA
48011 BILBAO (SPAIN)
WEB: http://www.iberisa.com

### RE: Tutorial 17 Femap Frequency response of tower

(OP)
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.

### RE: Tutorial 17 Femap Frequency response of tower

hello!,
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.

~~~~~~~~~~~~~~~~~~~~~~
Blas Molero Hidalgo
Ingeniero Industrial
Director

IBERISA
48011 BILBAO (SPAIN)
WEB: http://www.iberisa.com

### RE: Tutorial 17 Femap Frequency response of tower

(OP)
hello,

ok Blas, perfect explanation, i´ve understood now

thanks again.

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