Vibrations and rotational inertia

[bongoyozo]

I've analyticaly solved natural frequencies of a beam with shear model, and then Timoshenko model (that includes rotational inertia).
I've then solved the same beam in Nastran using Lanczos method and coupled mass. Results are following shear model that doesn't include rotational inertia. Ansys solver for the same model gave good results like Timoshenko.

So how do I include rotational inertia in Nastran ??

Thanks

 

[BlasMolero]

Hello!,
Please note that NX NASTRAN CBAR elements (BAR elements in Femap) have no torsional inertia mass if the CBARS are colinear, the Rigid Body Mass Matrix (QRR) don't have mass at all six Degrees of Freedom (DOF). Please consider to use the NX NASTRAN CBEAM (BEAM element in Femap), this element type has torsional inertia mass. If the CBAR model is not made up of all colinear elements, then the non-colinear CBARS will provide an inertia Mass at the appropriate Degrees of Freedom (DOF).

Best regards,
Blas.

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

IBERISA
48011 BILBAO (SPAIN)
WEB:

http://www.iberisa.com

Blog de FEMAP & NX Nastran:

http://iberisa.wordpress.com/

 

[bongoyozo]

Hello Blas,

Problem is I do use beam elements, as mentioned in question. And it's 1D model (only vertical vibrations). And yet the results are not satisfying.
It's really fishy Nastran results are same as shear model and not Timoshenko like Ansys. And yes, model is made in Femap with choosing different solvers, so model is the same in both solving.

 

[BlasMolero]

Hello!,

According to remark 13 (see below) of the PBEAM card in the NX NASTRAN V8.1 Quick Reference Guide you just have to set K1 and K2 to 0.0 and any beam ( e.g. CBEAM) referencing the PBEAM card will be a Bernoulli-Euler beam:

"...13. The shear stiffness factors K1 and K2 adjust the effective transverse shear cross-section area according to the Timoshenko beam theory. Their default values of 1.0 approximate the effects of shear deformation. To neglect shear deformation (i.e., to obtain the Bernoulli-Euler beam theory), the values of K1 and K2 should be set to 0.0. ..."

Best regards,
Blas.

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

IBERISA
48011 BILBAO (SPAIN)
WEB:

http://www.iberisa.com

Blog de FEMAP & NX Nastran:

http://iberisa.wordpress.com/

 

[bongoyozo]

Hello Blas,

of course when factors are set to zero, I get correct Euler-Bernoulli beam results. Again, when I leave them to default values, natural frequency results are larger than Timoshenko theory, obviously missing rotational inertia. I guess Nastran authors reference Timoshenko method, but it's actually the shear model.

One more hint:

http://www.sdtools.com/help/p_beam.html

"... No correction for rotational inertia of sections is used."


Regards