Autodesk Robot Structural Analysis

[#1]

Converges of Eigen period

Hey! I have modeled a beam (450X450mm ) length 4500mm in robot using beam elements. And got convergence of the Eigen period when I used 20 elements. I have solved the same problem using the differential equation for the beam without shear def.  Inn the result I get some differences and I have concluded that it must be the shear deformation. When I tried to model the same beam with length 200 m. I got some difftenses and the Eigen period got even bigger than the analytical solution.

I tough that the FEM solution always gave a stiffer construction than the analytical tough you impose a fictitious stiffness upon the construction.?????? The mass of the systems are the same.

[#2]

If I remember correctly that depends on the mass matrix type and the convergence is from 'above' or from 'bottom' respectively.

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