Modal analysis of cantilever beam - by hand and by Robot

[sergey.churilov]

Hello all,

It might be trivial, but I can't figure out the differences obtained when calculating the problem by hand and by Robot.

The problem is very simple. For the given cantivever beam with uniform loading, one should calculate the modal properties.

The results calculated by hand and by Robot are equal with respect to the stiffness and the mass, but the period, frequency and pulsation are different.

Please check the screen-shots.

[Rafal.Gaweda]

[sergey.churilov]

Thank you Rafal for going through my question.

It seems that acording to Roark the calculation is corect.

I will check one more time my calculation, but I am pretty sure that the hand calculation was Ok.

[t.sautierr]

When you calculate k = 1/delta I think you missed to convert force to mass ... : F = k delta -> mg = k delta -> k = mg/delta and then the results follows  no? (because if F=1 then m is not equal to 1) no?

[sergey.churilov]

Well I don't think so. The stiffness was obtained following the concept of deflections resulting from unit forces.

The force to mass conversion was done later when calculating the mass.

What I think causes the differences is that we can not use the concept of single degree of freedom system when dealing with systems with continuous mass distribution as in our case with the uniform load.

Any other comments would be helpful.