Different moment of inertia

[Engineer3636]

Hi,

I am trying to calculate the moment of inertia of a section with additional stiffener plates. I have attached the section I am trying to imput into the section definition.

The profile is aluminium with steel plates. When I calculate I get different values than robot. Can you please explain?

A quick reply would be appreciated

Thanks

[marcinrakus]

Hi,

I briefly ran through manual calculations at attached pdf I did not come up the modification of moments of interia of particular parts to the new center of gravity of combined section (using Steiner theorem). 

[Rafal.Gaweda]

Just one hint: all results with * are weighted results calculated with respect to base material set on first tab.

[Engineer3636]

Thanks Rafel,
So you think the manual calculations are incorrect and I should use Robot?
The value with * is the moment of inertia I should be using then as this is the equivalent aluminium section (combined aluminium and steel)?
D

[Rafal.Gaweda]

I am afraid your hand calculations are too simple or I do not see part of it.

for example Iytotal = Iym + n*Iyst1 + n* Iyst2

But you use these values as calculated for each separate part but you need to transform them to center of gravity of whole section before adding.