Moment of inertia should be in [mm4] but Fusion show it in [g mm2]

[giorgio.demurtas]

I have a C shaped beam made with a sheetmetal.

I want to knwo the moment of inertia (second moment of area) with respect to the x-z neutral plane (horizontal plane). I need this value (Jxx) to calculate how much bending moment can the beam hold.

The property window reports the "moment of inertia", I think I should read Ixx, but the unit of measurement is wrong: it should be mm^4, but I see g mm^2.

What am I looking at?

[algor_neil]

Moment of Inertia is not the same as the Second Moment of Area.

Moment of Inertia (Mass Moment of Inertia) is a 3D property with units [mass]x[length^2]

Second Moment of Area (Area Moment of Inertia) is a 2D property with units [length^4]

The use of I for both is always confusing.

[giorgio.demurtas]

You area right, I got confused by "I".

Is Fusion360 able to tell me the second moment of area of the section of the beam? I need it to evaluate its stiffness.

I projected the shape of the section on a sketch, but I don't find an option to get the second moment of area from the sketch.

Online I found that it is possible in Inventor: https://forums.autodesk.com/t5/inventor-forum/moment-of-inertia-second-moment-of-area/td-p/2724121

and I found it in the idea station of F360 https://forums.autodesk.com/t5/fusion-360-ideastation-request-a/2nd-moment-of-inertia/idi-p/7171229

[brooksaeromobile]

There is a work around to get 2nd moment of area in F360. Assuming metric units,

Take your cross section, extrude it 1mm. Set the physical property to water (SG 1.0 of course)

Multiply the I values by 1,000 and you have mm^4

[thomas8LC]

area moment should be a feature in the section properties.  why are simple tasks so difficult in fusion?

[christoph_bradl]

Can you discribe where I can find the section proporties exactly and than the 2nd moment of inertia?

[brooksaeromobile]

As far as I know you can't. Fusion gives mass moment of inertia in g/mm^2
not second moment of area.

The work around is to make the part out of water, extrude a section 1mm
thick, get the mass moment of inertia, multiply by 1000 and that's gives
you the second moment of area in mm^4

[bayderh]

How would you do it in Empirical Units?