Moment of Inertia of a bolt group 8

[Amar-Dj]

Hello everyone,

As a mechanical engineer working as a structural engineering, I still need to gather and understand the structural engineering theory.

I have a hard time to understand the moment of inertia of a bolt group (I_x and I_y).

Let's say that you have two columns of 3 bolts each. The zero is centered on this pattern. ''Y'' being the vertical axis and ''d'' is both the vertical and horizontal distance between bolts.

I have read that I_y= d²+(-d²)+d²+(-d²) = 4d²

Can anyone give me a reference explaining the math behind this and where does it come from?
Also, how come the units are mm² instead of mm⁴?

Thank you all for your answers.

 

[Amar-Dj]

I got my answer.

Thank you all