Understanding the Value of the Second Moment Of Area

Nov 2019
Hi all

I was hoping someone could help shed some light clearing some doubt on 2nd Moment of Area.

I know that if i had a beam that was loaded then the top of the beam would experience compressive forces.

As i moved down towards the neutral axis these compressive forces would become zero.

And i as i moved towards the bottom of the beam then the beam experiences tensile forces.

I know that the 2nd moment of area is how the area of a cross section is spread from the neutral axis i. e the more area away from the neutral
axis the less bending the beam will experience.

This is where i get confused.

Given a a rectangle (Breath 3m, Height= 7m), the equation for 2nd moment of area is given by bh^3/12 so my value for the second moment of area is 85.75m^4 as shown below:-


I am trying to visualize what this number means?

Does 85.75 mean that i have 87.75m^4 of area above AND below the neutral axis as shown in green above? Or Does 87.75 mean that my total area furthest away from the neutral axis is 87.75?

I guess i am trying to understand what 87.75 means other than its a value that describes the resistance to bending, its about HOW does this value represent the resistance to bending?

Finally does the equation Ixx = bh^3/12 only apply to a single point which is at the centriod or along the neutral axis in the cross section?

Can anyone help?

Thank you.