Steel Joists: Moment of Inertia??? I thought it might well be worth while asking here, as the Forum is awash with knowledge about most things. I am planning to do a loft conversion and I need to know about the strength of steel beams (Universal Joists, in fact). I keep seeing the expression 'Moment of Inertia' in literature and I couldn't see what it could have to do with a static structure. Then I realised that it must be to do with the cross section and how the bits further from the middle are more effective in the strength of the beam. So it must relate to the sum of each element of the cross section times the distance from the middle - which has the dimension L^4, the same as MI. It makes some sense but it just seems inappropriate, somehow. Does this terminology go back to the year dot or is it new?