Quote Originally Posted by pavlo View Post
One thing to bear in mind is that although aluminium is about a third of the density of steel, it's Young's modulus is also a third of steel's. This means that for the same geometry, you need to use three times as much aluminium to get the rigidity of steel. The benefit of aluminium is that the geometry of extuded sections have a high second moment of area (I) which gives good flexural rigidity.
That's what I was referring to, but you put it better.
Also steel is 1.7x more dense than aluminum, which is something to consider with regards to resonance.