Aluminium RHS rule of thumb?

[MarioGr]

Are there any approximate rules of thumb that engineers here use for aluminium RHS members vs steel RHS members with regards to bending capacity?

IE a 150 x 100 x 5.0 RHS member has a bending capacity (phi)Msx of 44.00 kNm. Therefore a 150 x 100 x 5.0 aluminium member's capacity would be (insert rule of thumb value here) 25%(?) of the steel value. Approximately 11.0 kNm.

Only asking because I've been asked to rough in some member sizes on the back of an envelope for a fabricator for pricing purposes.

Thanks very much.

 

[3DDave]

For the same section, the strength should be proportional to the yield strength ratio, but the deflection should be 3X and the weight should be 1/3.

The typical good substitute is a larger aluminum section to match the bending/deflection and reduce the weight over using steel so it will be a bit more than 1/3rd, but still a savings.

 

[JStephen]

One thing to watch is that a lot of the aluminum materials have higher yield strengths due to cold-working or heat-treating, but then if you weld them, the strength right at the weld is much lower- so it varies a lot depending on your detailing.