Aluminum 1/3 as stiff as steel. It flexes more for the same stress. A general rule of thumb, to achieve similar stiffness (which is different than yield strength), you need to triple the width of a section, or increase its height by 40%.
For example, a 3x3x1/8" angle in steel could be replaced with a 3x3x3/8" angle in aluminum. Because Aluminum is 1/3 as heavy as steel, you end up with the same weight.
Because the stiffness of a beam is proportional to its height squared, if you were designing the trailer from scratch you would be better off changing the other dimension of the beam - you only need to add 40% to the height of the beam. Our 3x3x1/8 angle example would be replaced with a 4.25x4.25x1/8. Doing it this way, the aluminum version is just as stiff, but weighs 1/2 as much.
Also, be aware that hardened or tempered aluminum (for example 6061-T6) becomes un-hardened or O-temper wherever you weld it. Depending on the design, some joints are high-stress, others are not. If the welded joints are in high-stress areas, they will likely not be strong enough in aluminum compared to steel. To get an idea of how significant the change is, Mild steel welds might have a yield strength over 50,000 PSI. Hard T6 Aluminum might be 30,000 PSI. Soft Aluminum welds will easily be down around 10,000 PSI.
The only way to re-harden the aluminum is to very carefully bake it at a temperature within a few degrees of its melting point for a few hours. This means you need a tempering oven big enough to hold the whole trailer - not cheap!
The solution is to add gussets and bracing to the design to reduce the stress on the welded aluminum joints. An alternative to welding like using bolted or rivited joints will maintain more of the strength of the aluminum in the critical areas.
- Rick
