This extra beam is -like the structure itself- not made with mechanical expertise. There are several reasons why the beam will not change the stiffness as much as a similar amount of material used in a more ideal way.

Yeh agree with Oekmont on all points. 

Hadnt really considered bolting anything - I am used to working with roll cages in race cars where bolt-in is far inferior to weld-in. It would be here too, but it wouldn't be so important. Properly designed plates (as Lester described) bolted on would work well.

+x

 

Lester Populaire - 2/17/2021 8:18:49 AM

the plates on the sides was exactly what i was referring to with the shear webs, which would be far superior to the proposed tube that is bolted on.

Thank you, I can't imagine really what you mean.
If you have any spare time, would you mind drawing on my picture your suggested idea?

If you add plates to the marked plane on both sides of the structure, and bolt them every few inches the stiffness should improve quite much. if you only find shorter plates you might just cover the middle area around the thinnest section.