alum tube stress cal-with 300 Lbs box

[cpsBureau]

I have a 2.0" X 2.0 X
.125" Alum tube top frame member,attached to a existing upper square
frame,
supporting a 300 Lbs load box. What is the shear stress load on the
single alum 2x2x.125" tube member. Only a rough estimate only, I just
wanted to see the
approx results of this member.
Tube top would be the 2x2x.125" square Alum, that the 300 Lbs box is
resting on. I drew this pic to give sample;
I realize this is only a partial picture, What formula would i use to
calculate this? Thank you very Much

 

[chicopee]

To calculate the shear stress depends on how your rectangular member is supported and how it is fastened to the square frame. It appears to be a classical case of strength of material problem. Since I dont have your picture I would suggest that you get a strength of material book to check out the beam diagrams and select the one that most resemble your set up, however, you may have to flip these diagrams upside down to match your situation.
These diagrams have two graphs, one graph is for shear(V) loads and the other graph for moment(M) loads. From the shear load graph select either the maximum or minimum shear load value which is to be divided by the cross sectional area of the tube for an average shear stress value.