The closed thin wall beam with the cross section shown below $($all dimensions are median distances in mm $)$ and a wall thickness of $3$ mm is subjected to the vertical force shown. If the webs are effective in bending as well as in shear, determine:

a$.$ The shear flow around the section

$(10$ marks$)$

b$.$ The bending stresses at the $4$ corners of a section on the beam located $300$ mm behind the one shown $(10$ marks$)$

$07-$Mec$-$B$9/$May $2013$: Page $3$ of $3$