A thin$-$walled, closed, two$-$celled cross$-$section with one rectangular and one

triangular cell is shown in Figure $3.$ The lengths of the midlines of the walls and the thicknesses of

the walls are shown on the figure. The cross section is subjected to an overall counterclockwise

torque T $=20$ kN$-$m $($not shown on the figure$).$ The shear modulus of the entire cross section is a

constant G $=25$ GPa.

a$)$ Find the shear flows everywhere $($i$.$e$.$ on each wall$)$ on the cross$-$section. Draw a clear

picture of the cross section to describe where and in what direction the shear flows act.

b$)$ Calculate the corresponding shear stresses everywhere on the cross section, as well.

c$)$ Find the angle of twist$/$length $,$ and find the torsional rigidity T$.$R$.$ of the cross section. Figure $3$