Problem $5$:

The dimensions of the double$-$box beam cross section shown at right are $\backslash ($ b$=200\backslash$mathrm$\{$~mm$\},$ d$=50\backslash$mathrm$\{$~mm$\}\backslash ),$ and $\backslash ($ t$=5\backslash$mathrm$\{$~mm$\}\backslash ).($Note that t is the thickness of all of the walls of the double box, not just of the one labeled in the diagram.$)$

a$.$ If the maximum allowable bending stress is $17$ MPa $,$ determine the maximum internal bending moment, $\backslash ($ M$\_\{$z$\}\backslash ),$ magnitude that can be applied to the beam.

b$.$ Assume that the maximum allowable bending moment is applied and the $\backslash ($ x $\backslash )-$axis points out of the page, toward the viewer. At the point where the maximum bending stress acts, draw a stress element in the x$-$y plane. Include coordinate axes and label the stresses, showing proper direction and magnitude. Also show the location of the stress element by drawing a dot on the diagram at right $($or your own$).$

The answer should be $748$Nm