$13.)$ A tractor weighing $3,000$ lb$,$ with a wheel base of $9$ft$.,$ carries $1,800$ lb$.$ of its load on the rear wheels. Compute for the maximum moment and the maximum shear when crossing a $14$ ft$-$span.Solve also the flexural stress in $($Pascal$),$ and horizontal shearing stress $($psi$),$ the cross$-$sectional area is $200$ mm x $500$ mm$.$