$6\mathrm{.}57$ A composite beam is made by attaching the timber and steel por$-$

tions shown with bolts of $16-mm$ diameter spaced longitudinally every $200$

mm $.$ The modulus of elasticity is $13$ GPa for the wood and $200$ GPa for the

steel. For a vertical shear of $16$ kN $,$ determine $($a$)$ the average shearing stress

in the bolts, $($b$)$ the shearing stress at the center of the cross section. $($Hint: Use

the method indicated in Prob. $6\mathrm{.}55.)$