Example $11\mathrm{.}4$

For the built$-$up section shown in figure $11\mathrm{.}9,$ calculate $($a$)$ the force per metre length in the weld if the two sections are welded together; $($b$)$ the pitch of the rivets if the rivets are $10$ mm in diameter and have an allowable shear stress of $81$ MPa $.$ For this case assume that the two sections are riveted together with a double row of rivets as indicated in figure $11\mathrm{.}9.$ Let the shear force at this section be $75$ kN $.$