Compute $(1)$ the minimum pull per pitch length which will rupture the joint and $(2)$ the efficiency of a double riveted double covered butt joint of plates having thickness of $20$ mm with $100$ mm pitch. The rivets are of $24$ mm in diameter. The permissible are tensile stress $=120$N $/$ m $*$ m$\_\{5\}^2$ crushing stress $=150$N $/$ m $*$ m $^2$ and shearing stress $=100$N $/$ m $*$ m $^2.$