Two steel plates $($E $=200$ GPa$),$ each with a thickness of $13$ mm and width $26$ mm$,$ are connected using a lap joint. The plates are separated by a $0\mathrm{.}1$ mm thick gasket. The joint is held together using two M$16(16$ mm nominal diameter$)$ bolts made of grade $4\mathrm{.}8$ steel $($yield strength of $420$ MPa$).$ Each bolt is tightened to a preload of $30$ kN$.$ How much external load can be applied before the joint starts to separate, i$.$e$.$ clamping load is zero?