$($b$)$ The $4\mathrm{.}0$ m high wall show below is constructed from clay bricks having a compressive strength of $20Nm{m}^{-2}$ laid in a $1$:$1$:$6$ mortar. Calculate the design load resistance of the wall assuming the partial safety factor for materials is $3\mathrm{.}5$ and the resistance to lateral loading is simple.

Fig. Q$1$