Q1 A single storey, single bay portal frame shown in Fig. 1Q1 having a mass m, a stiffness k and a damping coefficient factor & is given. By assuming that the mass is lumped on the centre of gravity of the rafter and that the columns and the beams are axially rigid, set-up the equation of motion in the case of a support excitation having a ground acceleration of ? (?) . Give a brief explanation. [30 marks] g