For the undamped structural frame shown in Figure Q1a, determine the equation of motion for free oscillations in the horizontal direction. Assume that the horizontal member is infinitely rigid with respect to columns and that the initial conditions are non-zero. Design data: Total mass: 13600 kg E=210 GPa I = 3.45 x 107 mm4 I2 = 7.08 x 107 mm 4.6m m=13600kg I I 6.1m Figure Q1a Determine the undamped natural frequency of vibration [in Hz] for the equation of motion developed under (a)