1. It is a simply supported steel beam with a span of l-4 m. The moment of inertia is l=7480cm", elastic modulus is E=2.1 x10 MPa, section coefficient W-534 cm'. Assuming the beam to be massless and has a motor at the mid-span, with weight G=35 kN and rotational speed n=500 r/min. Due to the eccentricity, the centrifugal force generated during rotation is Fp=10 kN, and its vertical component is Fpsinot. Questions: (a) Caleulate the natural circular frequency o of the beam. (b) Find the magnification factor B and the maximum normal stress omax of forced vibration of the steel beam under the above vertical simple harmonic load. F, sin Ot EI G 1/2 1/2