May 23, 1952:  A 96-foot, 200-ton span of 17 girders collapsed into Sullivan Square - two minutes after two M.T.A. buses loaded with passengers had passed under the 40-foot high structure. The overpass was being designed to carry traffic from Mystic Ave and Broadway, Somerville over Sullivan Sq. to Rutherford Ave. in Charlestown, and connect with the Boston Central Artery and Mystic River Bridge.