 Although the rolling lift bridge (a type of bascule bridge) was mechanically the most simple alternative, engineers soon discovered that the span applied its load in a rolling motion on its foundation, crumbling poorly built foundations.Ericson directed his staff to investigate other bridges in the U.S. and Europe, eventually finding that the Tower Bridge (1894) would be an appropriate starting point.With a model bridge that was patent-free, had a fixed center of gravity and simple mechanical motion, in 1900 Ericson prompted the Chicago City Council to allocate funding to replace five deteriorating swing-span bridges.