tim moore, senior structural engineer for washington's department of transportation says the cause of the collapse was torsional flutter or aerodynamic instability. >> that was the vice that took the bridge down. this was a condition that no structure can maintain for a significant period of time. >> gertie is replaced by a stronger bridge in 1950, ten years after the collapse. but a part of it still survives. >> this is the side span, the approach span. this is the original galloping gertie's 1940 bridge. you can see the two eight-foot-deep section. >> many lessons are learned from gertie's demise, including how critical aerodynamics are in building suspension bridges. >> there really wasn't a major suspension bridge built until galloping gertie collapsed until