Wednesday, May 14, 2008
Picture of Tacoma Narrow Bridge collapsing after 5 months from it’s traffic opening. The wind-induced collapse occurred on November 7, 1940 at 11:00 AM (Pacific time), due partially to a physical phenomenon known as mechanical resonance.
Shortly after construction finished at the end of June (opened to traffic on July 1, 1940), it was discovered that the bridge would sway and buckle dangerously in relatively mild windy conditions for the area. This resonance was transverse, meaning the bridge buckled along its length, with the roadbed alternately raised and depressed in certain locations — one half of the central span would rise while the other lowered. Drivers would see cars approaching from the other direction disappear into valleys which were dynamically appearing and disappearing. From this behavior, a local humorist coined the nickname “Galloping Gertie”. However, the mass of the bridge was considered sufficient to keep it structurally sound.
The failure of the bridge occurred when a never-before-seen twisting mode occurred, from winds at a mild 40 MPH.