The Tacoma Narrows Bridge collapsed due to high winds on November 7, 1940.
The Tacoma Narrows Bridge was built in Washington in the 1930s and opened to traffic on July 1, 1940. It spanned Puget Sound from Gig Harbor to Tacoma, which is 40 miles south of Seattle. The canal is about a mile wide where the bridge crossed the sound. Sleek and slender, it was the third longest suspension bridge in the world at the time, spanning 5,959 feet.
Leon Moisseiff designed the bridge to be the most flexible ever built. Engineers at the time believed the design, even though it exceeded the ratios of length, depth and width that were previously standard, was completely safe. In the aftermath of the collapse, it was revealed that engineers did not properly take into account the aerodynamic forces that were at play at the location during a period of strong winds. At the time of construction, these forces were generally not taken into account by engineers and designers.
On November 7, strong winds shook the area and the bridge swayed considerably. The first failure occurred around 11:00 a.m., when concrete fell from the roadway. A few minutes later, a 600 foot section of the bridge became free. By this time the bridge was rocking back and forth. At one point, the sidewalk elevation on one side of the bridge was 28 feet above that of the sidewalk on the other side. Even though the towers of the bridge were made of strong structural carbon steel, the bridge did not cope with the violent movement and collapsed.
Further investigations and tests revealed that the bridge was vulnerable to vibrations generated by the wind. When the bridge experienced strong winds from a certain direction, the frequency oscillations developed so much that collapse was inevitable.
A replacement bridge opened on October 14, 1950, after more than two years of construction. It is the fifth longest suspension bridge in the United States, 40 feet longer than the original. The construction of the new bridge took into account the lessons learned from the Tacoma Narrows Bridge collapse, as have all subsequent suspension bridges.
Today, the remains of the bridge can still be found at the foot of Puget Sound, where they form one of the largest man-made reefs in the world. The place has been listed on the National Register of Historic Places to protect it from waste pickers.
