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The bridge was solidly built, with girders of carbon steel anchored in huge blocks of concrete. It had a span of 5,939 feet. Preceding designs typically had open lattice beam trusses underneath the roadbed. This bridge was the first of its type to employ plate girders (pairs of deep I beams) to support the roadbed. With the earlier designs any wind would simply pass through the truss, but in the new design the wind would be diverted above and below the structure. Shortly after its construction in July 1940 (opened to traffic on July 1), it was discovered that the bridge would sway and buckle dangerously in windy conditions. This resonance was longitudinal, meaning the bridge buckled along its length, with the roadbed alternately raised and depressed in certain locations — one half of the central span would raise 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 the bridge gained the nickname "Galloping Gertie."

The failure of the bridge occurred when a never before seen twisting mode occurred. This is called a torsional rather than longitudinal mode (also see torque). In fact, it was the second torsional mode, in which the centre of the bridge remained motionless and the two halves of the bridge twisted in opposite directions. This vibration was due to aeroelasticAerospace engineering No aircraft structure is completely rigid, so when it is subjected to aerodynamic forces it will normally deflect by a small amount. This effect can become very important at high speeds because any change in the shape of the body can flutter. Flutter occurs when a torsional disturbance in the structure increases the angle of attackIn the diagram above, the black arrow represents the direction of the wind. The wing is shown end on. The angle α is the angle of attack. Angle of attack is a term used in aerodynamics to describe the angle between the wing's chord and the direction of the bridge (that is, the angle between the wind and the bridge). The structure responds by twisting further. Eventually, the angle of attack increases to the point of stallA stall is a (usually undesired) condition in aerodynamics and aviation. Normally, increasing the angle of attack between a wing and the airflow causes the lift produced to increase. This can continue until a point is reached where maximum lift is generat, and the bridge begins to twist in the opposite direction. In the case of the Tacoma Narrows bridge, this mode was negatively damped (or had positive feedbackPositive feedback is a type of feedback. Open systems (ecological, biological, social) contain many types of regulatory systems, among which are systems that involve positive feedback and its relative negative feedback. When a change of variable occurs in), meaning it increased in amplitude with each cycle. Eventually, the amplitude of the motion increased beyond the tolerance of the structure and the structure failed by the failure of the suspender cables. Once several cables failed, the weight of the deck transfered to the adjacent cables caused almost all of the central deck to fall into the water.

The bridge's spectacular self-destruction is often used as an object lesson in the necessity to consider both aerodynamicsFluid dynamics Aerodynamics is a branch of fluid dynamics concerned with the study of gas flows. The solution of an aerodynamic problem normally involves calculating for various properties of the flow, such as velocity, pressure, density, and temperature, and resonanceThis article is about resonance in physics. For other uses, see Resonance (disambiguation). In physics, resonance is an increase in the oscillatory energy absorbed by a system when the frequency of the oscillations matches the system's natural frequency o effects in structuralStructural engineering is the field of civil engineering particularly concerned with the design of load-bearing structures. In practice, it is largely the implementation of mechanics to the design of structures, such as buildings, bridges, walls (includin and civil engineeringIn modern usage, civil engineering is a broad field of engineering that deals with the planning, construction, and maintenance of fixed structures as they related to earth, water, or civilization and their processes. Most civil engineering today deals wit. However, it is a poor example of resonance since resonance normally implies forced resonance (as from the periodic motion induced by a group of soldiers marching in step across a bridge). However, in the case of the Tacoma Narrows Bridge, there was no periodic disturbance. The wind was steady at 42 mph (67 km/h). The frequency of the destructive mode, 0.2 Hz, was neither a natural mode of the isolated structure nor the frequency of blunt-body vortex shedding of the bridge at that wind speed. The event can only be understood while considering the coupled structural and aerodynamic system.

The collapse occurred on November 7, 1940. From the account of a driver stranded on the bridge during this event:

Just as I drove past the towers, the bridge began to sway violently from side to side. Before I realized it, the tilt became so violent that I lost control of the car... I jammed on the brakes and got out, only to be thrown onto my face against the curb... Around me I could hear concrete cracking... The car itself began to slide from side to side of the roadway.

On hands and knees most of the time, I crawled 500 yards or more to the towers... My breath was coming in gasps; my knees were raw and bleeding, my hands bruised and swollen from gripping the concrete curb... Toward the last, I risked rising to my feet and running a few yards at a time... Safely back at the toll plaza, I saw the bridge in its final collapse and saw my car plunge into the Narrows.

The final destruction of the bridge was recorded on film. The Tacoma Narrows Bridge Collapse (1940) is preserved in the US National Film Registry, and is still shown to engineering, architecture, and physics students as a cautionary tale. (1)

No human life was lost in the collapse of the bridge. However, Tubby, a cocker spaniel dog, was left in one of the cars on the bridge. Two people attempted to rescue Tubby, but the dog was too terrified to leave the car and bit one of the rescuers. Tubby died when the bridge fell, and neither his body nor the car were ever recovered [1].

The bridge was redesigned and rebuilt (using the original towers) with open trusses and stiffening struts and openings in the roadway to let wind through. The new bridge opened on October 14, 1950, and is 5,979 feet long--40 feet longer than its predecessor and the 5th longest suspension bridge in the United States.

Modern suspension bridges built with steel plate now use sharp entry edges rather than the flat plate sides used in the original Tacoma Narrows Bridge (see the bridge article section for an example).

The bridge was designed to handle 60,000 vehicles a day. It currently handles 90,000, and is projected to handle 120,000 by 2020. In 1998, voters in several Washington counties approved an advisory measure to create a second Narrows span. Construction of the new span, which will run parallel to the current bridge, began on October 4, 2002, and is scheduled to be completed in 2007.

An image of the original 1940 Narrows bridge is used in the logo of Kuro5hin. [2]