It thus acquired the nickname "Galloping Gertie. Strong winds caused the bridge to collapse on November 7, Initially, 35 mile per hour winds excited the bridge's transverse vibration mode, with an amplitude of 1.
This motion lasted 3 hours. The wind then increased to 42 miles per hour. In addition, a support cable at mid-span snapped, resulting in an unbalanced loading condition. The bridge response thus changed to a 0. The torsional mode is shown in Figure The torsional mode shape was such that the bridge was effectively divided into two halves.
The two halves vibrated out-of-phase with one another. In other words, one half rotated clockwise, while the other rotated counter-clockwise. The two half spans then alternate polarities. One explanation of this is the "law of minimum energy.
Nature prefers the two half-span option since this requires less wind energy. The dividing line between the two half-spans is called the "nodal line. The bridge collapsed during the excitation of this torsional mode. Specifically, a foot length of the center span broke loose from the suspenders and fell a distance of feet into the cold waters below. The failure is shown in Figure The fundamental weakness of the Tacoma Narrows Bridge was its extreme flexibility, both vertically and in torsion.
This weakness was due to the shallowness of the stiffening girders and the narrowness of the roadway, relative to its span length. Engineers still debate the exact cause of its collapse, however. Three theories are:. An early theory was that the wind pressure simply excited the natural frequencies of the bridge. This condition is called "resonance.
The turbulent wind pressure, however, would have varied randomly with time. Thus, turbulence would seem unlikely to have driven the observed steady oscillation of the bridge. Theodore von Karman, a famous aeronautical engineer, was convinced that vortex shedding drove the bridge oscillations. A diagram of vortex shedding around a spherical body is shown in Figure Von Karman showed that blunt bodies such as bridge decks could also shed periodic vortices in their wakes.
The bridge deck's own motion produced the forces. Engineers call this "self-excited" motion. It was critical that the two types of instability, vortex shedding and torsional flutter, both occurred at relatively low wind speeds. Usually, vortex shedding occurs at relatively low wind speeds, like 25 to 35 mph, and torsional flutter at high wind speeds, like mph. Because of Gertie's design, and relatively weak resistance to torsional forces, from the vortex shedding instability the bridge went right into "torsional flutter.
Now the bridge was beyond its natural ability to "damp out" the motion. Once the twisting movements began, they controlled the vortex forces.
The torsional motion began small and built upon its own self-induced energy. In other words, Galloping Gertie's twisting induced more twisting, then greater and greater twisting. This increased beyond the bridge structure strength to resist. Failure resulted. Early suspension-bridge failures resulted from light spans with very flexible decks that were vulnerable to wind aerodynamic forces. In the late 19th century engineers moved toward very stiff and heavy suspension bridges.
John Roebling consciously designed the Brooklyn Bridge so that it would be stable against the stresses of wind. In the early 20th century, however, says David P. Just four months after Galloping Gertie failed, a professor of civil engineering at Columbia University, J.
Finch, published an article in Engineering News-Record that summarized over a century of suspension bridge failures. The last major suspension-bridge failure had happened five decades earlier, when the Niagara-Clifton Bridge fell in And, in the s, aerodynamic forces were not well understood at all. The remains of the original Tacoma Narrows Bridge deck are still on the bottom of Puget Sound, forming an artificial reef, and its side spans were melted down for steel during World War II.
About 45 minutes before failure, a different kind of oscillation started. The reason for this change in oscillation is still debated, but one of the best suggestions has has to do with the aerodynamics of the bridge. Rather than a truss through which wind can flow, this shape of the Tacoma Narrows Bridge with the large steel plates on either side created some strange interactions with the wind.
Any amount of twist in the bridge created vortices, or areas of low pressure, in locations that actually amplify the twisting motion. As the bridge returned to its natural state, its momentum twisted it in the other direction where the wind could catch it and continue the twisting. This phenomenon is called aeroelastic flutter. This torsional flutter eventually created too much stress in the suspension cables, and the bridge failed.
One way that modern bridges avoid flutter is to include a gap in the center of the deck so that the pressures on either side can equalize. I cut a slot in my model, and sure enough the vibrations almost completely stopped.
Another option is just to make the bridge deck more aerodynamic to avoid creating vortices that push and pull on the structure. Take a look at the very first Practical Engineering video about Tuned Mass Dampers to learn about how wind-induced motion can be mitigated in skyscrapers. Cookie information is stored in your browser and performs functions such as recognising you when you return to our website and helping our team to understand which sections of the website you find most interesting and useful.
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