Why tacoma bridge failed

Pasternak, Alex. Photo: Washington Department of Transportation. The collapse of the Tacoma Narrows Bridge was driven by wind-generated vortices that reinforced the twisting motion of the bridge deck until it failed.

Librarians Authors Referees Media Students. Login Become a Member Contact Us. November 7, Collapse of the Tacoma Narrows Bridge. Here is a summary of the key points in the explanation. In general, the Narrows Bridge had relatively little resistance to torsional twisting forces.

That was because it had such a large depth-to-width ratio, 1 to Gertie's long, narrow, and shallow stiffening girder made the structure extremely flexible. On the morning of November 7, shortly after 10 a. The cable band at mid-span on the north cable slipped [and slid along the bridge]. This allowed the cable to separate into two unequal segments.

That contributed to the change from vertical up-and-down to torsional twisting movement of the bridge deck. Also contributing to the torsional motion of the bridge deck was "vortex shedding.

A small amount twisting occurred in the bridge deck, because even steel is elastic and changes form under high stress. The twisting bridge deck caused the wind flow separation to increase. This formed a vortex, or swirling wind force, which further lifted and twisted the deck. The deck structure resisted this lifting and twisting. It had a natural tendency to return to its previous position.

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Prof F. B Farquharson of the University of Washington was responsible for conducting experiments to understand the oscillations. On this day, the professor and his team recorded the movement of the bridge on camera, and we can find this today on YouTube. A three-dimensional scaled model of scale was built for wind tunnel experiments and to explicitly understand the reason for failure.

The experiments brought about a new theory: wind-induced oscillations. The image of the Tacoma Narrows Bridge collapse is shown in Fig. The shape of the bridge was aerodynamically unstable along the transverse direction. The vertical girders of the H-shape allowed flow separation, thus leading to vortex generation that matched the phase of oscillation. These vortices generated enough energy to push the girders out of their position.

The problem that caused the Tacoma Narrows Bridge collapse was not a new problem, but one which had been unspecified. Due to wind action, increased stiffness can be seen through various design methods such as adding a greater dead load, adopting dampers, stiffening trusses or by guy cables.

However, these factors were not originally considered and only became part of the later forensics. The Tacoma Narrows Bridge collapsed primarily due to the aeroelastic flutter. In ordinary bridge design, the wind is allowed to pass through the structure by incorporating trusses.

In contrast, in the case of the Tacoma Narrows Bridge, it was forced to move above and below the structure, leading to flow separation. Example: For a Reynolds number greater than , S is 0. In the case of the Tacoma Bridge, D was 8 ft. After the Tacoma Narrows Bridge collapse, the new bridge was redesigned based on lessons learned and rebuilt in Fig.

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. A problem with this theory is that the natural vortex shedding frequency was calculated to be 1 Hz, as shown in Section This frequency is also called the "Strouhal frequency.

This frequency was observed by Professor F. Farquharson, who witnessed the collapse of the bridge. The calculated vortex shedding frequency was five times higher than the torsional frequency. It was thus too high to have excited the torsional mode frequency. In addition to "von Karman" vortex shedding, a flutter-like pattern of vortices may have formed at a frequency coincident with the torsional oscillation mode.

Whether these flutter vortices were a cause or an effect of the twisting motion is unclear. Aerodynamic instability is a self-excited vibration. In this case, the alternating force that sustains the motion is created or controlled by the motion itself. The alternating force disappears when the motion disappears. This phenomenon is also modeled as free vibration with negative damping. Airfoil flutter and transmission line galloping are related examples of this instability.

Further explanations of instability are given in References [48], [49] and [50]. The following scenario shows how aerodynamic instability may have caused the Tacoma Narrows Bridge to fail.

For simplicity, consider the motion of only one span half. Assume that the wind direction was not perfectly horizontal, perhaps striking the bridge span from below, as shown in Figure Thus, the bridge is initially at an angle-of-attack with respect to the wind. Aerodynamic lift is generated because the pressure below the span is greater than the pressure above.