Architecture: The Collapse of the Tacoma Narrows Bridge

by Habina Seo

Many structures and buildings, especially bridges, have failed due to the effects of resonance. Undoubtedly one of the most famous examples is the Tacoma Narrows Bridge which collapsed in 1940, just four months after it was originally built. Not long after the construction of the bridge, it was found to dangerously sway and undulate along its length in normal windy conditions. On the day of the incident, it experienced winds of about 70kmph, and underwent stresses which caused a cable to fail, leading onto torsional motion which further allowed cracks to develop in the bridge. Its design, a suspension bridge, was a useful trend which allowed it to be efficient in spanning long distances with less material (and therefore a lower cost), resulting in the slim, slender appearance. However, this narrower form reduces the rigidity and stiffness compared to older designs, allowing unexpected factors like wind to have a major impact on the bridge.

After investigation of the collapse, it was found that the failure was predominantly due to aeroelastic flutter (it has been presented as a classical example of resonance, but that description alone can be misleading). Resonance occurs when the periodic force of high-speed winds has the same frequency as the natural frequency of the structure, causing the waves to add up (superpose) and create a rapidly increasing amplitude until the system fails and loses structural integrity. It is also dependent on the mass, where the resonance decreases as a structure gets heavier. Being relatively lightweight, the bridge was more prone to this resonance than heavier ones. However, in videos of the bridge curving, it is clear that there is something else which results in the twisting, side-to-side motion of the bridge (instead of just up and down motion). 

This is best explained by aeroelastic flutter. Although it can be used as a method of making use of energy from naturally occurring flow fields (such as wind and tidal currents), it has been a detrimental problem in engineering and has caused the collapse of many bridges and damage to buildings. The shape of the bridge, which was made of two large plates instead of a truss in which wind can flow through, created strange interactions with the wind coming from its sides. Any small amount of twist in the bridge created areas of slow pressure which amplified the twisting motion, and this cycle would continue due to the conservation of momentum by the time it returns to its original state, resulting in a continuous and increasingly gestural pattern. This flutter, intensifying the existing vibrations, eventually created too much stress in the suspension cables, leading to the collapse.

So how can these disasters be prevented? The effects of resonance can be reduced by installing absorbers and dampers, which reduces the amplitude of the waves by moving in the opposite direction of the resonance oscillations, by means of springs, fluids or pendulums. For instance, the Millennium Bridge in London was seen to be wobbling not long after its opening in 2000 due to the oscillations generated by people’s steps exaggerating existing motion. After it was shut down later that year, 37 fluid-viscous energy-dissipating dampers were installed to alleviate horizontal movement, and 52 mass dampers to limit vertical movement. In the case of the Tacoma Narrows Bridge, the newly designed bridge proposed stiffening struts and open trusses which allowed the wind to flow freely through, which reduced the twisting considerably. Now there are two bridges- Eastbound and Westbound- which carry an average of 90,000 vehicles a day.

The failure of this Bridge revealed the limitations of the ‘deflection theory’, which explained how deck and cables deflect together under gravity loads, so that as spans become longer and the structure becomes heavier, the stiffness required of the deck decreases. This theory influenced design in the 1930s as engineers attempted to design more graceful, lighter looking structures without compromising safety. However, after the collapse, it abruptly ended this trend in the desire for more flexible, light and slender suspension spans, as well as forcing the reconsideration of an entire generation of bridge engineering theory and practice. Nevertheless, it has become a valuable lesson and example to learn from for future designers and engineers, not just limited to bridge design but also linking to the effects of resonance in buildings, especially as design trends evolve to meet changing environmental, economic and aesthetic demands.