At the entrance to London's Albert Bridge, there is a sign that reads: "All troops must break step when marching over this bridge." This legend recalls the structure's tendency, opened in 1873, to vibrate when large numbers of people walked across it. This peculiarity led to its early nickname, the "Tembling Lady." The vertical swaying caused by troops marching across a bridge's deck was already a well-known fact. In 1831, the seventy-four men of a company of the British Army Rifle Corps—returning from military maneuvers—noticed a slight swaying as they walked across Broughton Bridge in Manchester. Within seconds, the vibrations increased in intensity, and a few minutes later, one of the four pillars supporting the chain that supported the weight of the bridge gave way, and the structure collapsed. Physics has taught us that an action with a frequency similar to that of the structure can be capable of collapsing it, even though the value of the action is much smaller compared to the action for which it was designed. We know this phenomenon as resonance. The fluttering was to blame. The same fate befell the Angers Bridge on French soil. The incident occurred in 1850 while a battalion of French soldiers were crossing it, despite having broken the passage and increased the distance between them. In this case, it was the action of the wind that caused a strong oscillation of the deck, preventing the soldiers from maintaining verticality. To avoid falling into the river, the Gauls continually moved from the lowest to the highest part of the deck, which, being heavier, descended abruptly, forcing the soldiers to run back to the opposite side. The only thing these swings achieved was to increase the resonance of the deck. The collapse of the bridge caused the death of more than two hundred people. The cause of the collapse was a phenomenon known as wing flapping, a self-induced vibration that occurs when a supporting surface flexes under aerodynamic load. Wing flapping is perhaps best understood by recalling a sensation we've all experienced while driving. If we stick our left hand out the window while the car is moving and place it parallel to the ground, we'll cut through the air. But if we rotate our wrist a few degrees, raising the thumb area, we feel an invisible force pulling our arm upward. If we then rotate our wrist in the opposite direction, the effect is the opposite: our hand feels pulled down toward the ground. This phenomenon is precisely wing flapping. Synchronous Lateral Excitation In 2000, the Millennium Bridge, a steel suspension bridge for pedestrian use that connects Bankside with the City of London, was opened. An unexpected and excessive lateral vibration due to a resonant structural response caused its closure two days after its opening. After extensive diagnostic testing, experts concluded that it had suffered a "synchronous lateral excitation." In plain English, the deck had swayed slightly as pedestrians passed by, but the oscillations increased when the intimidated pedestrians adjusted their stride to the bridge's movements. This "corrective" maneuver not only fed back the effect, but intensified it. The problem was solved after installing a series of additional shock absorbers on the structure. The collapse of the Tacoma Narrows Bridge. Under normal conditions, when the load is reduced, the deflection is subdued, and the structure's original shape is restored. In the most harmless cases, there may be a slight rumbling in the structure, but in the most violent cases, it can suffer irreparable damage. One of the most well-known extreme cases was that of the Tacoma Narrows Bridge. In the summer of 1940, the Tacoma Narrows Bridge in Washington State opened to traffic, becoming the third-longest suspension bridge in the world, behind the Golden Gate and George Washington Bridges. From the very beginning, the deck moved vertically in windy conditions, prompting workers to quickly nickname it "Galloping Gertie." Despite several cushioning measures, the bridge collapsed the same year it opened after being hit by winds of up to 40 mph (64 km/h). At that speed, the bridge stopped oscillating vertically and began to twist sideways, like a drying towel. MORE INFORMATION news No NASA confirms the success of the DART mission: the spacecraft managed to deflect the asteroid Dimorphos from its orbit news No A 'strange pair' of stars detected, the closest to each other seen so far A swarm of curious onlookers approached to contemplate the spectacle, among them Barney Elliott, a local photographer who recorded the sequence of movements on video. In the film, which can be seen today on the internet, oscillations of an amplitude close to seven and a half meters can be observed. ABOUT THE AUTHOR Pedro Gargantilla Pedro Gargantilla is an internal medicine physician at El Escorial Hospital (Madrid) and the author of several popular books.
