The Humen Pearl River Bridge spans Humen town and Panyu district in Guangdong province, China. On May 5, 2020, the suspension bridge that was completed in 1997, oscillated in an abnormal manner raising fears regarding its structural integrity. The shaking was caught on camera and the footage is impressive. Click on the video below to watch it.
Structural systems, including bridges, are designed to sustain some degree of oscillation when dynamic forces (e.g. earthquake loads, strong winds or waves) are applied. However, there are certain limitations regarding the amplitude of the tolerant shaking. Those limits are mostly established considering the functionalism of a structure and the response of its users.
Nevertheless, in extreme conditions, some bridges are designed to oscillate violently instead of being highly damaged, a fact that would probably lead to a total collapse.
According to experts, the Humen Pearl River Bridge may experience limited oscillations when wind velocity is about 8 m/s. But that was not the case in the current oscillation pattern.
Some experts stated that the structure was shaking violently as it was oscillating at its natural frequency. All structures have a natural frequency which is basically the number of swings they may naturally experience per second. If the external dynamic force applied has a frequency same to the natural frequency of a structure, it will experience intense oscillations.
The bridge was still occupied by the time the strong winds struck and it was later closed. Fortunately, there were no casualties.
The structure is currently being inspected and monitored while the causes of the extreme oscillations are under investigation.
Authorities also stated that the structure is currently under maintenance while a wall that will halt the winds' course is being constructed. When winds pass over the span, an air vacuum is created and the surface of the bridge is prone to up and down movements. While this is not noticeable on a regular day, the phenomenon can be devastating when strong winds strike. “It is reported that the Humen Bridge is under maintenance, and a 1.2-meter-high retaining wall has been added to the bridge deck, destroying the streamline of the section and causing vortex vibration," a release from China’s state media, mentioned.
According to recent updates issued by engineers, the incident has not affected the structural integrity of the bridge and the aerodynamic modifications applied will resolve the issue.