A sea bridge in South China's Guangdong Province was temporarily closed on Tuesday due to abnormal shaking which local authorities attributed to strong winds. The bridge swaying back and forth made some netizens nervous, saying it was the first time they had seen anything like that.

Several online videos showed that the Humen Bridge connecting Guangzhou, capital city of Guangdong and the city of Dongguan, a manufacturing hub in southern China, was shaking dangerously. Some experts were quoted in media reports as saying that the bridge was hitting its resonance frequency. When the bridge experienced abnormal vibrations, footage showed there were many cars on it, causing a potentially dangerous situation.

Local police said the shaking was due to strong winds and they imposed traffic control around 3pm on Tuesday.

When bridges are being designed, engineers should consider ways to avoid the impact of wind or other outside forces on the stability of the bridge, including a scenario in which the bridge wobbles violently in high winds, said a Beijing-based structural engineer who declined to give his name.

According to the engineer, the sudden wobbling could have been triggered by barricades, which changed the force of the wind on the bridge. "But this assumption can only be confirmed if the bridge remains stable when barricades are dismantled. If not, the builders and engineers should reconsider force structures of the bridge to avoid potential safety hazards."

Also, some experts said when the wind speed reaches about 8 m/s, it causes limited vortex vibration of the bridge. Humen Bridge is now under repair. The bridge deck added a 1.2 meters high retaining wall (water horse), thus destroying the section streamline caused by vortex vibration. At present, the retaining wall is being removed, according to media reports.

Guangdong Transportation Group said it has organized maintenance technicians to inspect the bridge and experts to study it. Preliminary indications are that the main structure of the bridge is not damaged, and possible causes of the swaying are under investigation, reports said.

Though some netizens left messages such as "it's horrifying" after they saw the post of this unusual scene, some came up with jokes, for example, suggesting the bridge "feels excited" as Wednesday (May 6) would be the first day for China to resume charging highway tolls since the outbreak of COVID-19 as the epidemic is now gradually being brought under control in China.