By G9ija

The main cause of mysterious shaking in the SEG Plaza two months ago was a vortex-induced resonance caused by the two long masts on top of the building, say Shenzhen’s official investigators.

They said that while the building was safe overall, necessary rectification work – including removing the long antennas and repairing “damage accumulated” over the past two decades – was expected to take over a month, according to an official announcement issued on Thursday morning.

The investigators’ verdict came nearly two months after the 350-metre (1,150 feet) tall landmark skyscraper began swaying on the afternoon of May 18, prompting authorities to evacuate thousands of occupants and shoppers at the plaza. The building was shut for safety inspections after more tremors were reported in the following days.

After two months of tests and experiments, investigators have now ruled out factors such as the subway running below the plaza in the city’s famous Huaqiangbei electronics district, vibration caused by air-conditioning units and nearby construction projects, the announcement said.

“The perceived vibration of the SEG Plaza building is caused by a combination of vortex-induced resonance of the rooftop masts, and changes to the dynamic characteristics of the building,” it said.

It said investigators conducted 63 vibration tests which showed that when the masts vibrated at a frequency of 2.12 Hz, “bending and torsion vibration” of the building occurred.

The expert team confirmed that the building had suffered “accumulative damage” over the past two decades, especially on floors connected to the rooftop masts. However, the experts stressed that such damage did not affect the building’s overall safety.

Based on the conclusion, the officials proposed a two-pronged rectification plan: including removing the rooftop masts; and repairing damaged areas.