Harbin authorities said on Wednesday that overloading caused the collapse of a bridge that killed three and injured five on Aug 24.

Four overloaded trucks, weighing more than 480 metric tons altogether, were on the right side of a 122-meter ramp of the Qunli viaduct project, said Liu Xingming, deputy head of a team investigating the accident.

That made the ramp tilt to one side and crash to the ground, plunging all four trucks to the pavement below, Liu told a news conference.

Liu, who is also deputy director of the Harbin Administration of Work Safety, said the investigation found that the ramp's designer, contractor and supervisor all have top-level qualifications, and no problems were found during the ramp's design, construction and supervision process.

Liu noted that when construction of the ramp finished in November, the designer, contractor, supervisor and some experts conducted a trial using seven vehicles, 30 tons each, to test the quality of the ramp.

"The experiment showed that the ramp could meet all the standards of the preliminary design," he said.

Xiao Rucheng, a bridge professor at Shanghai-based Tongji University, said the designer, builder and supervisor of a bridge would usually carry out a load-bearing test that is around 1.1 times heavier than the weight the bridge is designed to hold when the project is finished.

"From the test we could tell that the ramp that collapsed in Harbin was designed to hold less than 210 tons, so the four trucks, weighing more than 480 tons, far exceeded the capability of the ramp," he said.

"It was in line with our analysis before the investigation result came out."

Three of the trucks had been illegally modified so they could haul more cargo, authorities said.

But Liu said road management authorities had noticed the overloading before some of those trucks entered Harbin.

The official said three trucks loaded with stones from Jilin province were found overloaded and fined 450 yuan ($71) when passing a tollgate on their way to Harbin in the early morning of Aug 23.

Liu said road management authorities in Jilin's Dehui city, where the toll gate was located, should bear partial responsibility for the accident because they fined the trucks for being overloaded but did not ask them to unload the extra goods.

The dereliction of duty of traffic police in Heilongjiang's Shuangcheng city and road management authorities in Harbin also indirectly caused the accident, he said.

Those violating the law will be punished, he said.

The five injured are still in the hospital, including one in serious situation, according to Ren Ruichen, vice-mayor of Harbin.

The collapsed ramp is 3.5 km from the main span of the $300-million Yangmingtan Bridge, which was in use for less than a year.

The latest bridge collapse in Harbin renewed worries over the quality of Chinese infrastructure amid a construction boom across the nation.

Some 37 bridges have collapsed across the country since 2007, killing 182 people and injuring 177, reported International Finance News, a newspaper affiliated with People's Daily.

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