The collapse of an overpass in Wuxi, East China's Jiangsu province, which killed three and injured two, is under investigation with an overload of four trucks considered the main cause.

Photos and videos from the site show that the overpass overturned on one side and collapsed.

Zhu Lei, a professor in civil engineering at the Beijing University of Civil Engineering and Architecture, said it was a typical accident that happens mostly to single-pier bridges, which have only one pier located on their central lines.

That kind of design used to be popular because it saves space underneath, but now there are more double-pier bridges with two piers on two sides.

Zhu said that single-pier bridges are safe as long as they meet standards. However when overloaded trucks frequently drive on these bridges, one side of the pier may be damaged, which in the long run can cause an overturn.

In extreme cases, trucks can outweigh one side of the bridge and cause it to overturn.

Bridge bodies range in weight from hundreds to thousands of tons, and if one single truck carries 400 tons of goods, some bridges will be at risk of overturning.

Zhu's comments were echoed by the research team of Lu Xinzheng, a professor on civil engineering at Tsinghua University.