China on Wednesday completed the main structure of the world's longest cross-sea road-rail bridge in its southeastern province of Fujian.

The last steel truss girder, weighing 473 tonnes, was bolted on the Pingtan Strait Road-rail Bridge, another mega project in China, on Wednesday morning.

Hundreds of bridge builders clad in orange overalls, as well as government officials, hailed the completion on the bridge deck, with several rounds of fireworks being set off to celebrate the moment.

With a staggering span of 16.34 km, the bridge connects Pingtan Island and four nearby islets to the mainland of Fujian province.

The bridge, which is expected to open to traffic next year, can help shorten travel time from two hours to half an hour between Fuzhou, capital city of Fujian province and Pingtan, a pilot zone set up to facilitate trade and cultural exchanges across the Taiwan Strait.

"Of all the bridges being built across the world, this is no doubt the most challenging," said Wang Donghui, chief engineer of the project, adding that it is China's first and the world's longest cross-sea road-rail bridge.

The project has attracted worldwide attention from the start of construction in 2013 as it spans an area off the coast of southeast China long seen as a "no-go zone" for bridge-building.

The region has strong gales and high waves for most of the year and is known as one of the world's three most perilous seas along with Bermuda and the Cape of Good Hope.

Workers had to battle the notoriously strong winds, choppy waters and rugged seabed in the region to drill 1,895 piles into the ocean.

More than mega project

The road-rail bridge has a six-lane highway on the top and a high-speed railway at the bottom, which is designed to support bullet trains traveling as fast as 200 km per hour. It is a part of the 88-km Fuzhou-Pingtan railway.

In the past, Pingtan was a backwater island of humble fisheries. It did not even have a bridge connecting it to the mainland until 2010 when the Strait Bridge began operating for cars only.

In 2010, China established the Pingtan Comprehensive Pilot Zone to facilitate cross-Strait exchange and cooperation, ramping up its efforts to improve the island's infrastructure.