Built around 605 in China's Sui Dynasty (581-618), Zhaozhou Bridge (also known as Anji Bridge and Great Stone Bridge) is the oldest existing bridge in China. It stretches across the Xiao River, 2500 meters from the southern gate of Zhao County, Hebei province. Li Chun, a Sui Dynasty architect, presided over its construction.
In the Sui Dynasty, Zhaozhou, or the Zhao prefecture, connected north and south as a major transportation hub. With the Xiao River breaking the south-north traffic, the construction of a bridge was sorely needed. Given that the river dried up in spring and winter but flooded in summer and autumn, the bridge had to be designed to withstand severe hydrologic conditions.
The single arch stone bridge is 50.82 meters in length and 37.37 meters in span. The stone arches of Chinese bridges, such as the Marco Polo Bridge (Lugou qian), are generally semi-circular (called semi-circular arches), but the arch of the Zhaozhou Bridge is segmented to make the slope of the bridge floor gentle and to facilitate the passage of vehicles and horses (if the arch were semi-circular, the road surface would be stepped, making it very difficult for vehicles to use the bridge).
The Zhaozhou Bridge bears two pairs of small arches at each side of the central arch. The larger pairs of arches are 3.81 in span and the smaller are 2.85. All of the arches are circular and built of stone. Zhaozhou Bridge is the world's earliest open-spandrel arched bridge, 700 years earlier than its European counterparts.
The single-span open-spandrel arch, compared with a filled spandrel arch, minimized water blockage and increased the flow area by 16.5%, especially when the Xiao River was flooded. Besides, as the bridge was single span, the weight of the whole bridge and its traffic load was shouldered by two arch feet, causing great pressure on the abutments. The four smaller arches in the spandrels help to transfer the load on the large arches by more than 500 tons (equivalent to one fifth of the bridge's weight) and save more than 200 cubic meters of stone.
Since its inauguration in the Sui Dynasty, the bridge has been repaired several times due to wars, earthquakes and other natural and man-made events. The earliest renovation occurred about 200 years after the bridge was completed. But the most thorough one was an overhaul in the mid-1950s, during which modern cement mortar perfusion technology was used. Stone components and accessories excavated from the riverbed or replaced were relocated indoors for preservation. They joined the permanent collections of the local museums and the National Museum of China.