Segmental Arch Bridge

A segmental arch bridge carries its roadway on an arch that is shallower than a full semicircle, using only a segment of a circle's curve, which lets the bridge span a wide opening with a lower, flatter profile and less overall stone than a full semicircular arch would need, while open spandrels, arched openings built into the solid masonry above the main arch's haunches, further reduce the bridge's weight and let floodwater pass through during high water. The oldest known example, and the oldest standing bridge in China, is the Anji Bridge across the Xiao River at Zhao County in Hebei province, built between 595 and 605 AD during the Sui dynasty by the craftsman Li Chun; its two small side arches above the main span reduced the structure's overall weight by roughly fifteen percent while easing floodwater through the bridge. The design was centuries ahead of its time by proving that a full semicircular arch was not needed to distribute a roadway's weight safely, and the American Society of Civil Engineers recognized the Anji Bridge as an International Historic Civil Engineering Landmark in 1991.