The segmental arch bridge

UNESCO Courier, Oct, 1988

A conceptual breakthrough occurred when a Chinese engineer realized that an arch did not have to be a semi-circle. A bridge could be built which was based not on the traditional semi-circular arch but on what is known as a segmental arch. The way to envisage this is to imagine a gigantic circle embedded in the ground, of which only the tip shows above ground level. This tip is a segment of a circle, and the arch it forms is a segmental arch. Bridges built in this way take less material and are stronger than ones built as semi-circular arches.

This advance took place in China in the seventh century AD. It was the concept of Li Chun, the founder of an entire school of constructional engineering whose influence lasted for many centuries. We are fortunate that his first great bridge, built in 610, survives intact and is still very much in use today. Called the Great Stone Bridge, it spans the Jiao river near Zhaoxian at the foot of the Shanxi Mountains on the edge of the North China Plain.

Four small whole arches were incorporated within the structure of the main bridge. They were an innovation of great consequence in bridgebuilding, for they were the world's first arched spandrels. Li Chun found that by punching these holes in the ends of the bridge he could accomplish several things at once: flood waters could rush through them, lessening the chance that the main bridge would be swept away at its supports in a sudden flood; the total weight of the bridge could be lessened, thereby diminishing the tendency to buckle by the ends sinking down into the river banks; and vast quantities of material could be saved, which would normally have gone to make solid ends for the bridge.

The Great Stone Bridge has a span of 37.5 metres. The largest surviving Roman whole arch bridge, the Pont Saint Martin near Aosta, spans 35.5 metres. But the average whole arch Roman bridge spanned between 18 and 25 metres, whereas whole arches in Roman aqueducts had an average span of about 6 metres.

The greatest segmental arch bridge in China is the famous "Marco Polo Bridge', often so named because Polo described it at length. just west of Beijing, it crosses the Yongding river at the small town of Lugougiao, and is 213 metres in length, consisting of a series of eleven segmental arches extending one after another across the river, each with an aveage span of 19 metres. It was built in 1189 and is still heavily used by modern truch and bus traffic. Marco Polo though this bridge "the finest in the world."