Engineering

Part 5 - Bridges

Engineering developed before the Neolithic revolution as people found ways to make their lives easier. The use of fire and the invention of the spear were followed by boats, levers, wedges, ramps, wheels and pulleys and these were used to build large structures like buildings, monumental structures, such as the pyramids, and bridges.

Remains, dated to about 4000 BCE, are the earliest evidence of bridges. They were stepping stones and board-walks across marshes. Prehistoric piles, tree trunks driven vertically into the bed of the upper Lake Zürich in Switzerland, date back to 1500 BCE originally supported a wooden bridge.

The Arkadiko Bridge is a Mycenaean stone arch bridge part of a former network of roads, designed to accommodate chariots, in southern Greece. Dating to the 13th century BCE, it is one of the oldest arch bridges still in use.

The Romans built arch bridges such as the Alcántara Bridge in Spain and many aqueducts that have survived to the present. The secret of their durability was a cement, called pozzolana, consisting of water, lime, sand, and volcanic rock.

Although large wooden Chinese bridges were built during the Warring States period (475 to 221 BCE) the oldest surviving stone bridge in China is the Zhaozhou (or Anji) Bridge, built from 595 to 605 CE, it is the world's oldest surviving, open-spandrel, stone segmental arch bridge.

Long distance transport of goods by land south from the North China Plain to the Central Plains was important to the Sui dynasty, in 581 CE, so they started huge infrastructure projects including the excavation of the 2400-kilometer-long Grand Canal. But but the overland route crossed the Xiao River near Luanzhou (later Zhaozhou and now Zhaoxian). Flowing west to east the Xiao River, in today's Hebei province, was an impediment to overland transport from the north and so Li Chun was assigned to design and build the Zhaozhou Bridge.

The innovation of the segmental stone arch bridge occurred near the end of the 6th century CE and it disproved conventional wisdom that a semicircular arch was necessary to transfer the weight of a bridge vertically onto the foundations. Instead some of the bridge weight was transferred horizontally to abutments at each end. It was an achievement not matched in Europe for centuries.

The bridge is about 50 metres (160 ft) long with a central span of 37.37 metres (122.6 ft). The arch covers a circular segment less than half of a semicircle (84°) and with a radius of 27.27 metres (89.5 ft) reducing the weight of the bridge. Iron dovetails were used to join the 28 thin, curved limestone slabs making up the central arch. This allowed the arch to adjust to shifts in its supports and prevents the bridge from collapsing even if a segment of the arch breaks.

An inscription left on the bridge by Tang dynasty officials seventy years after its construction reads in part: 'Its convexity is so smooth, and the wedge-shaped stones fit together so perfectly . . . And besides the mortar in the crevices there are slender-waisted iron cramps to bind the stones together. The four small arches inserted, on either side, break the anger of the roaring floods, and protect the bridge mightily.'

The bridge survived at least eight wars, ten major floods and numerous earthquakes, the most recent being the 7.2-magnitude Xingtai earthquake in 1966. Yet, the support structure remains intact and the bridge is still in use.

In 1991, the American Society of Civil Engineers designated the Zhaozhou bridge as an International Historic Civil Engineering Landmark.

Rope bridges, precursors to the suspension bridge, were used by the Inca civilization in the Andes mountains in South America, just prior to European colonization in the 16th century.

Hans Ulrich and Johannes Grubenmann designed many timber bridges during the 18th century and the first book on bridge engineering was written by Hubert Gautier in 1716.

The Iron Bridge in Shropshire, England opened in 1781, was the first cast iron arch bridge in the world. In the 19th century, wrought iron truss systems were developed for larger bridges, but iron did not have the tensile strength to support large loads. The development of higher tensile strength steels permitted much larger bridges.

Robert Stephenson's High Level Bridge across the River Tyne in Newcastle upon Tyne, completed in 1849, is an early example of a double-decked bridge. The upper level still carries a railway and the lower level is used for road traffic. In 1927 welding pioneer Stefan Bryła designed the first welded road bridge in the world, the Maurzyce Bridge which was later built across the river Słudwia at Maurzyce, Poland, in 1929. In 1995, the American Welding Society presented Poland with the Historic Welded Structure Award for the bridge.

Built from a mixture of crushed stone and cement mortar in 1905, the Friedensbrücke in Plauen is a railroad stone arch bridge having an arch with a span of 90 m (295 ft). With a span of 220 metres (720 ft), the Solkan Bridge over the Soča River in Slovenia is the second-largest stone arch bridge in the world and the longest railroad stone bridge. It was completed in 1905 by assembling more than 5000 tonnes (4,900 long tons; 5,500 short tons) of stone blocks in just 18 days.