Arch shapes on early bridges

In 1675 Robert Hooke provided the principles of catenary theory but the practice of arch building had long preceded him.  The Etruscans, the Romans, the Persians and the Chinese Dynasties all built masonry arched bridges. Many Roman bridges remain intact in Europe, today, but in Scotland nothing remains.  In Europe, arch building became a lost art for 600 years but re-emerged in the 11th century. The Romanesque semicircular arch then became the fundamental solution for Norman ecclesiastical architecture as well as for bridges; a semi-circle had been the Roman mainstay for arches, and it was this that they copied. Some Romanesque bridges can be seen in Europe, but in this period most were made of wood and some of those in masonry may have had a late Roman origin.  Fountains Abbey Infirmary Bridge in Yorkshire is a rare example of an English  Norman (Romanesque) arched bridge.   Exeter Bridge (below) was recently uncovered. There are no Norman bridges in Scotland.    

Screen Shot 2017-11-01 at 09.49.04

Romanesque architecture gave way to Gothic in the 12th and 13th centuries. (The oldest known pointed arch is in ancient Byzantium and dates from the 6th century.)   An ogive or Gothic arch has a geometry which is closer in shape to a catenary/parabola.  Consequently, cathedral arches and rib-vaults could be given slimline voussoirs, and bridges built in the gothic shape could be slimmer yet strong. They need less buttressing. 

Screen Shot 2016-04-18 at 14.02.59
Screen Shot 2016-04-18 at 14.18.21

 There is an odd additional stratagem here:  a weight on the tip (crown) of the arch alters the geometry of the thrust line,  bringing it more into line with an ogive shape, which confers stability.  This additional stablity was known about in medieval times but seldom seen on bridges.   However, below is the Puente del Diablo near Barcelona, a 13th century 37m arch on the River Liobregat . Note the remarkably slim voussoirs. 

Screen Shot 2016-04-18 at 14.20.29

Screen Shot 2016-04-18 at 14.28.14

Not all medieval bridge arches from this period are of ogive shape; there are some semicircular and segmental arches.   Segmental arches have a flattened profiles and a  parabola or a catenary can still be drawn within the voussoirs, particularly if they are wide enough.  Interestingly, the flatter the arch,  the more a parabola coincides with a circle-segment. Substantial lateral buttressing abutments are needed.  This was clearly known a long time ago,  as in France and  Italy there are many medieval segmental bridges. In Scotland, there are only a few.   

The Renaissance brought an end to the building of pointed bridge arches. Rounded arches predominated thereafter.      

Early  bridge building                     Arches General                  Force Lines

Dec. 2012                                      Site last updated  Feb 2019