John Smeaton
During Smeaton's short foray into legal training in London, he had seen Old Westminster Bridge (begun 1739) being constructed over the Thames. When one of its piers settled more than 600mm into the riverbed in 1746, his engineering instincts began to emerge. He submitted a speculative design for a cofferdam in 1748 to enable repairs in the dry. Five years later, when he was 29, he decided to make a career from what he called "engineery".
In all, Smeaton completed 12 bridges. Two are of brick, the rest stone — he didn't embrace the use of cast iron when it came along in the 1770s. All Smeaton's masonry bridges were far less massive, more elegant and therefore more economical than those of his contemporaries, such as Robert Mylne (1733-1811), James Paine (1717-89) and John Gwynn (1713-86).
From 1753 to 1760, he was involved in the planning of the original Blackfriars Bridge, although the commission went to Robert Mylne. Smeaton's design for Glasgow Bridge was also not used. However, in February 1760 he published a dissertation on his principles of bridge design. In it he emphasised the importance of foundations to the stability of a bridge's superstructure — an issue that would haunt him in later years.
His first completed crossing is the five-arch Coldstream Bridge over the River Tweed, designed in 1763 and completed in 1767. River piers are effected by riverbed scouring and Smeaton's solution here was to place rubble mounds around the foot of each pier. The decorative black stone discs he used at the centre of each spandrel were to appear on all his major bridges.
The Coldstream design proved a useful blueprint. Smeaton amended it to seven arches for his bridges over the Tay and the Deveron. Perth Bridge and Banff Bridge have respectively larger and smaller maximum arch spans than Coldstream. Rubble mound defences against scour were successful for both these bridges.
Between 1770 and 1773, Smeaton designed and built two single-arch canal aqueducts. The first is Camelon Aqueduct (demolished), which crossed a road at Falkirk. The second crosses Luggie Water at Kirkintilloch. Although it was usual to place up to 1m of clay beneath the canal bed of an aqueduct to prevent leakage, Smeaton's drawings show that at Kirkintilloch the clay is just 150mm thick over the crown of the arch, and at Camelon there is no clay layer.
Four smaller stone bridges followed, designed between 1767 and 1776. Two of them are single-arch spans — Stonehouse Creek Bridge in Plymouth (1773) and Altgran Bridge. The other two were built for the Duke of Queensberry at Amesbury — the five-span segmental arch Queensberry Bridge and the three-span classical Ornamental Bridge.
In 1777, Smeaton returned to his Coldstream design, this time modifying it to nine spans for Hexham Bridge over the River Tyne. Site investigation led him to believe that the gravel riverbed would support the piers without piling, and five of them were so constructed. However, soon after completion one settled and needed rebuilding. Scouring was a problem too.
An unusually violent flood in March 1782 caused serious erosion of the riverbed. Four of Hexham Bridge's gravel-founded piers and two of the piled ones were undermined: some of the arches were swept away. Smeaton observed sadly in a letter (6th June 1782) to the bridge's resident engineer Jonathan Pickernell, "All our honours are now in the dust! It cannot now be said, that in the course of 30 years' practice ... not one of Smeaton's works has failed".
In October the following year, Smeaton was asked if he would rebuild the bridge but he declined, asking that another "who has not got the Horrors of the River Tyne painted upon his imagination" be appointed. The reconstructed Hexham Bridge was completed in 1795, founded this time on a piled timber platform stretching from one bank to the other. This is the bridge that still stands.
Smeaton's two brick bridges are very different from each other. The first, Newark Bridge (1770) over the flood plain of the River Trent has 74 arches! Cardington Bridge is a five-arch red brick structure in Bedfordshire.
It's a testament to Smeaton's robust design work that all but the Camelon Aqueduct and the original Hexham Bridge still exist today. His practice of visiting quarries in person to select durable stone for the masonry certainly paid off.
All items by John Smeaton  •  Everything built ... 1724 - 1792
main references  BDCE1, DNB, JS
painting of Smeaton  courtesy Institution of Civil Engineers

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Drawings for Smeaton's first-completed bridge — Coldstream Bridge over the River Tweed.
Queensberry Bridge
Drawings for Hexham Bridge over the River Tyne. It was to stand for only two years before being swept away in a flood The current bridge is a reconstruction.
Section through one of the river piers of Hexham Bridge showing its founding directly on the riverbed.
Hexham Bridge arch and pier details.