Early Bridge Building
It is unclear whether the early master masons understood catenary theory, although the concept may have been known in China. In Europe, early-medieval writing was in Latin and largely the preserve of clerics, who seldom took an interest in engineering. We do know, however, that a scientia, or body of knowledge, existed in Gothic times and later. Gothic and early Renaissance architects certainly applied principles of geometrical equilibrium. There were strict prescriptions of proportion and, occasionally, formulae empirically recommending maximum ratios for the sizes of arches, piers and foundations. Proportions such as these may be the essence of very ancient masonic secrets.
Palladio (Venice, c.1560) and Serlio (Bologna, c.1540) both recorded the average arch thickness and span of Roman antiquities. The most prolific author and architect was Leon Battista Alberti (Genoa, c.1450), whose De Re Aedificatoria was the first printed book on architecture. On bridges, he recommended that pier width should be one quarter of the span and that voussoir width should be one fifteenth. He also recognised the importance of segmental arches and proposed a range of proportions for them.
By the standards of modern engineering design, informed by plastic theory and limit analysis, these ancient empirical solutions may seem simplistic. Yet Roman arches still stand today, and medieval cathedrals remain safe despite the complexity of their vaults and flying buttresses. Perhaps we should not be surprised. Heyman’s (Ref) modern thesis remains pertinent: masonry design is fundamentally a problem of geometry, the calculation of stress is of secondary concern, and it is the shape of a structure that governs its stability. On this view, medieval empirical rules were largely sound, and only the proportions themselves are open to scrutiny, since they were derived from practice. It is significant that the ancient builders learned not only from their mistakes but also from the extensive use of proportionally accurate models.
The early mason was builder, architect and engineer rolled into one. Master masons were the consultants of their day: prosperous, middle-class professionals who directed other craftsmen, supported by qualified journeymen (paid by the day, or journée) and apprentices. A master mason served as both architect and master of works, and well-known names were sought after, even internationally. Journeymen specialised as bankers, carvers or fixers. Bankers dressed and shaped the stone, both on site and at the quarry. Carvers produced the artistic and sculptural work, though they were seldom employed on bridges. Fixers raised and positioned the stones. Sandstone and granite blocks were sourced from a local quarry whenever possible, since transport costs could rival those of extraction. Skilled quarrymen, though not members of a guild, worked in teams called “marrows”, assisted by unskilled labourers, to extract (or “win”) the stone. Bonfires were lit against the rock to make it expand, and crowbars, feathers and wedges were then used to split it. The finer ashlar blocks were cut with saws, using a sprinkling of iron filings and sand as an abrasive. Each block was then “sounded”, or lightly struck with a hammer: a stone that rang like a bell was good, while one that gave a dull thud was rejected. Typically only about a third of the extracted stone was suitable for ashlar. The rest was classed as rubble or waste, and rubble blocks were often roughly dressed and squared with a hammer (known as scabbling). In bridge-work, ashlar was usually reserved for the voussoirs (arch stones), while rubble was used for the spandrels and abutments. Mortar was made from quicklime or slaked lime mixed with sand. It took some time to set, as the lime slowly turned back into calcium carbonate. (See the Mortar section.)
Independent designer architects did not emerge until around 1700. In England, Christopher Wren (1670s) may be thought of as the first independent architect. He had never worked on a building site and tended to keep his hands clean. The title of Master Mason then evolved to Master of Works. In 1716 Henri Gautier (Ref), a French doctor, mathematician and architect wrote the first book on the construction of bridges. His Traité de Ponts addressed all aspects of design, choice of site, construction and contemporary regulations. He reported on the great 17th century Italian masters, and using examples both wooden and masonry he provided operational instructions for the size and shape of piers, cutwaters, arches, spandrels and parapets. An interesting perspective is that in 1716 Gautier devoted as many pages to wooden bridges as he did to masonry. Many of these principles were later endorsed by Charles Hutton (Ref) in his very mathematical treatise, The Principles of Stone Bridges, published in 1772.
The foundations of a stone arch bridge are the single most important factor in determining longevity. If they are sound, the bridge may last for centuries; if they are poor, it may survive only a few years. Foundation failure is the most common cause of bridge collapse.The line of thrust, generated by gravity, must remain within the masonry of the arch. It then passes through the abutments into the foundations. Half of the bridge’s accumulated thrust is concentrated at each end of the arch. If the arch-ends are allowed to spread apart or settle, the line of thrust will drift outside the masonry, and collapse becomes inevitable.
Choice of location was crucial. A straight stretch of river avoided unpredictable flow. Shallow sections were preferred, and a low tidal range made pier building easier. In Southern Europe, some rivers dry up almost completely in summer, but this was not the case in Scotland. A narrow section of river was best for single-arch bridges, though not necessarily for multi-arch bridges, where restricting flow causes even more problems. Hard or rocky banks and riverbed were an advantage. Plans also had to include an analysis of seasonal change and spate levels. A single low wide-spanning arch arch was always preferable to having piers in the river. .
Even a simple semi-circular voussoir arch cannot be built without faux-works or centering because the structure will not stand up until every voussoir is in place. Temporary centering structures were made of wood, and the carpenters often erected, dismantled and re-erected the same centering on different rings of the arch, across the barrel, until each arch was completed. More frequently, an entire arch falsework was constructed of wood. Hutton notes that timber was a significant part of the cost, and that the same profile could only be used if all the arches were the same size. Hence differently sized arches were rare and expensive. It was always known that abutments had to be constructed in solid anchored masonry; frequently ashlar was used and sometimes short wooden piles were driven into the foundation layer in advance of the masonry.
Semicircular arches will stand alone if the barrel is sufficiently thick. Each arch can be built separately, and the wooden centering can then be dismantled and moved on to the next arch. Segmental arches with slender piers (less than 1/4 of span) will usually rely on the lateral resistance buttressing from the immediate neighbouring arch. The obvious implication is that false-works are required for the entire structure, all at the same time, to avoid complete collapse. Once completed, however, the advantage of low flat segmental arches lies in reduced height for a given span, so that fewer piers are needed. There is also more flexibility and room for diversity with a slimmer more elegant profile and a clear advantage of less steep gradients on the approach. The disadvantage lies in that very interdependence; if flood-waters engulf these flatter arches they may well all collapse at the same time.
Multi-arched bridges posed a particular challenge: the construction of piers in mid-stream. Gautier discusses this in broad principles, while Ruddock (Ref) provides a modern, detailed analysis.
A small tidal range in shallow water might still permit the Roman approach of building a wooden cofferdam on the river bed (Old Scots: bulwark; Old French: bâtardeau). A gin and ram was used to drive piles into the mud, side by side, until they formed a semi-watertight double-walled circle, perhaps 50 ft in diameter. The central pool was then emptied by chains of men with ladders and scoops, or by Archimedean screw pumps. The considerable advantage of this method was that the pier (and its cutwaters) could be built on the dried-out riverbed itself, either on bedrock or on a rubble foundation over piles. The piles were cut off at the lowest water line and capped with masonry, since timber rots rapidly at the air-water interface but scarcely at all when fully submerged. The result was a much higher-quality foundation, and a much narrower one, which obstructed the river’s flow as little as possible.
A wider tidal range usually required starlings: these were artificial islands. The construction was similar to a cofferdam, but piles were driven into the river bed and a chamber was constructed, sometimes with additional piles the middle; rubble was then poured in to the chamber on each receding tide. The stone piers were built on top of the starling. The piles were left in place. This arrangement was more commonly seen below English bridges and was clearly recorded at Berwick and in London, The French called this an encaissement and filled it with a crude concrete mix (The word ‘pile’ is confusing; the French used it to describe a pier, whereas, in French, a pile is ‘un pieu’ or 'un pilastre') Starlings were so large that they caused a considerable obstruction to the flow of the river. The old 14th-century London Bridge had a large 'fall' (afflux) of water level across it and a torrent between the arches at maximum ebb. Young men used to ride the rush for fun, in small boats. Going upriver was only possible with slack or flood-tide times.
A more common approach, seen more often in Scotland, and also useful for a significant tidal height, was to sink a large wooden frame or ‘brander' made up of longitudinal planks. This frame would be filled directly with rubble and sunk on the required spot. In this way, a different form of starling was created which had no vertical piles. The French called it a 'crêche’, the English, a‘ grating’. There are reliable records of brander foundations at Ayr, Dee, Perth and Stirling. Hutton’s late 18th-century glossary describes a Caisson, not as a cofferdam as would be expected, but as a wooden boat on which the pier was constructed in ashlar only to be sunk when it reached low-tide level height. This appears to be a later version of a grating.The technique is particularly well described at Ayr Bridge.
Gautier described a further approach: that of diverting the whole river while foundations were being built. This may have been less of a last resort than one might think: Hughes (Ref) , in 1839, describes a variation suitable for a meandering river on a lowland basin; the bridge would be built on dry land in advance of a permanent diversion of the river, as shown. There are no known examples in Scotland but this may be because of the absence of records. In 1808, Telford partially diverted the River Tay when building the bridge at Dunkeld, although this was more in the nature of a huge cofferdam.
One more approach, not relevant to Scotland, was found at Piercebridge Roman bridge on Hadrian's Wall. A paved road was created across the riverbed on which the bridge was built and to which the cutwaters were bonded. We can presume it was successful.
Multi-span bridges need piers with triangular cutwaters, facing the current, to provide the best protection against scouring (undermining of the foundations). In the 15th-century Alberti pointed out that these were just as important on the downstream side. In earlier times, ‘framing and setting’ of short piles overlaid with masonry formed a defence which was installed some yards upstream of the pier. This divided the current and was more disposable and renewable. The single most important element of maintenance was the constant attention to the risk of scouring.
On top of the piers, the centering for all the arches would then be built, and this would be followed by the voussoir arches in solid dressed masonry. If ribs were used these were almost always in ashlar. Puddled clay might be used as a waterproofing over the top (arch-back) of the arch. Sometimes a second order of voussoirs was built, often called a counter-course. The spandrels were last to be constructed with infill rubble to provide a plane for the cobbled surface. It was good practice to keep the centering in place until the spandrels were complete. Parapets were an optional extra, sometimes deliberately omitted; for example, the narrow old packhorse bridges required that nothing should impede the large panniers on each side of the horse.
It is now known that most bridge arches experienced some outward shift (spreading) of the abutments, probably when the centering was removed. This minor settlement caused cracks in the arch in three places, usually at the crown and at both haunches. An arch structure is no less stable because of these hinges but any deterioration leading to a fourth hinge would be unsustainable. This three-hinged state is so common that it can be considered a natural state.
Jean Randolph Perronnet was the most eminent late 18th-century bridge builder, and the architect of the huge segmental Pont de Neuilly in Paris (1783.) He and his contemporaries were very critical of medieval engineering, but their criticism was misplaced and lacked perspective. Many Medieval structures have remained standing for 500 years, and many more were replaced in the 19th century, not because they were at risk, but only because they were too narrow for increasing traffic. Rochester's Medieval bridge on the Medway was so solid that it had to be blown up in order to be dismantled. These old bridges frequently spanned wide, deep, fast-flowing rivers with frequent flooding. Their longevity was largely a function of repair to the foundations and constant attention to starlings and branders to avoid scouring, and regular inspection of abutments which are always at risk of displacement. However, some design features were pivotal to this long-term success: segmental arches with their lower profile have to be built with a very high standard of masonry if they are to survive the years; however, semicircular and Gothic arches create less dependence between arches so that the whole structure can survive the collapse of a single arch; also, they have more height, ensuring that the river banks will burst before water levels reach the crown, an event which is frequently fatal. Many old bridges have flood-arches on dry land which also helps. These design features have been incorporated repeatedly, over 2000 years. That, in itself, is a recommendation.
Today these old masonry arch bridges are often expected to carry modern traffic. Eight of the pre-1600 bridges still remain on the Scottish road network along with a further sixty, which are structurally from before 1750, and still taking traffic. In all cases, the implied live loads are far beyond the expectations of the original builders. Since the 18th century, engineers have attempted to answer questions about the safety of existing bridges and about their carrying capacity. Danyzy’s (Ref) elegant experiments in 1732 identified the hinging failure leading to collapse. Castigliano (Ref) introduced non-linear elastic stress analysis and proposed the middle third rule. Pippard’s (Ref) elastic method in the 1930s was further developed into the MEXE system which has been in general use since the 1940s. Heyman (Ref), in the 1970s, introduced Plastic Theory and Limit Analysis, references which were later deployed extensively in computer programs such as ‘Archie’. Most of these systems are to some degree underpinned by line-of-thrust theory and computations around expected end-point 'hinge' failures of the old arches. This is not the only approach. Recently, the important strengthening role played by the backfill has been identified which has led to the development of some new modelling strategies. Today, 3D non-linear finite element analysis (FEA) has become a respected, computer-assisted, graphic approach to stresses and displacements. It is probably fair to say that despite the crucial importance of arch safety assessment there still exist uncertainties and inconsistencies in this whole area of research.
A useful animation of early bridge building can be found here .
Page last updated Oct. '26