Serendipity played a role in how, just as the need arose for a structural material of tensile strength and durability, an improved type of iron under development during the previous 80 years had just been perfected. When the British iron industry had found itself facing three crippling shortages at the beginning of the 18th century, it had responded in a very modern way by using technology to find a solution. Just how they applied their ingenuity and how that new iron would eventually make possible the Industrial Revolution, is a fascinating story of which the focus of this discussion is but one aspect. {3,5}
The iron made by the new process had a tensile strength suitable for heavy construction. Of equal significance, it was suitable for making fine cast iron which meant that mass production was now possible on a scale that had never before been imagined. British iron was strong and versatile and available locally at a good price. The civil engineers transforming British transportation with roads and canals began immediately to test its potential. {3, 6}
The British iron industry was equally enthusiastic about their new product. Abraham Darby III (1711-1763) and his fellow iron masters were of the mind that there was nothing that could not be improved by being made of iron. When architect Thomas Pritchard (1723-1777) approached them with the idea of building an iron bridge at a previously unbridgeable location in the Severn Gorge at Coalbrookdale, they realized immediately that this would be the very project to demonstrate the attributes of their new cast iron in a bridge which, in addition to being durable and strong, could be quickly and easily assembled at a modest cost. The estimate in this case was too modest - the over-runs almost bankrupted Darby who had personally guaranteed the cost - but in every other respect the project exceeded expectations.
This first modern iron bridge retained the appearance of the ancient bridges. It was basically a traditional proto-Roman arched bridge, but composed of spaces instead of solid stone: like a line drawing in the air fabricated in iron. What was not traditional is that it had an unprecedented span of 120 feet between abutments. Its 70 foot long cast iron half-arch sections, each weighing over five tons, were floated to the site on barges and hoisted into their final location from a temporary scaffolding. The foot of one of the arched half ribs was inserted into the iron base plate on the masonry abutment. Its twin was similarly placed on the other side and then the two halves were joined with vertical and horizontal pins in a cast iron crown piece. The joinery used in this first generation of iron bridges followed the traditional wood working methods such as dovetail and wedge joints. {2, 3, 7}
The bridge was assembled from some 800 separate castings essentially in three months without any serious accidents and without interrupting the barge traffic on the Severn River beneath it. Once the approach roads were completed, the toll bridge was open to traffic on New Year's Day 1781. From the moment it was opened, it caused a sensation among the public and the engineering profession. 230 years later, restored for its bicentennial, it is still startling: a Georgian cobweb of iron arching effortlessly over a rocky river gorge. (Fig. 1)
The early iron bridges were built during a very short period of time - roughly 75 years between the 1770's and the 1850's. They enjoyed such a brief moment of fame precisely because of the irresistible momentum of innovation that had been harnessed to create them. Their success spawned their successors in steel. They are virtually unique to the British Isles because, with a few exceptions, bridges built overseas were just enough later to be of steel, as were the second generation of British bridges that were built to accommodate the railroads. {3, 6}
Iron Bridges and Aqueducts
The arched cast iron bridge with decorative spandrels remained popular in many guises and sizes. The stiffening grid of struts and cross braces between the parallel arches acted as a decorative element as well when the sun light reflecting off the water animated their geometry. Certain iron works had their signature motif such as the diminishing circles in the spandrel that denotes a Coalbrookdale provenance, but Thomas Wilson (ca.1750-ca.1820) and others used them too. Gothic themes were popular during the Regency period. (Fig. 2: Eaton Hall Bridge, Chestershire 1824) {8}
The road and foot bridges needed to span the canals would be of a known span and so were candidates for replication as needed from a master design which the cast iron process made so economical . They are handsome: often with an elliptical arch and usually solid spandrels. They were often painted black with white trim, perhaps to make them more visible in the valley mists. Thomas Telford (1757-1834) designed some bridges (Fig. 3) as well as a series of handsome aqueducts for carrying the canal across a road in an iron black box with spare Regency white trim and railings between the massive embracing arms of a Georgian masonry fore-court abutments. There are a number of these similar aqueducts: Nantwich 1830 and Congleton 1831 and Stretton 1832-5 . (Fig 4) {4, 9}
Those who prefer less theatrical bridges will find them among those built for use in the mining and iron industries. In Wales, there evolved a utilitarian design for tram bridges using only a few heavy castings. The Rhyd-y-Car Tram Bridge 1815 designed by Watkin George, chief engineer at the Cyfarthfa Works at Merthyr Tydfil, has combined the structural braces and arches of the parapet into one massive casting. (Fig. 5) The Robertstown Bridge 1811 tram bridge spanning the River Cynon in Abedare uses one casting for its arch and the spandrel trusses. (Fig. 6) Their decks were made of cast iron plates. Very few such bridges were saved when their works were demolished as no one valued them aesthetically. {1,10}
As engineers sought ways to span greater distances, they began experimenting with suspension bridges whose chains were wrought iron rods hung from substantial masonry pylons. Suspension bridges could span great distances and subsequently at greater height above the water, but were subject to oscillation under stress by wind or other extreme conditions. These suspension bridges are the most spectacular and best loved of all the early iron bridges. Thomas Telford's 1848 Conwy Suspension Bridge at Gwynedd, Caernarvonshire (Fig. 7) and his nearby Menai Straits Bridge 1826 or I.K. Brunel's (1806-1859) posthumous Clifton Suspension Bridge 1836-64 in Somerset (Fig. 8) are world famous. {4, 7, 8, 13}
A self- taught and innovative bridge designer was a brewer in Bath, James Dredge (1794-1863). Dredge patented his design for a cantilever bridge that he proudly claimed was able to resist the oscillation problems of suspension bridges since each half of his bridge is structurally independent. The two triangles creating the bridge touch in the center of the span. They are formed by wrought iron rods which descend from the iron pylon caps in decreasing numbers as they approach the center from each end of the bridge. At each diminution rod link along the way, hanger rods run down at varying angles to support the wooden bridge deck. The visual effect is that of a suspension bridge. (Fig. 9)
Perhaps because he lacked credentials in the profession or in political circles, Dredge lost a number of high profile commissions for which he submitted proposals including the bridge at the Clifton Gorge. He was popular with property owners who needed an inexpensive, quickly built bridge for their farm roads or for access over a canal to their property. Dredge is thought to have built at least fifty of them, some abroad. There are evidently only seven left today. {1, 12}
As the demand increased for longer and longer spans, some interesting and unconventional iron bridges were built at the beginning of the rail road era. Robert Stephenson (1803-1859) designed two tubular railroad bridges in northern Wales. Each bridge consisted of paired rectangular boxes for track in each direction. The 1848 bridge at Conwy spanned 400 feet (Fig. 10) and the one across the Menai Straits called the Britainnia Bridge built at the same time and in the same manner spanned 460 feet between masonry piers plus extensions of the parallel tubes for 230 more feet on either end. A disastrous fire in 1970 ruined the Britainnia Bridge and although the original masonry piers have been kept, the new construction with the addition of a motorway on top has lost the integrity of Stephenson's original bold design. {4,7}
The most radical of all iron bridges is I.K. Brunel's 1854-9 Royal Albert Railroad Bridge between Cornwall and Devon over the River Tamar. Three very tall granite pylons support two elliptical balloon-like arched tubes from which the bridge deck is suspended by tie rods and by suspension chains from the pylons to the decking. Since the chains cannot extend past the pylons to be anchored on the shore in the usual manner because of the long approach viaducts, they terminate in the pylon. The inward deflection on the pylons is counteracted by the great curved elliptical tubes. This bridge represents the ultimate in what the iron was capable of achieving when used by a designer of genius. (Fig. 11) {3, 13, 14}
The majority of the iron bridges of this era were modest carriage or foot bridges built by the local foundry or blacksmith usually for a domestic setting. The 1830 Abercamlais wrought iron suspension footbridge over the River Usk on the estate of that name in Wales is the only survivor of six built by Crayshaw Bailey for its neighbors along that spate river. This one was at the highest elevation by six inches. (Fig. 12)
Another 1810-1834 foot bridge at Eastnor Castle in Herefordshire was built by the local blacksmith to appear as if it were a cast iron bridge, copying the diminishing circles in spandrel design in welded iron bars. There are a pair of 1795 iron foot bridges in Lancelot "Capability" Brown's (1716-1783) first large commissioned landscape at Croome Park, Worcestershire. They were designed by James Wyatt (1746-1813) and made by the local iron smith John Machell. Built of square iron bar stock, their spare Palladian design belies their early date: they could easily be mistaken for pre-war Bauhaus modern. (Fig. 13)
Under the aegis of author's prerogative, this quick review of the various kinds of early iron bridges will finish with a tribute to the maestro, Thomas Telford. In addition to his genius as an engineer of great scope and imagination, Telford possessed an architectural perfect pitch for proportion and scale. He was especially a master at marrying iron with masonry without letting either of those assertive materials dominate the other. Telford orchestrates their interplay so that they enhance and balance each other. It is worth taking a closer look at how Telford achieved this, because of all the engineering works of this period, his are the best known and most admired.
Telford's 1805 Pontcysylite Aqueduct carries the Shropshire Union Canal at a height of 121 feet over the River Dee Valley. In contrast with his nearby Chirk Aqueduct 1796-1801 (Fig. 14a) which is all masonry with an imbedded iron trough holding the canal so that it is visually a masonry structure, at Pontcysylite (Fig. 14b) Telford decided not to hide the iron canal trough. The massive masonry piers required to support the great weight of the canal could easily have dwarfed the less significant bulk of the iron trough or conversely the bulk of the canal at that incredible height could have appeared top-heavy in less skillful hands.
To achieve the balance that gives this aqueduct its visual impact, Telford establishes a rhythmic repetition of the plain, tapering masonry piers and the arched voids between them to minimize their great height and bulk. The solid plate iron spandrels below the canal trough give it extra presence and also serve to highlight the spring of the elliptical iron arches from pier to pier which adds even more visual emphasis to the canal box that they are carrying. This effect is reinforced by the repetitive bold pattern of shadows cast by the seams of the bolted iron plates of the canal trough that catch the eye even at great distance. The result is that the two materials complement each other to achieve an impression of effortless, weightless stability. {1,4,7, 8}