Knowledge LibraryEngineeringCivil EngineeringKL-ENG-HIST-1612
Spanning the Gap: The Iron Bridge, the Truss and the Suspension Chain
Cast iron is strong in compression and brittle in tension, so an arch is exactly the right form for it. And at Menai, Telford tested the wrought iron first and designed to the result — which makes the factor of safety a decision rather than a tradition.
01Executive summary
Three milestones in which metal replaces masonry as the primary structural material, and in which someone tests it before building with it.
The arched cast-iron ribs erected over the Severn at Coalbrookdale in 1779 are the first use of structural metal as the principal material of a bridge rather than as concealed reinforcement. Palladio had experimented with the timber truss in the sixteenth century, the first essential change in bridge construction since Rome. And for the Menai suspension bridge in the 1820s, Telford tested the wrought iron in advance and designed to the results.
02What the masonry arch could and could not do
For roughly two thousand years after Rome, a bridge was a masonry arch, and the variation was in form rather than in principle. Medieval builders chose freely among segmental, pointed and elliptical arches, used brick or stone, and varied pier widths and heights as they saw fit — the Saint Esprit over the Rhône, built between 1265 and 1309, ran some 820 metres with twenty-six arches and spans of 26 to 34 metres; the bridge at Trezzo of about 1375 had for centuries the longest single arch in the world at over 70 metres.
The limitations were structural and operational together. A masonry bridge is heavy. Its foundations are usually secure but its piers are massive, so they obstruct a large fraction of the waterway. Old London Bridge — begun by Peter of Colechurch in 1176, completed in 1209 after thirty-three years and his own death four years before the end — had nineteen irregular pointed arches whose openings amounted to no more than a third of its length. It was called a pierced dam rather than a bridge, and at certain stages of tide navigation through it was impossible for hours. That is what a masonry crossing costs: it obstructs what it crosses.
Successive generations piled houses, shops, towers and superstructures onto London Bridge with little thought for the strength of the materials underneath. On one occasion a whole row toppled into the stream. An extreme frost in 1282 cracked the masonry and brought down five arches. The structure nonetheless lasted six hundred years, until 1831 — a genuinely remarkable service life achieved with no analysis whatever, by building in massive compression with enormous redundancy. Robustness by mass is a real strategy, and it is the one available when you cannot calculate.
03The iron bridge: a material becomes structural
In 1775 the third Abraham Darby and John Wilkinson had improved the coke furnace enough to cast the arched ribs for the first iron bridge, erected over the Severn at Coalbrookdale in 1779. It is the point at which metal stops being a concealed accessory to masonry — the Greek cramps, the medieval tie rods — and becomes the material the structure is made of.
Cast iron is a compression material
Cast iron is strong in compression and brittle in tension. An arch loads its material almost entirely in compression, so it is exactly the right form for the metal available. The choice was correct, not conservative.
Members instead of masses
A masonry arch is a continuum of small units. An iron arch is a small number of large, manufactured members with defined properties, made in a foundry and assembled on site. That is prefabrication, and it changes the whole construction process.
The other structural line running through this period is the truss. Palladio experimented with it in the sixteenth century, and the source is explicit that this was the first essential change in bridge construction after the medieval arch forms. A truss resolves a span into members carrying pure tension and compression, which means each member can be sized to its force — and it requires knowing what those forces are, which is precisely what Stevin's triangle of forces made possible. The truss and the analysis of statics arrive within a few decades of each other, and that is not a coincidence.
04Menai: testing before building
A suspension bridge inverts the arch. Where an arch works in compression and pushes outward, a chain or cable works in pure tension and pulls inward on its anchorages. Wrought iron is the opposite of cast iron — tough and strong in tension — so the two forms and the two materials pair naturally.
What makes Telford's Menai crossing of the 1820s a milestone in this series is not the form but the method. The materials for the bridge were tested in advance, and a large quantity of material was saved during construction as a result.
Every structure before this in this series was proportioned by precedent, by rule, or by building it and seeing. Testing the actual material, establishing its strength, and then designing to that figure is a completely different intellectual activity: it makes the factor of safety a decision rather than a tradition, and it makes economy possible without recklessness. The saving of material is the proof that it worked. This is the practical beginning of design by calculation from measured allowables, and everything in the three following series — permissible stress, limit states, statistical characteristic values — is downstream of it.
The same movement appears elsewhere in the period. After a series of boiler explosions in the United States during the 1830s, a committee of the Franklin Institute in Philadelphia carried out extensive testing. The pattern is consistent: measurement of material properties entered engineering practice through structures that had to be economical and through machines that had been killing people.
05Takeaways for current practice
- Match the form to the material's strength in the right direction. Cast iron into an arch, wrought iron into a chain.
- Test the material and design to the result. It converts the factor of safety from a tradition into a decision, and it pays for itself in material.
- Robustness by mass is a real strategy when you cannot calculate. London Bridge lasted six hundred years on it.
- Count what a structure costs the thing it crosses. A pierced dam is a navigation obstruction that happens to carry a road.
- Analysis and the forms that need it arrive together. The truss is not useful until member forces can be found.
The modern descendants sit in AS 5100 for bridge design, AS 4100 for steel and AS 1170 for structural actions. Cited by number for orientation only — verify currency.