ch 1b_4_1 historical development
DESCRIPTION
A detailed view design, production, and erection of steel structures according to the new European code EC3.TRANSCRIPT
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STEEL CONSTRUCTION:
INTRODUCTION TO DESIGN
Lecture 1B.4.1: Historical Development of
Iron and Steel in Structures
OBJECTIVE/SCOPE
To appreciate how steel became the dominant structural material that it is today, it is
essential to understand how it relates to cast iron and to wrought iron, both in its properties
and in the way that all three materials evolved.
PREREQUISITES
None.
RELATED LECTURES
Lecture 1A.2: Steelmaking and Steel Products
SUMMARY
The properties of the three ferrous metals, cast iron, wrought iron, and steel, are described
and the evolution of their production is summarized. The evolution of their structural use
is also given and the prospects for further development introduced.
1. PROPERTIES OF THE THREE FERROUS
METALS: CAST IRON, WROUGHT IRON AND
STEEL
Cast iron, as the name implies, is "cast" or shaped by pouring molten metal into a mould
and letting it solidify; a wide variety of often very intricate forms is thus possible. It is
very strong in compression, relatively weak in tension, much stiffer than timber, but
brittle.
Wrought iron is strong both in tension and compression and ductile, thus making it a much
safer material for beams than cast iron. Its main disadvantage is that, never reaching a
fully molten state, it can only be shaped by rolling or forging, thus limiting its possible
structural and decorative forms.
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The properties of mild steel are similar to those of wrought iron but it is generally stronger
and can be cast as well as rolled. However, it has a lower resistance to corrosion than
wrought iron and is less malleable and thus not so suitable for working into elegant,
flowing shapes.
These properties, in terms of strength and carbon content, are shown in Figure 1; the
values shown should be considered as indicative rather than absolute limits. They do not
include malleable or ductile cast irons which have strengths in tension considerably above
those shown.
2. EVOLUTION OF FERROUS METALS
2.1 Blacksmith's Wrought Iron
Iron has been known and used for more than three thousand years, but it was not until the
development of the blast furnace around 1500 AD that it could be produced in molten
form. In China, molten iron goes back much earlier but this is not generally thought to
have been known in the Western World until well after the independent invention of the
blast furnace. There is slender evidence that the Romans knew how to produce cast iron
but, if they did, the knowledge was certainly lost.
Before the blast furnace, iron was extracted from ore by chemical reduction in simple
furnaces or hearths. Inevitably, the scale of the operation was small and the process quite
laborious, the iron coming in a hard pasty form, far from liquid, which was then refined
and shaped by hammering. Essentially, this was 'blacksmith's iron'.
2.2 Molten or Cast Iron
Although possible in the 16th Century, molten or cast iron was hard to produce on a large
scale before the change from charcoal as a fuel to coke. With charcoal, the practical size of
furnace was limited by the crushing of the fuel by the weight of the charge of the ore and
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thus the stifling of the blast. Abraham Darby I is generally credited with the mastery of
coke smelting and, even though this was in 1709, coke smelting did not dominate the
industry until about 1750 in Britain and considerably later in other parts of Europe.
2.3 Industrialised Wrought Iron
Large scale wrought iron, as opposed to blacksmith's iron, became possible mainly as a
result of the developments culminating in Henry Cort's puddling furnace patented in 1793.
In this furnace, the carbon in cast pig iron was burnt off in a reverbatory furnace while the
impurities were drawn off by 'puddling'. As the process continued and the iron became
purer, its melting point rose and the furnace charge became more viscous, eventually
being removed in a stiff plastic form for rolling or forging. It was the enlarged scale of the
operation which was significant rather than any change in the actual material which was
effectively the same as the blacksmith's variety.
The modernising of wrought iron depended not only on the puddling process, but the idea
of grooved rollers which made possible the economic production of angle and tee sections,
and later channels and joists. Here again, Henry Cort, who patented the grooved rollers in
1784, gets the credit although the due financial rewards eluded him.
2.4 Steel
Although steel-type iron had existed for many centuries, steel as used today dates from the
18th Century. It was produced either by cementation, a process by which bars of pure
wrought iron absorbed carbon during prolonged heat treatment, or after about 1750 in
molten form by Hunsman's crucible process. Cementation was largely confined to the
cutlery and tool trades and has no real relevance to construction. Crucible steel continued
to be made, although at a decreasing level of production, until after the Second World
War; however it is uncertain how much of this was used structurally in construction
works.
It is a common fallacy that the use of steel dates from Bessemer's converter of the mid
1850s; not only did Kelly in America get there first with an almost identical process, but
the amount of steel already being produced was quite substantial. Some 60,000 tons of
steel were produced each year around 1850 in Britain alone which is far from negligible,
except perhaps when compared with an annual world production of 2,5 million tons of
iron in the same period. Bessemer's steel was certainly cheaper and could be made in
larger quantities, but its quality was uncertain. It was not until the perfection of the
Siemens-Martin open-hearth process in the 1880s that steel moved in a big way into the
construction and shipbuilding industries.
Today, very little truly structural cast iron is being used and no wrought iron is being
made. Steel is wholly dominant. There are, however, some signs of a limited revival of
cast iron, particularly in the new ductile form only available since the 1940s.
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3. ACHIEVEMENTS WITH STRUCTURAL IRON &
STEEL
In looking at the structural achievements with iron and steel in the last 250 years, it is
convenient to class these in relation to the period, or age, when each of the three ferrous
metals was dominant. Inevitably, these periods overlap and it is significant that in each
case it took quite a long time - up to 50 years - before what was found to be possible
became commercially widespread. The periods are broadly as follows:
Cast Iron Period 1780-1850 (Columns up to 1900)
Wrought Iron Period 1850-1900
Steel Period 1880 - Present Day
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These dates are essentially based on Britain where the iron industry was more developed
in the first half of the 19th Century than elsewhere. In France, there was no real cast iron
period, while in America both cast iron and wrought iron were comparatively little used
before the middle of the 19th Century, after which there was a positive explosion in their
application. Steel on the other hand, became popular at roughly the same time throughout
Europe and America. Figure 2 emphasises how short the overall period of structural use of
iron and steel has been in relation to man's knowledge of iron.
4. THE PERIOD OF CAST IRON (1780-1850)
Given availability, new materials are introduced either for greater economy or to solve
specific problems.
4.1 Cast Iron Arched Bridges
All the early cast iron bridges were arched forms in which cast iron merely replaced
masonry, the advantages being greatly reduced weight and horizontal thrust, economy and
speed of erection. The first iron bridge of any magnitude was the famous Coalbrookdale
one completed in 1779 and spanning some 33 metres (Slide 1), a structure full of apparent
illogicalities mixing carpenter's and mason's detailing but still standing proudly today. The
construction of this bridge was followed by a whole succession of cast iron arch bridges in
Britain, including Thomas Wilson's Wear Bridge of 1792-6 with wrought iron strapping to
the cast voussoirs and a span of 72 metres (Slide 2) and Rennie's Southwark Bridge of 73
metre span completed in 1819. The climax, but by no means the last, cast iron bridge, was
perhaps Telford's Mythe Bridge at Tewkesbury (1823-26) with a span of only 52 metres
but great lightness and total structural logic (Slide 3).
Slide 1
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Slide 2
Slide 3
In other parts of Europe, cast iron arch bridges were a rarity until well into the 19th
Century, the number of schemes greatly exceeding the number built. Le Pont des Arts in
Paris of 1801-3 by Cessart was, perhaps, the most famous, now, alas, replaced by a not
wholly convincing welded lookalike. There were several early cast iron arch bridges in
Russia.
4.2 Cast Iron in Buildings
With all buildings, fire was a recurring problem with timber structures. It was almost
certainly the reason for one very early application of cast iron, the columns supporting the
vast cooker hood and chimney of 1752 at the Monastery of Alcobaca in Portugal. In
Britain, cast iron was used in the early 1770s in churches, partly for the cheap
reproduction of Gothic ornament, but also for structural columns. In Russia architectural
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cast iron was used extensively throughout the 18th Century but it is not clear to what
extent it was also used to support floors and roofs.
It is hard to see any trend arising from these early applications of iron to buildings. It was
in the multi-storey textile mills in Britain in the 1790s that cast iron was first shown to
have a major future in building structures. The disastrous fire at Albion Mill in 1791 was
perhaps the biggest incentive for change. Bage and Strutt were the great pioneers.
Between them, they developed totally incombustible interiors in cast iron and brick but
with floor spans still of only about 2,5 to 3,0 metres in each direction, as had been the case
with timber interiors. Later, this iron mill construction spread to warehouses with a
gradual increase of spans.
While fire was the main reason for change in the mills, there was a growing desire in
public buildings and large houses for long-span floors which did not sag or bounce.
Timber had generally proved inadequate for spans above 6-7 metres. Between about 1810
and the early 1840s there was an increasing interest in cast iron floor beams, some with
spans of 12 metres or more such as those in the British Museum of the early 1820s (Figure
3). Sometimes these castings were used as simple substitutes for the main timbers in
essentially timber flooring, but in other cases brick jack arches, as in the mills of around
1800, or stone slabs were combined with long span cast iron beams to give rigidity, sound
insulation and fire protection. Another form of 'fire proofing' consisted of wrought iron
plates within the ceiling space arching between the cast iron beams. The climax of the
development of cast iron flooring was reached in Barry's Palace of Westminster of the
1840s. Up to the mid 1840s, cast iron was seen as the wonder material everyone was
looking for.
It is tantalising how little is known about who actually fixed the size and shape of the
beams used by Nash, Barry and other architects of this period. Thomas Tredgold's book on
cast iron of 1824 was undoubtedly influential but dangerously in error in some respects. In
most cases, it is probable that proof-loading of beams, which was widely used, provided
the main safeguard against misconceptions and poor workmanship.
Apart from the mills and the long span floors, there was a whole range of new uses of cast
iron between 1810 and 1840, sometimes on its own for complete structures as in
Hungerford Market of 1836, or Bunning's highly decorated Coal Exchange of 1847-49. In
Russia, there was also a considerable quantity of cast iron building construction in the first
half of the 19th Century, as in the Alexandrinsky theatre of 1829-32 and the Dome of St
Isaacs Cathedral (1837-41).
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Towards the close of the 1840s, cast iron had lost much of its golden image and was being
seen as an unreliable material, especially for beams. The progressive collapse of five
storeys of Radcliffe's Mill in Oldham in 1844 and the failure of the Dee Bridge in 1847
were both highly damaging to its image.
4.3 Composite Cast and Wrought Iron in Building
Not all iron in the 'cast iron period' was cast. Some of it was composite cast and wrought
iron and some simply wrought iron. There is little evidence of steel being used structurally
in this period.
In Britain, cast iron was sometimes used in combination with timber as at New Tobacco
Dock of 1811-14 or with wrought iron, as in the 1837 roof at Euston Station (Slide 4).
Slide 4
After 1840, the scale of iron construction and the proportion of wrought to cast iron in
composite structures, increased substantially. The Palm House at Kew 1844-47, by
Richard Turner and Decimus Burton, was a marked advance on earlier glasshouses and
arguably incorporates the world's first rolled I sections. Wrought iron roofs of increasing
span on cast iron columns proliferated both in the naval dockyards and for railway stations
culminating in Turner's roof of 47 metres span at Lime Street, Liverpool (1849).
In France, some highly innovative wrought iron floors and roofs had been built before the
Revolution, such as Victor Louis's 21 metre span roof of 1786 at the Palais Royal Theatre
in Paris (Figure 4). In this roof, as in the case of the bridge at Coalbrookdale, the structural
logic is not altogether clear. However, the flooring system of arched wrought iron flats
devised by M. Ango in the 1780s (Figure 5) is clearly understandable and derivatives of
this system continued in use until they were largely replaced by a number of 'fire-proof'
systems, still based on wrought iron, in the late 1840s. Cast iron impinged in France to
quite an extent in the 1830s and after, notably in the great iron roof of 1837-38 at Chartres
Cathedral and the Bibliotheque St Genevieve 1843-50, but it seems that wrought iron
always retained its dominance.
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Composite construction featured quite widely in Russia. In St Petersburg, a form of
riveted plate girder was devised in 1838 for the repair of the Winter Palace after the fire of
1837. This development was just ten years before the independent development of riveted
wrought iron beams in Britain.
4.4 Suspension Bridges
Some of the most creative work on the suspension bridge dates from the 'cast iron period'
but is wholly related to wrought iron, although Tredgold did have the temerity to suggest
cast iron support cables. In most fields of construction, America clung to timber rather
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than iron in the first half of the 19th Century, but must be given credit for introducing the
level deck suspension bridge, as patented by James Finley in 1808 with wrought iron
chassis (Slide 5). Thereafter, there was a minor battle of principles on the form of cable.
Britain favoured wrought iron chains with eye-bar links, as had Finley, while the French
preferred wire cables, the difference being largely due to the states of the iron industries in
the two countries.
Slide 5
By 1850, France had built several hundred suspension bridges, mainly due to the
enterprise of the Seguin brothers, while Britain could claim scarcely more than a dozen. If
the French had confined the wires to the sections of the cables above ground, all might
have been well, but they did not. Corrosion became a major problem brought to a head by
the collapse in 1850 of the Basse-Chaine suspension bridge with a death toll of 226.
Thereafter, substantial remedial works followed and the building of suspension bridges all
but stopped in France for many years. Nevertheless, based on French influence, wire
cables did take over from eye bar chains in America and became virtually standard
throughout the world.
5 THE WROUGHT IRON PERIOD (1850-1900)
5.1 Wrought Iron in Bridges
The wrought iron period was primarily the period of the riveted wrought iron beam which
dates from the late 1840s, although by then wrought iron had established a fairly firm
position in composite construction. Seen in the long term, wrought iron beams owe their
birth, in part, to growing doubts both on the safety of cast iron in bending and in part to
successful experience with iron ships. However, by far the biggest single contribution, not
only to the development of riveted beams, but to the whole establishment of wrought iron
as the dominant material of the period, was the design and construction of the Britannia
and Conway tubular bridges, particularly the former.
The key figures here were Robert Stephenson, engineer to the Chester and Holyhead
Railway; William Fairbairn, the practical man with experience of iron ships; and Eaton
Hodgkinson, the theorist and experimenter.
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Faced in 1845 with the then seemingly impossible task of taking trains over the Menai
Straits, when shipping interests ruled out arches and suspension bridges as they had been
shown to be inadequate for railway loads, they developed a new structural form, the box
girder, and demonstrated it on a large enough scale for trains to run inside (Slide 6).
However, it was not the bridges which mattered so much as the understanding which
resulted from the crash programme of research and testing which made them possible.
Slide 6
Between them, these three men dispelled the initial belief that wrought iron was weaker in
compression than in tension, proved that a rectangular tube was stronger in bending than a
circular or oval one, isolated the problem of plate buckling, and showed how to counteract
this behaviour with cellular flanges and web stiffeners. Thus, these three men and their
assistants established riveted wrought iron as a calculable material for beams of almost
limitless size. Further, they demonstrated the benefits of continuity in beams, even for
deadload (based on theoretical work from France) and proved that the strength of rivets
depended on clamping as much as on dowel action. The extent of material and model
testing for these bridges was prodigious.
The speed of the work was almost as remarkable as the result. The problem of crossing the
Menai straits was posed early in 1845, the Conway Bridge was opened in December 1848
and the Britannia Bridge in March 1850. In both cases, work on the supporting masonry
started in the spring of 1846 well before all the problems of the spanning structures had
been solved. Other smaller wrought iron bridges of the same period, with cellular
compression flanges were, it seems, all spin-offs from this basic development.
It is, perhaps, worth noting that concurrently with this major innovative work, Stephenson
was responsible for a mass of other railway construction, including the six-span Newcastle
High Level Bridge with cast iron tied arches of 1846-49 (Slide 7) and the ill-conceived
Dee Bridge at Chester based on trussed
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Slide 7
cast-iron beams, which collapsed disastrously in 1847 soon after it was opened. The
pressure on the leaders of the engineering profession at this time are hard to imagine and it
is no surprise that, sometimes, relationships became strained, as they did between
Stephenson and Fairbairn.
The evolution of the plate girders of today from these beams with cellular compression
flanges took place largely in the 1850s. Figure 6 shows some steps in this transformation.
The rationalisation of truss forms and their full structural evolution is another feature of
the 1850s. Many of these forms derived from timber construction in America but given
riveting and wrought iron the scope opened up enormously. The Britannia Bridge has been
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criticised for wasting material in comparison to an equivalent structure with open trussed
sides, but this is unfair when one considers how little was known about true truss action in
the mid 1840s. Figures 7a and 7b show typical intuitive and mathematically rational truss
forms of this period. There were many variations on these forms.
Numerous wrought iron bridges of all forms and sizes followed in all countries. In Britain,
I.K. Brunel's Saltash Bridge completed in 1859 and Thomas Bouch's fatal Tay Bridge
opened in 1878, stand out for very different reasons. In France, Gustave Eiffel's great
arches at Oporto and Garabit, of 1875-7 and 1880-84 respectively, are now world famous.
In America, Charles Ellet's Wheeling Suspension Bridge of 1847-9, Roebling's Niagara
Bridge completed 1855, and James Ead's St Louis Arch Bridge of 1867-1874 are all
rightly famous, although one must add that the last of these is partly of steel.
5.2 Wrought Iron in Buildings
In buildings the scope for drama in the use of iron was generally more modest, the largest
outlet being in flooring systems both in Britain and in other parts of Europe. It was almost
certainly the development of these flooring systems in France in the late 1840s and early
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1850s which provided the impetus for the commercial development of rolled joists,
regardless of whether the first ones of all were rolled there or in Britain. The size of the
joist sections gradually increased but until liquid steel took over, size was limited by the
problems of handling large quantities of puddled iron.
Cast iron continued to be used extensively for columns well after 1850. In America there
was a great vogue for cast iron facades which lasted for several decades. Bogardus and
Badger were the two main suppliers. Internally, the structures vary, with iron, masonry
and timber all represented.
Apart from these useful, but often unseen, applications of iron to traditional buildings,
some spectacular iron build structures, mainly long span roofs, were built in all countries.
Most commonly, but far from exclusively, they were over railway stations. They included
the ribbed iron dome of the British Museum Reading Room (1854-57), the 73 metre
wrought iron arches at St Pancras Station (1868) and the dome of the Albert Hall (1867-
71). These buildings were matched in France, for instance, by the Bibliotheque National
(1868), Les Halles (1854-68) and the Bon Marche Department Store (1867-78); and in
America by the dome of the Capitol in Washington (1856-64).
Throughout this period most buildings, particularly those of more than one storey,
depended on masonry walls for stability, whether or not the floors and roof were of iron.
The route to full structural framing in iron or steel is uncertain. It is often stated that the
Home Insurance Building in Chicago of 1884-85 was the first fully framed tall building
which formed part of a continuing development. Perhaps the earliest example of a stiff-
jointed frame was Godfrey Greene's four-storey Boat Store at Sheerness of 1858-60. The
Great Exhibition Building in London of 1851 and the Chocolat Menier Factory outside
Paris of 1870-71 have also been claimed for this 'first', but they both had diagonal bracing
and, anyway, had no apparently direct influence on the multi-storey steel construction of
today.
6. THE STEEL PERIOD (1880-PRESENT DAY)
Steel is not only stronger than wrought iron, but being produced in a molten state made
larger rolled or forged units practicable. However, it is not easy to identify which is which;
for several decades, steelwork was fabricated by riveting in the same way as wrought iron
and, when riveted, the two look almost exactly the same. The Forth Bridge in steel and the
Eiffel Tower in wrought iron, were completed at almost exactly the same time (1889-90).
Looking at them, who could tell the difference?
Figure 8 shows how steel took over in quantity from wrought iron in Britain. Figure 9
shows how the proportion of open-hearth steel increased until it had all but cornered the
market by 1920. The biggest incentive for change to steel lay in the ship-building industry.
Lloyds Register allowed steel plating of 4/5 the thickness of wrought iron and, by 1908,
Lloyds was insisting that all steel for shipbuilding should be produced by the open-hearth
process.
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In bridges, the steel period was mainly one of increasing size and span. Here the initiative
shifted away from Britain mainly to America where the need for major bridges, was
greatest at this time. All the great suspension bridges up to 1945 (Golden Gate, George
Washington, Transbay, etc.) were built of riveted steel with spun cables of high tensile
steel wire.
In buildings the 'Skyscraper' came of age in steel, again with the initiative mainly in
America. Long span roofs also took a leap in scale with steel both in France and America.
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First there were the great three-pin arch structures over the Philadelphia railway stations of
1893 (79 and 91 metre spans) followed by the Galerie des Machines for the 1889 Paris
Exhibition of 111 metres span - over 50% up on St Pancras. These spans, in turn, have
been dwarfed by the post-war domes over sports arenas. The span of the Louisiana
Superdome of 1975 at 207 metres is more than 3 times that of the Albert Hall.
The one big change in technique with steel was the introduction of welding, mainly from
the 1930s, although possibly earlier. Today, the rivet is as dead as the production of
wrought iron. Now welds and bolts dominate all construction in steel.
In all fields, new developments tend to follow new needs and this certainly seems to have
been the case with bridges. Since the Second World War, most new thinking on
suspension bridges, especially aerodynamic design and weight-saving, has been in Britain
while Germany has led the field on the design of cable-stayed bridges.
7. PRESENT TECHNIQUES AND FUTURE
PROSPECTS
One of the most noticeable moves in construction in the last ten years, in Britain certainly,
but it seems elsewhere in Europe as well, has been towards a revival of structural steel for
bridges and buildings. Fashions change in constructions, as in clothing, and so do needs
and costs. It is, thus, interesting to look at some of the recent variants on normal structural
steel and at rival materials to see how they have fared and to speculate on what may
happen in the future.
Weathering steel (unpainted with stabilised corrosion) and exposed steelwork fire-proofed
by water in hollow sections are both innovations of the 1960s but neither shows signs of
wide adoption. On the other hand, stainless steel, although in itself much more expensive
than mild steel or even high tensile steel, is being found to be increasingly worthwhile
when the cost of maintenance is considered.
Plastics have yet to make any significant impact except as a protective coating or for
architectural trim.
Aluminium was once thought to be a dangerous rival to structural steel but, so far, it has
made little impact in bridge or building structures. Reinforced concrete - still dependent on
steel - has been a strong and growing competitor of fabricated steelwork since the 1890s,
largely because of its in-built fire resistance, helped in the 1950s and 1960s by an
architectural desire to 'expose the structure'. This trend is now being reversed and, since
1980, there has been a vigorous rebirth of structural steel. The increasing use of structural
steel has been encouraged by the pursuit of 'fast-track' construction and the realisation that
reinforced concrete is not a maintenance-free material. There has also been a swing in
taste from visually expressed concrete to 'high tech' styling or to the complete wrapping of
buildings in glass or masonry.
Future developments with structural steel in buildings are likely to be associated with fire
protection. Thin intumescent coatings which froth up when heated and form a protective
layer, are becoming still thinner - more like paint - but the need for such protection may be
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substantially reduced by the development of fire engineering. This development could lead
to a new era of exposed steelwork with increasing attention to the shape and form of
members and the appearance of joints. Castings of steel or ductile iron could well be in
demand once more.
8. CONCLUDING SUMMARY
The use of iron and steel in structures evolved through development in the
production and properties of the three ferrous metals, cast iron, wrought iron and
steel.
Cast iron is formed into its final shape from molten metal a liquid which is poured
into a mould and solidifies. Wrought iron never reaches a fully molten state and is
shaped by rolling and forging. Mild steel can be cast as well as rolled but has a
lower resistance to corrosion than wrought iron.
Iron has been known and used for more than three thousand years but it is only in
the last 250 years that new production methods have allowed the large scale use,
first of cast iron, then wrought iron and finally steel. Cast iron was widely used in
bridges and buildings in the period between 1750 - 1850.
Wrought iron became popular during 1850 - 1900 allowing the construction of
many novel bridges and building structures of increasing size and span.
Steel came into increasing use from about 1880, and being stronger than wrought
iron, has been used to build even larger structures. The introduction of welding of
steel was a major innovation in connection techniques which facilitates the wider
use of steel.
For the future, stainless steel is being found to be increasingly attractive despite its
greater cost. The development of fire engineering may lead to a new era of
exposed steelwork together with a wider use of coatings of steel or ductile iron.