Developments in Europe
The story of the development of the modern clear span building really begins with the first major long span iron structure in Europe, the bridge at Ironbridge.
It seems appropriate to begin the story of the development of the clear span building with the first major long span iron structure, the bridge at Ironbridge. The bridge, spanning 100.5ft. (30.63m), with a rise of 45ft. (13.72m), was designed by Thomas Pritchard, a Shrewsbury architect, and built by Abraham Darby, the iron founder, between 1775 and 1778. It demonstrated, for the first time, that considerable distances could be spanned by a relatively light iron structure and, as a consequence, began a line of technological development which has spawned today's clear span building structures.
Ironbridge marked the turning point from it being regarded as: "secondary material used only occasionally for cramps, tie-rods and hinges, to becoming the dominant component of...some complete buildings". However, we should note that (mirroring the story of the Roman invention of concrete) developments had taken place in China centuries before what we refer to as the revolution at Coalbrookdale. Iron suspension bridges were actually developed in China before the 8th Century and iron was also used in the construction of Pagoda buildings from at least the Sung Dynastic period.
The early developments in iron and steel and their structural uses were largely in Europe.
In the eighteenth and nineteenth centuries the most significant advancements which took place in the technology of iron and steel production, and in structural design theory related to iron and steel structures, took place in Europe. Although the technology of iron and steel production did advance, making iron available in greater quantities, and at lower cost, the quantity of iron actually used in building remained fairly constant up until 1850. Most designers preferred to be cautious, working with the materials which they had come to be familiar with: brick and timber. There were, however, notable cases of experimentation and invention by certain designers who were determined to respond to the availability of this new material and to use it in clear span buildings.
One of the earliest uses of wrought iron for roof construction was by Victor Louis.
In 1786 the celebrated French theatre builder Victor Louis designed a 26m span roof for the Theatre-Francais which was both elegant and daring: qualities which characterised French iron structures for more than a century afterwards. This was one of the earliest uses of wrought iron for roof construction.
The form of the roof is a tied arch spanning about 26m, with horizontal iron ties serving to counteract the horizontal thrust produced by the arch. Five of the fabricated elements are bowed to become larger at their centre than at their ends. For the smaller elements this appears to have been done to give good resistance to bending, and in the case of the larger element to give enhanced resistance to buckling. In this respect this structure is particularly sophisticated since there was, at this time, no formalised theory indicating the advantages of increasing the depth of cross-section of structural elements (to increase the second moment of area). In contrast it is a little surprising, that in such an advanced structure the fabricated elements are not triangulated to form more efficient trussed beams and arches.
Many of the early clear span iron structures borrowed ideas and principles from contemporary masonry and timber construction.
The stone arch was copied in many cast iron bridges, in some cases by a literal replacement of the stone voussoir by the cast iron voussoir. Timber structures were emulated in a similar way. The wooden cupola of the Granary at Paris, which had been destroyed by fire in 1802, was replaced in 1811 by an iron structure but the new roof structure was as far as possible a direct replacement of the original timber structure and the construction techniques effectively the same as that used in timber .
During this period the French were at the forefront of technical, educational and scientific understanding and it was not until the mid 1800s that efforts to address this problem were made in Britain.
It is no coincidence that many innovative iron structures were built in France in the first half of the nineteenth century. The French were at the forefront of technical, educational and scientific understanding and it was not until the mid 1800s that efforts to address this problem were made in Britain (it was not until 1849 that a paper describing the analysis of trusses appeared in the UK). But the rapid accumulation of technical knowledge in the mid 1800s led to a further change: specialisation was becoming inevitable. Being a universal designer/engineer became increasingly difficult.
Consequently we see that much of the pioneering and brave invention in long span building took place between 1780 and 1850. Although important steps were made in the latter half of the nineteenth century it is in the seventy years before where we see the giant leaps made.
