Introduction

There are a number of strands to the story of the development of the clear span building. Many early developments took place in Europe where designers, in trying to use iron in the structure of glasshouses, exhibition halls, market halls and railway stations, developed solutions which we have adopted and developed in today's clear span buildings. In the early twentieth century the industrial developments in the USA meant that this became a breeding ground for further innovation and advancement of the technology of clear span building in steel.

But it is interesting to reflect on how and why the structural developments described here took place. It is easy to be deceived into thinking that the development of the clear span structure, (and other structural developments) took place in a definable and rational order. This is not the case. Nor is it true that the appearance of a new structural form heralded the universal adoption of that form by designers. For example one of the first waterfront sheds at Liverpool docks had a roof which was wholly iron frame. However the subsequent sheds built there reverted to the earlier form which combined together wrought iron and timber.

The idea that economics is the principal dictator of structural choice is also wrong. In this respect it is interesting to compare the cost of similar structures. When Barlow described his roof of St. Pancras Station he mentioned that the contract price for Lime Street Station was only £17/10s (£17.50) per ton compared with £24/5s (£24.25) per ton for Cannon Street and Charing Cross Stations. There are clearly other practical considerations in addition to cost which dictate the use of a particular material or form.

In long span open roofs architectural considerations may well be paramount, either because the desired form is available only in one material or because the material or form is desirable in itself.

With these factors in mind we can consider the changes and advances in the development of clear span iron roofs. These changes are rarely clear-cut and are often related to technical advances in areas outside the building industry.

It is particularly interesting to look at the developments which have taken place in long-span roof design for two reasons.

·        Firstly to gain a better understanding of the buildings which we have inherited from previous generations.

·        Secondly those buildings can prove to be important generators of new design ideas.