1. INTRODUCTION
The historical development of bridges is the field which best illustrates the progress of structural engineering from ancient times up to the present century. In particular, the development in steel bridges equates with the progress in structural analysis, theory of strength of materials, and materials testing, since all of them were increasingly stimulated by the need for bridging larger spans and building more economically with the new construction method. Fortuitously, mechanics and mathematics had reached the threshold of modern engineering science just when the technology of constructional steelwork was being developed.
However, at the time when the new material - iron, and later steel - was ready for use in larger structures, there already existed a quite highly developed technology in bridge building, namely for bridges made of timber and bridges made of stone. During the years 1750 - 1770 approximately, a new method of coke smelting produced larger amounts of iron at a cost which provided the basis for application of iron in engineering practice.
It is important to mention that the technologies of bridge building at that time were based on individual intuition of outstanding 'masters' and on the experience passed down through the generations rather than on rules of mechanics and mathematics. The significance of preserving the knowledge of bridge building and of extending it was closely connected with military purposes and the interests of trade in ancient times. The Romans even established a separate group - the 'pontifices' (bridge makers) - who later were raised into the rank of priests, headed by the 'pontifex maximum', which was also one of the titles of the Roman emperors. Similar reasons motivated the French King Louis XV to support the new engineering school (Ecole Royale des Ponts et Chaussés in 1747).
Thus, the building of steel bridges was founded at the beginning on the then well-tried principles and construction methods of timber and stone bridges. Stone bridges provided the arch type while wooden bridges demonstrated mainly fine-structured trusses. According to the typical material properties of cast iron - the first type of iron available - iron bridges were first built as arches. Later, when steel was available, which is capable of acting in tension, various structural systems were developed on the basis of the principles of wooden trusses. Due to the superior material properties of steel and the advantages of the new construction method, a rapid development of bridge structures led to a large variety of efficient, inventive systems for any kind of span.
In this lecture, the history of steel bridges is subdivided according to three types of bridge:
- arch bridges
- beam structures, including trusses, plate/box-girder bridges, and all kinds of supported bending structures, such as cable-stayed bridges and tied arches
- suspension bridges
There is, of course, much overlap in chronological order concerning the three types of bridges through the period of time considered. However, this classification seems to be most appropriate to an engineer's understanding, being based on the main bearing behaviour of bridges rather than on aspects of shape or statical system.
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