The first bridges must have consisted of nothing more than a log thrown over a stream. In time, more artifice was involved as man made simple beam bridges, usually in timber, sometimes in stone. Under the Normans, the local community was responsible for the building and maintenance of a bridge - a duty which was frequently neglected and thus taken over by the Church which employed the technology it acquired with the experience of building churches and cathedrals. The comming of canals in the 18th century and the railways in the following century produced a new breed - the civil engineer and the building of bridges became a very sophisticated occupation.
Few if any bridges were constructed of stone during the Saxon period before 1066. The Noram Conquest was followed by a tremendous burst of building activity - initially castles to control the new realm and cathedrals, then churches and other structures like bridges. This great age of building saw the development of skills in the bridging openings with arches. First the semi-circular Norman arches of the Romanesque type, by the 12th century, the pointed Gothic arch appeared.
Under the Normans, the local community was responsible for the building and maintenance of bridges. The Church stepped by encouraging the execution of these frequently neglected duties as acts of piety. In time, the Church with its wealth of construction experience gained in the building of ecclesiastical buildings stepped in to carry out the works, raising funding for the projects by the sale of indulgences. It was inevitable that advances such as the pointed Gothic arch would be incorporated into bridge-building.
Whilst the semicircular arches are easy to set out and are intrinsically strong, they throw a great deal of the weight horizontally and, where bridges or any other structure is concerned, are very susceptible to failure should there be any movement of the foundations. The pointed Gothic arches exert less horizontal force and, although visually disfugured, remain structurally stable even if there is a slight shifting of the foundations.
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The same technique was employed in bridge-building as in the construction of the vaults of stone roofed buildings; a series of intermediate ribs are first constructed, the rest of the arch vault is then inserted to fill the span between the ribs. |
Early medieval bridges are typified by semi-circular arches of small span or narrow pointed arches, both on massive piers which are protected from possible undermining by the scouring action of the flow by pointed cutwaters. The large size of the piers enabled the arches to be constructed a single span at a time minimising the use of skilled labour and allowing the re-use of temporary centering.
