Bridge Loads

A bridge may be defined as a structure used to carry loads over an opening,which may take the form of a valley or stream, a road or railway. One way to cross a valley is to build an embankment which effectively closes the opening.

Such a structure is not a bridge, and it is important to recognise that the opening crossed by the bridge generally performs a function in itself, which must be maintained.

The loads mentioned in this definition include the weights of trucks and pedestrians for a road bridge, and of locomotives and rolling stock for a railway bridge. These will be called primary loads, for they express the purpose for which the bridge was required.

 There are other senses in which the term ‘load’ may be qualified, such as in the terms ‘service’, ‘design’ and ‘legal’ loads. The second of these is of paramount importance in this book, for before a bridge can be built it is necessary for the designer to choose design loads that are used for the selection of member sizes. However, the choice of design loads cannot be separated from a study of the other two: the service loads, applied to the bridge during its service life, and the legal limits, intended to govern these loads.

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