ICE - Institution of Civil Engineers - Virtual Library

The great majority of highway bridges are of small to medium spans for which plate-girder construction is usual, and it is with these that the Paper is mainly concerned.

Factors influencing bridge design generally and welded construction in particular have been briefly surveyed. They include a comparison of the live loads for designing highway bridges in different countries as an explanation for the differences in economy of construction encountered. Reference is also made to the use of welded construction to overcome certain problems concerning the alignment and clearances of bridgework. Since this subject would not be complete without a mention of bridge decks suitable for bridges of this type, the more commonly used designs are included.

The welding described is based on good metal-arc welding practice in Great Britain and, associated with this, the characteristics of steels suitable for welded bridgework. Past experience and research in the construction of welded bridges is referred to because it has affected design practice and, in turn, problems connected with fabrication. Since the preparation of economical design is closely related to fabrication, this has been described more fully, together with remarks on testing procedure and inspection.

The Paper also describes how the available methods of erection will influence the choice of design and deals with the maintenance of steel bridges, the economics of welded construction, and their influence on the trend towards improved aesthetic merit in bridge building.

According to the plastic theory, the collapse load of a ductile structure in the absence of instability due to axial stresses may be predicted by reference solely to the requirements of statical equilibrium. Since it is found unnecessary to consider the behaviour of the structure during the increase of the load to the collapse value, the assumption that the plastic theory will, in all cases, lead to a correct estimate of the collapse load and the mode of failure demands a rigorous proof. That assumption is vindicated in this Paper by reference to four formal propositions which are of fundamental importance in the plastic theory of structures.