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The design of steel and composite bridges: part 2

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The design of steel and composite bridges: part 1


To introduce steel and composite bridges. To discuss bridge components and structural systems. To describe the common types of steel bridge - plate girder, box girder, and truss girder bridges.


The fundamentals of bridges are described. The basic components of a bridge structure are given, and the types of bridge structural systems are discussed in the context of their uses. General aspects and deck systems of steel bridges are described prior to discussion of plate girder, box girder, and truss girder bridges.


Bridges have been built by man in order to overcome obstacles to travel caused by, for example, straits, rivers, valleys, or existing roads. The purpose of a bridge is to carry a service such as a roadway or a railway.

Bridges play an outstanding role in structural engineering, deserving the denomination of 'ouvrages d'art' in latin languages. Some bridges are also considered 'iconic' and symbols of the city in which they have been built (e.g. Golden Gate Bridge in San Francisco, Viaduc de Millau near Millau Town, Ponte Vecchio in Florence).

The choice between a steel or composite bridge and a concrete bridge (reinforced concrete or prestressed concrete) is a basic decision to be taken at a preliminary design stage. Several technical factors influence this decision. For example:

  • spans required
  • execution processes
  • local conditions
  • foundation constraints

The decision should be based on comparisons of:

  • structural behaviour
  • aesthetics
  • solution competitiveness and, more and more commonly, environmental indicators and LCA

When comparing costs, both initial costs and costs associated with maintenance during the life of the structure should be considered. The time required for execution, which in steel bridges is generally shorter than in prestressed concrete bridges, may also influence the decision.

100 years ago, concrete bridges could not compete with steel bridges for medium and long spans due to the lower efficiency (strength/dead load) of concrete solutions. Until the 1980s with the development of prestressed concrete, it was not a straightforward decision to decide between a concrete and a steel solution for medium span (about 40 to 100m) bridges. Even for long spans between 200 and 400m, where cable stayed solutions are generally proposed, the choice between a concrete, steel, or composite bridge superstructure was not an easy task.

The choice between a steel and a concrete solution is sometimes reconsidered following the contractors' bids to undertake the bridge works.

Generally speaking, steel and composite solutions have the following advantages when compared to concrete solutions:

  • reduced dead loads
  • more economic foundations
  • simpler erection procedures
  • shorter execution time
  • easier inspection and maintenance procedure
  • confirmed durability and robustness

A disadvantage of steel when compared to concrete was the maintenance cost for the prevention of corrosion. However, it is now recognised that concrete bridges also need inspection, maintenance, and repair.

Although maintenance costs and aesthetics play a significant role in the design decision, the initial cost of the structure is generally the most decisive parameter for selecting a steel or a concrete bridge solution. Solutions of both types were generally considered, at least at a preliminary design stage.

With the development of steel and concrete composite bridge solutions during the last 30 years in Europe and the demonstration of the undisputable competitiveness of this technology, this is generally no more the case. A composite steel and concrete solution has become the preferred solution, unless there are specific local constraints.

In Figure 1, the principal components of a composite bridge structure are shown. 

The two basic parts are:

  • the substructure
  • the superstructure

The former includes the piers, the abutments, and the foundations.

The latter consists of the deck structure itself, which supports the direct loads due to traffic and all the other permanent and variable loads to which the structure is subjected.

The connection between the substructure and the superstructure is usually made through bearings. However, rigid connections between the piers (and sometimes the abutments) may be adopted, particularly in frame bridges with tall (flexible) piers.

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