Federal Highway Research Institute

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Composite steel bridges

Composite steel bridges comprise main steel girders and overlying carriageway pavement made of concrete. Shear connectors are used to form the link between steel and concrete. Such connectors have a function similar to nails on wooden structures. They are welded on the steel girders and serve to firmly bind the steel and concrete after the latter has hardened.

The picture shows a bridge with open cross-section Bridge with open cross-section

Two different cross-sections are usually employed on composite-steel bridges: Steel I-profiles (open cross-section) are used for short and medium spans. Steel box girders (closed cross-section) are used for long spans such as those of valley bridges. One benefit of steel box girders is their greater ability to absorb longitudinal torsion. For this reason, such girders are often employed on bridges which are curved or whose supports need to be awkwardly positioned.

The picture shows a bridge with closed cross-section Bridge with closed cross-section

A major advantage of composite-steel structures is their simple and quick assembly. As soon as the steel girders rest on the supports, traffic routes beneath the bridge can be released again. Completion of the concrete carriageway pavement does not require any further assembly struts which would impair traffic, so that this design is especially suitable for traversing existent roads and railways.

A characteristic detail of composite-steel bridges is the link from their main steel girders to an abutment or cross member. Cross members are usually made of steel but can also comprise concrete. The main girders are subjected to very high strains and stresses which must be transmitted to the cross members.

The Federal Highway Research Institute prepares guidelines and standards concerning composite-steel design and investigates the practical outcomes of new regulations. The institute also sponsors basic research toward new developments in composite-steel design. For instance, the institute is trying to ascertain whether upper steel flanges are dispensable. Tests here involve a replacement of such flanges by horizontal shear connectors welded to the web. This saves material and lowers construction costs.

Thanks to their light weight, composite-steel bridges can be made very slim. In combination with appropriate colour schemes, this makes for attractive bridge design.