Bridge technology

Bridges are often long and slender; thus the effect of lateral loading may become very significant and oscillations induced by the wind may cause unacceptable motion or even collapse.

Wind is a particular problem with suspension and cable-stayed bridges where there is usually a lightweight deck hanging from a primary structure that has limited lateral rigidity. Oscillations induced by the wind may cause unacceptable motion or even collapse as in the infamous case of the 853m span Tacoma Narrows Bridge, which collapsed in a relatively light wind in its year of construction, 1940. Modern bridge decks are designed to an aerodynamic profile to reduce lateral loads and to ensure that wind induced oscillations are controlled. Alternatively, a rigid truss system is used to support the deck, to transmit lateral wind forces to the pylons and control oscillation.

Many bridges span over water and/or are subject to the effects of de-icing salts. This can lead to severe corrosion of the steelwork unless adequate protection is applied. The ease of future maintenance should be considered at the initial design stage.

Corrosion protection can be provided in the form of a conventional multi-coat paint system, high-build coating system, by the use of weathering steel and/or by enclosure to reduce contact with atmospheric contaminants. Most bridges are designed to last for at least 120 years, therefore, adequate provision should be made for the inspection and maintenance of the finishes. Access is highly important in this respect as corrosion often occurs in hidden locations (such as around the bearings when water penetrates through expansion and movement joints). The Forth Rail Bridge is a good example of this maintenance problem as it requires almost continual painting to prevent excessive corrosion.

Basic painting systems are based on the use of conventional blast primers, primers, undercoats and finishes.

For highway bridges, the systems specified by the UK Highways Agency involve the application of up to six coats to produce a total dry film thickness of 200-300 microns following appropriate surface preparation. Selection of a suitable paint system is determined by the environment in which the bridge is situated (marine or inland) and the accessibility of the component in the bridge (ready access or difficult access). For example, in a "marine" environment with "ready access", painting after blast cleaning would involve:

Workshop applied

• 1 coat zinc phosphate acrylated-rubber/alkyd blast primer
• 2 coats zinc phosphate acrylated-rubber/alkyd blast undercoat
• 1 coat micaceous iron oxide acrylated-rubber undercoat

Plus Site applied

• 1 coat acrylated-rubber undercoat
• 1 coat acrylated-rubber finish

giving a minimum total dry film thickness of 300 microns.

High-build painting systems, based on 2-pack, chemically cured resins have been developed mainly for steel protection in the severe environment of the North Sea oil and gas extraction structures but this technology may also be applied to the protection of steel bridges.

These coatings are thicker (usually having a dry film thickness of 1000 microns) and are applied in one spayed coat after blast cleaning of the steel. Slightly more expensive than conventional systems the high-build systems typically use materials such as elastomeric urethane and glass flake epoxy polyester coatings.

Weathering steel is an alternative that contains up to 3% of alloying elements, which, on exposure to air, form an adherent protective oxide coating.

During the early part of their life, weather resistant steels corrode in a similar manner and at a similar rate to mild steels. As the protective layer develops, however, the corrosion rate falls to a low level and depending on the environment within which the steel is placed, a brown to almost black, stable oxide patina is produced after about two years. Corrosion products from the weathering steel may stain adjacent surfaces such as concrete piers and abutments, therefore, suitable drainage should be provided. The thickness of plates within the structure should be increased by 0.5 to 2mm depending on the degree of exposure to corrosive environments to allow for loss of material. Weathering steels are not recommended for highway bridges in certain more severe corrosive environments.

The concept of corrosion prevention by enclosure of steel relies on the fact that clean steel does not corrode significantly at relative humidities up to 99% if environmental contaminants are absent. Beams sheltered by a concrete deck may, therefore, be enclosed in rigid plastic or other sheeting.

Enclosure reduces exposure to the atmospheric contaminants that produce corrosion. Results of tests suggest that painted steel within the enclosures may remain maintenance free for decades and that the life of unpainted and weathering steel will also be extended.