Orthotropic deck

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Fabricating an orthotropic deck section
Fabricating an orthotropic deck section

An orthotropic bridge or orthotropic deck is one whose deck typically comprises a structural steel deck plate stiffened either longitudinally or transversely, or in both directions. This allows the deck both to directly bear vehicular loads and to contribute to the bridge structure's overall load-bearing behaviour. The orthotropic deck may be integral with or supported on a grid of deck framing members such as floor beams and girders.

The same is also true of the concrete slab in a composite girder bridge, but the steel orthotropic deck is considerably lighter, and therefore allows longer span bridges to be more efficiently designed.

The stiffening elements can serve several functions simultaneously. They enhance the bending resistance of the plate to allow it to carry local wheel loads and distribute those loads to main girders. They also increase the total cross-sectional area of steel in the plate, which can increase its contribution to the overall bending capacity of the deck (i.e. the deck plate acts as a top flange in a box or I beam girder). Finally, the stiffeners increase the resistance of the plate to buckling.

The Golden Gate Bridge now has an orthotropic deck
The Golden Gate Bridge now has an orthotropic deck

Some very large cable-supported bridges (cable-stayed bridges and suspension bridges) would not be feasible without steel orthotropic decks. (The steel deck-plate-and-ribs system may be idealized for analytical purposes as an orthogonal-anisotropic plate, hence the abbreviated designation “orthotropic.”) Thousands of orthotropic deck bridges are in existence throughout the world. Despite the savings and advantages (up to 25% of total bridge mass can be saved by reducing deck weight, as the weight reductions extend to cables, towers, piers, anchorages, and so forth), the US has only about 60 such bridge decks in use as of late 2005. Many of them are in California, including the San Mateo-Hayward Bridge (1967) one of the first major bridges in the US to be built using an orthotropic deck.

However, orthotropic decks are common in railway bridges, as they permit a very shallow deck depth which reduces the steepness of approach gradients and hence their costs. The form is also widely used on bascule and other moveable bridges where significant savings in the cost of the mechanical elements can be made where a lighter deck is used.

The unpopularity of the orthotropic deck relates mainly to its cost of fabrication, due to the amount of welding involved. In addition, it must be prefabricated rather than assembled on site, which offers less flexibility than in-situ concrete decks. Orthotropic decks have been prone to fatigue problems, and to delamination of the wearing surface, which like the deck is also often of a very thin material to reduce weight.

It is possible to refit a bridge originally designed with a concrete or non structural deck to use an orthotropic deck. For example San Francisco’s Golden Gate Bridge completed in 1937, the landmark bridge spanning the bay between San Francisco and Marin County, California, originally used a concrete deck. Salt carried by fog or mist had reached the rebar, causing corrosion and concrete spalling. In 1985, the Golden Gate Bridge was restored using steel deck panels. The project not only restored the bridge to prime condition but also used fewer materials and reduced the deck weight by 11,160 metric tons (12,300 tons).

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