Alternative University

Transportation Infrastructure

Florida Keys Hurricane 1935

Bridge Design

Dikes and water blocking causeways must not be built for the purpose of transportation across estuaries. Modern affordable bridge construction should be used for that, e.g., low-cost long-span box girder bridges.

The most affordable box girder bridges have room for two lanes of traffic, and space for pulling over. These can be built of concrete, in sections on shore with portable prestressing rigs, and floated into position with a barge crane. Newer methods allow transport of the bridge segments over previously installed bridge segments (instead of being floated out).

These affordable bridges can be high enough to clear storm surges, with longer spans to allow more laminar flow of water beneath the bridge.

Figure 1:  Section of a prestressed concrete box girder bridge deck, used in the Long Key bridge built in 1982, with room for two lanes of traffic (one lane in each direction) and room to pull over. Each section was precast on shore in a prestressing rig against previous section for better installation fit. This design was engineered to use longer spans than was built, but the spans were shortened at the request of state officials to line up with the old viaduct (which should have been removed instead).

Figure 2:  Example installation (not in the Florida Keys) of a prestressed concrete box girder bridge section, for multiple lanes of traffic in each direction, showing how the sections fit together.

Figure 3:  Prestressed concrete box girder bridge, built in 1985, with one lane of traffic in each direction, crossing Mjøsa (lake in Norway), similar to the Long Key bridge but with longer spans (bridge info). This type of bridge can be used in estuaries to allow laminar water flow and storm water flushing.


In temperate and boreal climates, biomaterials (wood, hemp, etc.) could be used to replace some of the concrete in a box girder bridge, reducing life cycle emissions of the bridge.

Figure 4:  New box girder bridge design that will soon be built at Mjøsa, with two lanes of traffic in each direction. Concrete usage will be reduced by using wood instead of concrete for much of the box girder construction.


 1.  Reyn O’Born, “Strategies and solutions for including life cycle emissions in planning Norwegian road infrastructure”, May 2019. pdf

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Bridge Design (this page)
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