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New Bridges

Labor Day Hurricane 1935

Page 5:  New Bridges

New prestressed concrete bridges have been built parallel to the viaducts in the Florida Keys, and are now in use for vehicular traffic instead of the viaducts. The new bridges cause much less turbulence of water flow, and are recommended for use in estuaries.

Figure 5.1:  Dante B. Fascell Long Key Bridge.

The Dante B. Fascell Long Key Bridge, shown above, replaces the Long Key Viaduct, although the viaduct has not yet been removed. In the photograph of Fig. 5.1 the viaduct is hidden behind the new bridge (you can see its shadow underneath).

Dante B. Fascell (pronounced FAH–SELL) was a popular member of the U. S. House of Representatives (D–Miami).

The Fascell Bridge is also known as the Long Key Channel Bridge or simply Long Key Bridge. The 3.7 kilometer (2.5 mile) long bridge crosses the Long Key Channel, and is of box girder construction, using precast prestressed concrete sections.

Figure 5.2:  Box girder section of the Long Key Bridge.

Using a portable assembly line, each section was economically precast and prestressed against the previous section, on the side of the road near the bridge, barged out, and lifted into place with a floating crane.

Connecting the sections essentially forms a long road on top of a long square tube. The square tube actually has a trapezoid cross section, with a wider top than bottom. The trapezoidal square tube underneath the road is narrower than the road, with the road sticking out each side (Fig. 5.2).

“A total of 370 tons (336 metric tons) of post-tensioned strands were required for the structure, with an additional 150 tons (136 metric tons) of pretensioned strand in the top slab. Combined, these quantities represent only 2.3 lbs of prestressing steel per sq ft (10.9 kg/m²) – a very economical value for bridges and an indication of the significant savings offered by the system.”
Thomas M. Gallaway, Design Features and Prestressing Aspects of Long Key Bridge (PDF)

The length of each span between bridge piers depends on the size of the trapezoidal box. Longer individual spans between piers require that the trapezoid be larger.

The size of the trapezoid that was used in the Long Key Bridge was actually for longer spans between piers than was constructed, and would have required less piers if constructed as designed, further reducing costs by building less piers.

However, the State of Florida asked for the spans to be shortened to line up with the viaduct instead of removing the viaduct. That was a mistake. The Fascell Long Key Bridge as now built (its current box trapezoid size) would have allowed longer individual spans and less piers if built as designed.

Following are other examples of box girder prestressed concrete bridges, in other Florida estuaries (not the Florida Keys).

Figure 5.3:  Mid-Bay Bridge.

Figure 5.4:  Acosta Bridge.

The Long Key Bridge (Fig. 5.1 and 5.2) and the Mid-Bay Bridge (Fig. 5.3) are both two-lane highways (one lane each way) with room to pull over on the bridge deck.

Bridges with more than two traffic lanes cost more and require two boxes (Fig. 5.4).

For many estuaries, a two-lane highway is enough, and costs less.

We look again at the new Channel 5 bridge in the Florida Keys, which is a two-lane highway with room to pull over on the bridge deck.

Figure 5.5:  Fishing at the Channel 5 Bridge.

Figure 5.5 shows the new Channel 5 bridge behind the old viaduct that has not been removed yet. Individual spans of the new bridge are much longer than each span of the viaduct.

“The precast, post-tensioned box girder design was used since it was cheaper than traditional concrete beam designs.”
“The New Channel 5 Bridge”, Bridge Mapper

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