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A Pratt Truss is named after Thomas and Caleb Pratt who patented the design in 1844. The diagonal forces, except for the very center ones hold the bridge’s tension, while the shorter vertical members handle the bridge’s compression. This design allows for thinner diagonal members and thus a more economical construction cost. Originally built with a combination of wood and iron, the Pratt Truss has also been able to make the transition from wood to iron and now to steel construction.

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Bridge Design

Public input clearly expressed the desire to maintain the look and feel of the existing swing span bridge, which has become an icon for the area. The rehabilitated bridge is being designed within the existing bridge right-of-way and vertical profile.

Rendering of rehabilitated bridge as viewed from the Intracoastal Waterway
Rendering of rehabilitated bridge as viewed from the
Intracoastal Waterway

The Swing Span

The swing span is a center-pivot modified Pratt through truss. It matches the appearance of the original swing span, with the exception of the roadway widening and the addition of the new sidewalk. The elevation of the bottom and top chords of the truss are at the same elevation of the existing chords to maintain the bridge appearance and navigational channel clearances. The portal bracing will be reconfigured to allow for additional vertical clearance for motorists crossing the bridge.

The Bridge Deck

The width of the new bridge deck is constrained by both the existing foundations and the navigational channel of the intercoastal waterway. However, the new layout is much improved over the current bridge. The new cross section will include two 14-ft shared use travel lanes. There will be a full width sidewalk (5.5 feet) on the harbor side of the bridge which closely aligns with the existing pedestrian/bicycle path along the causeway.

The bridge deck for the swing span uses a lightweight concrete and steel grating. The approach span deck and sidewalk is a conventional reinforced concrete deck using lightweight concrete. Grooving of the deck improves drainage run-off, and grinding of the deck enhances the smoothness of ride for the traveling public. The improved sidewalk for the swing span has solid aluminum deck panels and the steel railing system is similar in appearance to the existing bridge.

The Control House

The new control house is being located above the roadway at the same location as the existing. Its octagonal design is similar to the existing control house. The new space is larger than the existing space to maximize the controller’s view corridors, enable the addition of a new bathroom, and allow for new electrical equipment space requirements.

Approach Spans

The six approach spans on either side of the swing span are being replaced and will be supported by new seismic isolation bearings mounted on the existing concrete piers.

Roadway Improvements

The approach roadway is being improved for approximately 400 linear feet on each end of the bridge. Transitions from the existing multi-use path are being constructed to allow bicyclists and pedestrians easy access to the new bridge sidewalk.

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