Print Case Study
J. F. White Contracting Company
June 01, 2007
When an existing drawbridge had to be replaced on Water Street, J. F. White Contracting Company needed to provide temporary pedestrian access to the shops and ferry landing served by the old structure. They chose a retractable Continental® truss bridge.
The design called for the structure to be built in two 60’ sections. One six foot wide section rests in the channel on steel piling and is anchored to the shoreline. The other, slightly narrower span nests inside of it and includes a track welded to its undercarriage. This track rides on steel wheels that are an integral part of the wider section, and retaining wheels above the track keep the section securely in place. The wheel and rail system allow the inside span to be extended over the channel and reach the opposite shore.
The bridge had to be operational during severe weather and power outages. To make this possible, a redundant drive system was designed to extend and retract the span. A powerful electric motor is the primary drive unit and a pneumatic system is the back up. If electrical power would fail, the bridge operator can quickly switch to the generator powered pneumatic system.
The bridge will be in operation for only about nine months. Once J. F. White completes the new Water Street Bridge project, the temporary pedestrian bridge will have served its purpose and will be removed. In the interim, pedestrians will have direct access to the retail shops and the Martha’s Vineyard Ferry without the lengthy detour around Eel Pond. Boaters will also have easy access to Nantucket Sound, with a bridge tender on site to open the new structure for passage.
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