Suspended Span Deck Replacement for the Throgs Neck Bridge in New York. Courtesy Wikimedia.
As part of a joint venture, Thornton Tomasetti is progressing to final design for the replacement of the deck system on the suspended spans of the Throgs Neck Bridge. The existing deck system, a concrete-filled grid with relief joints at every sixth-floor truss, is quickly approaching the end of its useful service life. In addition, the relief joints are leaking, causing corrosion on the stringers, floor trusses, and lateral bracing below. Two jointless deck options were considered: an orthotropic deck system and a “floating” grid deck system supported on continuous stringers. The orthotropic deck system is being progressed to final design.
In addition to the redecking design, load ratings were prepared to determine strengthening requirements for the floor and stiffening trusses, which included the option for a future seventh lane on the bridge. Ancillary improvements include modular joints to replace the existing finger joints, a new steel median barrier and parapets, a new fire standpipe system, and new drainage, LED roadway lighting, and signage.
A preliminary wind study and analysis were conducted to check the bridge stability against a 10,000-year return period. The predicted mean hourly wind speeds at the deck level of the two jointless deck alternatives far exceeded the 10,000-year wind speed (115 mph), meaning the bridge is very stable against strong winds. Wind tunnel testing will also be performed for the proposed structure, which will include the impacts of staging and wind on trucks.
At the towers and anchorages, a cast-in-place concrete deck is being proposed. Special attention will be taken during the development of the design staging and construction details to ensure that the cable and anchorages are protected during construction. The final product will be watertight and sealed so that the dehumidification system can be added under a future project.
Construction staging and maintenance and protection of traffic are critical to the project’s success. The recommended staging includes a five-lane scheme that provides three lanes in the peak and two lanes in the non-peak direction, with the provision for a movable barrier.
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