The Bronx-Whitestone Bridge in New York City. Left image courtesy Wikimedia.
Transportation has led a series of interrelated reconstruction programs on the Bronx-Whitestone Bridge linking Queens to the Bronx in New York. This suspension bridge, which opened in 1939, has a main span of 2,300 feet.
In 1988, after extensive aerodynamic evaluation, Transportation designed and provided construction support services for a tuned mass damper system to stabilize the bridge against possible damaging wind vibration. As part of this project, the firm designed the maintenance walkways for servicing this system. The damping system operates by means of its own inertia, elasticity, and friction, with no need for manual intervention, power supply, or control system. It uses 240-foot-long torsional springs, mass blocks weighing 25 tons each, and multiple caliper disc brakes to dissipate more than 200 horsepower. In order to fit the system into the existing bridge envelope, the damper is wire-rope driven.
In 1995, Transportation performed an in-depth inspection of the entire bridge, and established the vertical clearances of the bridge’s navigation channel under varying conditions of traffic, live load, temperature, and tide. During the inspection, the firm’s personnel noted corrosion and other defects in the wires of the main cable, and severely corroded eye bars in the anchorages.
Subsequently, the cable was unwrapped, wedged, oiled, coated with red lead paste, and rewrapped, with Transportation on hand to inspect the internal condition of the entire length of both main cables. Sample wires were selected and tested for strength. A surprising result was the large number of wires containing stress corrosion cracks. Transportation developed a statistical model to determine the overall strength of the cables. For the anchorages, we recommended rehabilitation schemes to strengthen the eye bars, including a system of girders and anchor rods to relieve loads. The anchorages were enclosed, and a new dehumidification system installed.
The need to increase the load-carrying capability of the main cables, coupled with Transportation’s previous aerodynamic studies of the Bronx-Whitestone Bridge, led the firm to a plan to strengthen the bridge by making it lighter. Heavy trusses had been added to the bridge in 1946 to stiffen it against potentially damaging winds across Long Island Sound. The trusses, combined with progressive deck rehabilitations, had greatly increased the dead load of the bridge. If the deck could be lightened and the trusses removed without compromising aerodynamics, the cables could then carry a much greater load.
We designed a deck replacement for the entire suspended span. The project involved replacement of the original concrete-filled steel grid with an orthotropic deck, removal of the stiffening trusses, new lateral system, and new lighting to the bridge. We re-evaluated its stability under the revised configuration and designed wedge-shaped Fiber Reinforced Polymer (FRP) fairings on the bridge, which will stabilize it in winds up to 120 mph. As part of the design, a prototype deck panel was installed to confirm constructability and compatibility with MPT requirements. The field work was conducted in conjunction with a full-scale laboratory test program of the orthotropic deck, which was completed in 2002.
To address seismic concerns, we conducted a seismic performance evaluation to determine the effects of installing the new orthotropic deck together with other structural modifications. The analyses for the seismic evaluation phase included Transportation’s in-house SHAKE program utilizing the hard rock motions specified in NYCDOT’s 1998 Seismic Guidelines; and SASSI modeling and analyses of the tower and anchorage foundations to obtain the seismic motions input for the bridge analyses. We performed linear and non-linear time-history analyses to obtain the seismic demands. The seismic retrofit included an innovative design using Low Yield Strength steel to change the behavior of the structure, dissipate energy, and eliminate the vulnerabilities.
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