Manhattan Bridge Reconstruction in New York City.
Transportation led a 30-year program to rehabilitate this signature New York City bridge. The firm’s engineers first inspected the Manhattan Bridge in 1978 and found persistent cracking and corrosion. Analysis showed that twisting from one-sided train loads was the primary cause of the cracking, exacerbated by stick-slip behavior of partially frozen expansion bearings between the roadway framing and the suspended structure. Along with the analysis, a repair scheme was developed by our engineers that promised to correct the structure’s design insufficiencies, restore its lost capacity, and extend its life span. The work included stiffening the bridge trusses to resist cracking and twist (after evaluating 13 remedial schemes), replacing non-functioning pin-and-link hangers with multi-rotational bearings, and redecking all roadways including replacement of stringers and floorbeams. A 108-foot panel was installed and the bridge was load-tested and monitored with strain gages to confirm the plan’s effectiveness and constructability. All work was designed to be done while maintaining both highway and transit traffic.
As part of the rehabilitation of the main cable and cable anchorage chamber C, three-quarters of a main cable was cut and re-anchored to new anchors installed inside the anchorage to relieve loads from the severely corroded eyebars. Transportation provided design and construction engineering services for this operation, which was performed with trains and vehicular traffic operating on a portion of the bridge. At the most critical times, office personnel interpreted strain gage readings during stressing of the newly transferred strands with hydraulic jacks to make certain that permissible wire loads were not exceeded and directed tuning of the cable strand forces in the finished work. Heavy structural steel and concrete work alterations to accommodate the new system were included in the scope. Transportation also designed a dehumidification system to be installed in all eight cable chambers to control moisture condensation on the eyebars.
The reconstruction of the bridge was staged in four phases to maintain the continuity of both highway and subway traffic. The first phase involved the stiffening of the suspended side spans and approach spans on the north side of the bridge, which was completed in 1988. The second phase included the rehabilitation of the entire south side of the bridge. In 2000, the third phase of the reconstruction started. Under this phase, the final tightening of the upper laterals in the main suspended span was completed, along with the roadway deck replacement in the main span and subway framing replacement for the north side of the bridge. Currently, Transportation is preparing the final phase of construction plans and specifications including the replacement of the lower roadway deck and framing for the entire length of the bridge.
As part of the final phase of the rehabilitation of the suspended span, Transportation is providing seismic evaluation and retrofit recommendations for this 90-year-old suspension bridge. The new assignment includes analysis and preparation of seismic retrofit details. Transportation is also performing a soil-structure interaction analysis. The study includes a time history analysis of the bridge, incorporating inelastic material behavior and geometric discontinuities. The earthquake ground motion is provided by a expert panel, convened by Thornton Tomasetti and NYCDOT to provide seismic design uniformity throughout the city with concurrence on rock motions.
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