When it opened to traffic in December 1955, the Tappan Zee Bridge was built to handle 100,000 vehicles per day. Today, because of population and economic growth on both sides of the Hudson River, the bridge carries a daily average of 140,000 vehicles and will frequently get as high as 170,000 vehicles per day.
State officials determine that, as it entered its 55th year, the Tappan Zee Bridge required major work. Specifically, a significant portion of the deck needed to be replaced. However, given its high level of use, significant lane closures and round-the-clock construction work were not politically feasible. The New York Thruway Authority instead awarded the contract to Perini Construction Company and its joint venture partner, Tutor-Saliba.
At the conclusion of the 2008 construction season, 1,080 pre-cast concrete panels had been placed on the Tappan Zee Bridge, the remaining 12 panels awaiting placement at a staging area in a parking lot off of the NYS Thruway.
The contract, awarded to Perini in August 2006, included the innovative approach of pre-casting deck panels. Due to existing deck deterioration, the NYS Thruway Authority requested Perini accelerate the contract so that the new deck was in place by the end of 2008. Under the $157 million contract, the two outer lanes in each direction of the Tappan Zee Bridge across the west causeway, west truss and the main truss span have been replaced.
Prepping Prior to Redecking Saves Time
Michael R. Fliescher, executive director of the New York State Thruway Authority, explained the Authority’s decision to go with Perini when the contract was originally awarded.
“In the end, Perini’s plan to measure, conduct surveys, and do the majority of the prefabrication work prior to the actual redecking should further reduce the amount of actual construction time on the bridge, thus reducing the associated construction impacts to both drivers and nearby residents.”
Over the two-year construction period, the panels were fabricated off-site, fit-up to assure paired panels matched, and stored in Perini’s storage yard where the pre-fabricated guard rails were installed on the panels for the outside lanes. The complete panels were trucked to the bridge or staging areas where they were on-hand to install. During overnight work hours an existing deck section was cut away and a new deck section put into place using cranes. For the west deck truss, barge-mounted cranes were used.
To assure each pre-fabricated panel fit in the place where an existing deck section was removed, a baseline survey was performed to establish vertical and horizontal controls on the existing bridge and verify the controls depicted on the contract drawings. Subsequently, an in-depth, as-built survey of the west causeway, west deck truss, and main span truss was performed to establish the existing deck elevations, bottom of steel stringer elevations and bearing and pedestal elevations, as well as verify the horizontal geometry.
Because the bridge curves and has a cross slope for drainage, the panels required working with three dimensions at the same time, with panels weighing an average of 50 tons and the heaviest weighing in at 100 tons.
The as-built surveys were used to analyze the design drawings, converted into shop drawings for fabrication of the structural steel and pre-cast panels, and then converted into erection drawings to provide the field erection crew with directions on how to erect and assemble the structural steel and pre-cast panels.
Utilizing Off-Site Fabrication
Fabrication of the panels began with the steel. Each panel contained an assortment of wide flange steel stringers, wide flange and channel diaphragms, connection T sections and plates and a variety of bolts and nuts sizes. The fabricated structural steel was delivered to a galvanizing plant where it went through a hot-dipped galvanizing process to prevent corrosion.
At Perini’s off-site assembly and storage yard, the structural steel frames for each precast panel were erected. Once assembled, the frames were transported to the precaster’s fabrication plant in New Jersey where the concrete panel was fabricated and cured for 45 days.
The completed panels were transported back to Perini’s yard and prepared for delivery to the bridge. This final preparation included installing the permanent barrier and painting temporary traffic line striping.
Placing the panels required carefully planned logistics and coordination of many activities that needed to work in unison to successfully place deck panels and re-open the bridge by 6 a.m. Some preliminary activities occurred during the day to prepare for the removal and installation.
Regardless of the panel location, typically MPT was put in place to close one lane of traffic beginning at 8 p.m. with the second lane closed by 9 p.m. and the third lane closed at 10 p.m. after the movable barrier was shifted.
Two hydraulic cranes were mobilized into pick positions. Connection between the existing superstructure and substructure was removed and rigging was installed on the existing deck slab.
The existing slab and stringers were removed by crane, placed on awaiting flatbed trailers and hauled off the bridge. The new deck panel was pulled onto the bridge, rigging was installed on the new deck panel and it was hoisted into place using the cranes. Bearing and diaphragm connections were installed. All MPT was removed to restore traffic lanes by 6 a.m. With planning and precision, the Tappan Zee crew was able to place several panels a night.
This story was reprinted with permission from New Jersey Constructor Magazine, Issue 2, 2010.
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