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Civil Engineering Magazine THE MAGAZINE OF THE AMERICAN SOCIETY OF CIVIL ENGINEERS

Bridging the Hudson

By JEFFREY HAN, P.E., M.ASCE, MICHAEL MARTELLO, P.E., AND KENNETH WRIGHT, P.E.

Replacing New York State's Tappan Zee Bridge involved traversing more than just water; the team also crossed the hurdles of poor foundation conditions and the enormous scale of the crossing. Creative solutions included long-span plate-girder approaches, precast and preassembled components, giant waterborne cranes, and simultaneous fabrication paths that helped meet the project schedule.

In 1995, after the years of construction, the Tappan Zee Bridge opened to traffic in New York State. The crossing served as a vital artery over the Hudson River between Rockland and Westchester Counties, about 20 mi north of New York City, until its retirement on October 6, 2017. Its replacement—the visually striking Governor Mario M. Cuomo Bridge—opened to all traffic in September 2018.

The former bridge, which was intended to carry up to 100,000 vehicles per day, handled an average of 140,000 vehicles per day by its retirement. Traffic congestion and delays were common because of its narrow lanes and lack of emergency shoulders. According to the New York State Thruway Authority (NYSTA), the bridge's average accident rate per mile was double that of the rest of the authority's 570 mi long statewide Thruway system. In the decade prior to replacement, hundreds of millions of dollars went toward maintaining and repairing the bridge, and if it bridge had not been replaced, the maintenance cost would have rivaled the cost of a new bridge without the benefits a new structure would bring, according to the NYSTA. After years of study, New York governor Andrew M. Cuomo—a son of the bridge's namesake, Mario Cuomo—decided that replacing the old bridge was the best alternative. The project included the bridge itself as well as supporting facilities.

The project was designed and built by Tappan Zee Constructors LLC (TZC), a consortium based in Tarrytown, New York, that comprised a number of design, engineering, and construction firms. The consortium includes the global engineering and construction company Fluor Enterprises, headquartered in Irving, Texas; the heavy/civil construction company American Bridge, headquartered in Coraopolis, Pennsylvania; the infrastructure company Granite Construction, headquartered in Watsonville, California; and the heavy/civil contractor Traylor Bros., headquartered in Evansville, Indiana. The New York City office of HDR served as TZC's lead designer, its key subconsultants including COWI (formerly Buckland & Taylor), of Vancouver, British Columbia, Canada, for the main span; URS/AECOM, of New York City, for the foundations; and GZA GeoEnvironmental, of Norwood, Massachusetts, for geotechnical consulting. The TZC team was also supported by numerous specialty, local, and disadvantaged business enterprise subcontractors and subconsultants.

The project was advertised by the NYSTA as a best-value design/build project. Four teams were short-listed, but ultimately only three of these submitted proposals and bids. The NYSTA determined that the TZC team provided the best value and awarded it the contract.

The new bridge features two parallel 3.1 mi long crossings. Each has a 2,230 ft long cable-stayed main span and a total of 10 approach units, most comprised of five 350 ft span continuous steel-girder units. Each structure includes four general traffic lanes and a dedicated bus rapid transit lane, bounded on either side by emergency shoulders. The westbound bridge includes a shared-use pedestrian and bicycle path with six viewing platforms along the north fascia that is expected to be completed this year. Crossover bridges have been built to allow emergency diversion of traffic between the structures.

All-electronic toll-collection facilities were also built as part of the project. Additionally, the project included two new multistory buildings, one for NYSTA maintenance and one for New York State Police.

The signature feature of the twin cablestayed main spans are their eight 419 ft tall towers, which incline outward, as do their cables, which are arranged in fan shape. This solution achieves a clean appearance that does not unduly clutter sight lines within the Hudson Valley.

The bridge is designed for a 100-year service life before it would require major maintenance, which proved to be a significant driver of the design. Achieving this service life required special high-performance concrete mixes designed for various exposures—to the atmosphere, road salt spray, and brackish river water—along with the use of other corrosion-resistant materials. Those materials include galvanized reinforcement steel in the concrete structures; painted weathering steel in the superstructure; galvanized steel in other, miscellaneous structures; sacrificial steel as part of the piles; and a robust overlay system for the deck.

The crossings' foundation design addressed demanding geotechnical challenges, including poor foundation soils, significant variation in soil parameters, the potential for liquefaction, and ice loads. And the bridge's foundations needed to be designed with extra capacity to support a potential future commuter rail crossing that could be built between the two structures.

Ground conditions varied dramatically along the bridge alignment. Preliminary subsurface explorations performed by the NYSTA before and during TZC's proposal preparation revealed that the depth to bedrock (glacial till) along most of the alignment was 270 ft or more. For much of the western one-third of the alignment, the depth to the top of bedrock was more than 700 ft below the mean water elevation. The sloping bedrock surface leading down into this "canyon" was also a challenge with respect to construction of driven pile foundations.

To resolve these issues, the team designed the bridge to be supported primarily by steel pipe piles measuring from 36 to 72 in. in diameter. They were driven deep into the subsurface of the river bottom—with some friction piles approaching 400 ft below the pile caps. The piles supporting the cable-stayed spans as well as many of the approach span piers east of the cable-stayed spans are driven to refusal on rock.

To limit the number of piles, the design required H-pile capacities that were unprecedented for the ground conditions encountered along the new bridge alignment. The solution consisted of roughly 1,100 large-diameter, open-ended pipe piles, totaling about 50 mi in length. In the deep clay area on the western third of the alignment, the project used what the authors believe are one of the highest-capacity friction piles (2,100 tons) ever used in this type of soil. End-bearing piles with capacities of up to 3,600 tons were used where dense soil or bedrock was within 300 ft of the water surface.

The average pile lengths typically varied from about 230 ft long for the end-bearing piles to about 335 ft long for the friction piles. The 3 ft diameter pipe piles were designed with a wall thickness of 1¼ in. The 4 ft and 6 ft diameter pipe piles were designed with a wall thickness of 1 in. The 6 ft diameter pipe piles had 2 in. thickened end pieces that functioned as driving "shoes" to avoid pile damage as they were being driven into the glacial till and bedrock. The pipe piles were driven with hydraulic hammers with a large ram weight to limit the impact velocity and driving stresses. Noise and vibration levels were minimized by this very heavy hammer.

Near the riverbanks and on land, driven H-piles, micropiles, and drilled shafts were used. Here, the bearing strata were much closer to the surface, and these solutions helped minimize noise and vibrations near local residences.

The designers selected structure types with proven service lives and the ability to minimize foundation demands. The approach design maximized span lengths using long-span steel girders. The high-capacity friction piles used in the deep clay soils allowed the design team to achieve these very long approach spans.

The design of the approach spans was generally based on five-span continuous units. The steel framing for each crossing included five main girders and four substringers to minimize the foundation loads. Overall, 240 million lb of steel was used in the superstructure, with another 160 million lb of steel in the pipe piles.

Seismic isolation was achieved with the use of triple friction pendulum bearing systems (TFPBS), and to the best of the authors' knowledge, these represent the largest use of TFPBS on the east coast of the United States to date, with more than 500 individual bearing assemblies supporting the approach spans. The TFPBS were integral to achieving the 100-year service life, as they allow the bridge to resist the area's 2,500-year return period earthquake with minimal damage to the main substructure components.

The concrete deck includes nonprestressed, full-depth precast panels designed for a 100-year service life. This service life is provided by limiting crack widths in the concrete through design methods, using galvanized rebar and high-performance concrete mixes with low permeability, and employing an overlay system that comprises a waterproofing membrane overlaid with a high-density, low-permeability asphalt wearing course.

The typical section of the crossings included both conventional multigirder cross sections and girder/substringer cross sections. The steel framing supports 10¾ in. thick precast deck panels that are made composite with the girders through the use of shear studs, which were ultimately grouted simultaneously with the placement of the haunches. The precast panels are connected through transverse and longitudinal concrete closure placements. Continuity is achieved through all joints via conventional reinforcement through the joints.

Additional Insight: Saying Goodbye to the Tappan Zee 

Construction phasing required a wider westbound crossing to accommodate eight lanes of traffic, which were necessary during the intermediate phase of construction. This additional lane width will ultimately serve as the shared-use pedestrian and bicycle path.

Over the shipping channel, the crossings are twin-tower cable-stayed bridges with a span arrangement of 515 ft, 1,200 ft, and 515 ft. (See the elevation above.) The four towers of each crossing are paired and have modified H shapes in which the legs slope outward. This configuration provides several distinct advantages. First, it creates a striking appearance, which meets NYSTA's goal of providing an iconic structure for the community. Second, the outward cant of the tower legs keeps the stay cables outboard of the bridge deck, improving user safety. In northern climates, there have been instances of ice forming on stay cables and falling onto bridge decks. With the chosen configuration, falling ice will tend to drop outside the limits of the deck. Third, the outward cant obviated the need for an above-deck crossbeam, which is typical for H-shaped towers. The outward bending introduced to the towers due to their outward cant is largely balanced by the inward bending induced by the inward horizontal component of the stay-cable forces. Finally, the tower geometry allows for the future design and construction of the transit bridge between the two crossings.

The two cable-stayed decks were erected using the balanced cantilever method. Modules consisting of edge girders and the floor system were transported beneath the bridge by barge and lifted into position by floating cranes. The edge girders were fabricated with the stay-cable anchor pipes in place to limit on-site work. The erection times were minimized and the load imbalances during construction were limited by minimizing the amount of construction equipment on the bridge deck and avoiding deck-supported lifting. A significant advantage of the composite cable-stayed superstructure is the significant weight reduction over concrete superstructures, which allowed the use of smaller floating cranes for the balanced cantilever erection method, reduced the size and cost of the stay cables, and generated savings in the foundations.

Wind tunnel testing indicated that having two structures of this size and flexibility in such close proximity to each other meant that their aerodynamic behaviors would be closely tied to each other. So an open traffic barrier was used in the center spans to eliminate the need for wind fairings to control wind responses. Baffle plates were still needed under each cable-stayed crossing to ensure acceptable aerodynamic performance over the service life of the bridges.

The cable-stayed units are fixed at the anchor piers and allow expansion at the towers under normal load cases, with lock-up devices provided at the towers. These devices engage the towers in the longitudinal direction in extreme events, such as earthquakes or high winds, in which longitudinal loads are applied very quickly. Thus, all four supports in each crossing participate in carrying rapidly applied longitudinal loads.

The bridge was also designed for vessel collision, a critical load case, considering that ocean-going barges travel up the Hudson River to destinations well north of the bridge site. Given the probabilistic nature of the assessment of barge impacts, the main-span towers and anchor piers were detailed to have a common pile cap between the two bridges so that all the piles could be engaged to resist lateral loads together. This allowed the crossings to meet the design criteria without the need for a costly external fender system to absorb ship impacts. Ultra-high molecular weight polyethylene panels were installed around the perimeter of the pile caps for the cable-stayed bridges and the approach spans to protect against abrasion from ice floes or vessel collision.

TZC used extensive precasting of various concrete components in order to minimize work performed over the water, increase safety, and expedite construction. This simplified the construction of the pile caps, pier caps, and bridge decks and moved much of the forming, fabrication, and erection of these components onto land. Large sections of the approach superstructure framing were assembled at an off-site construction yard and barged to the bridge site for erection. A 1,800-ton supercrane, owned by TZC and nicknamed "I Lift NY," hoisted into place massive preassembled sections, up to 410 ft long and complete with utility conduits, supports, and catwalk/access systems.

Using the typical construction method of sheet-pile cofferdams to construct the water-level pile caps would have been tremendously labor-intensive and would have resulted in high levels of risk and unacceptable environmental impacts. So the design/build team developed a precast tub system to function as a form for the water-line pile caps. These tubs were unique because they were designed to be integral structural components of the finished pile caps rather than being sacrificial. A rigorous finite element analysis was performed for the tubs, focusing on stress levels and crack sizes that might be expected at various points during fabrication and construction. Tracking stresses throughout every stage of the construction was important in proving that a 100-year design life could be achieved.

The tubs were as large as 36 ft wide by 84 ft long and weighed an estimated 360 tons. Appropriate construction tolerances were incorporated into the design so that everything fit properly in the field. Heavy vertical shear reinforcement was anchored into the bottom slab of the tubs using custom-made threaded inserts to achieve composite action between the tubs and the infill concrete. A full 95 percent of the pile caps were fabricated off-site, yielding a huge schedule benefit through a reduced volume of in situ concrete placement-again, enhancing worker safety-and reducing the environmental risks associated with using cofferdams.

A similar precasting technique was used for the pier caps. Using precast shells for the pier caps allowed the contractor to limit the amount of marine activities to erection rather than forming and casting. Like the pile caps, the precast pier caps were designed to be part of the final section and were required to resist a portion of the final load demands. Limited prestressing was included in the walls of the pier caps to achieve a neutral stress case at the top of the pier caps after the infill concrete was placed as well as to limit cracking in the walls during shipment and erection. One challenge was threading the vertical column bars through ducts in the bottom slab of each pier cap, which required very careful planning and coordination. Ultimately a template was made of the bottom slab duct layout and shipped to the site to confirm any interferences before erecting the pier caps.

The length of the main-span tower and anchor-pier pile caps are roughly the length of an entire football field (including the end zones) and did not allow for the pile caps to be constructed in the same manner as the approach structure. The contractor built a cofferdam above the mean high highwater line that was temporarily supported on the permanent piles. The soffit, or floor, of the cofferdam was constructed using precast-concrete soffit slabs that were "stitched" together with cast-in-place closure placements and a cast-in-place structural topping. The side forms, or walls of the cofferdam, were then installed on top of the soffit slab system to complete the cofferdam. After that the entire cofferdam was lowered into the river, sealed, and dewatered to allow for installation of reinforcement bars.

Because of its size and complexity, the precast deck panel design for the approach spans was an engineering achievement in itself. The panels were designed with the roadway crown and superelevation transitions built into the geometry, with most panels spanning across multiple girders. The 6,806 precast deck panels were up to 48 ft long by 12 ft wide. The designer and contractor worked closely to develop a precast deck system that eliminated the need for soffit forms at the joints. Longitudinal joints were located at the centerline of a main girder to allow the girder top flange to act as the form. For transverse joints, a robust positive connection detail that included a concrete lip to serve as the bottom form was designed. Designing the joints to avoid external soffit formwork increased the deck production rate over conventional methods. Lifting lugs were preinstalled to allow for easy transportation and erection of the panels. Pockets were carefully located for the installation of shear studs once the precast decks were installed.

The main-span superstructure uses a composite deck system comprising edge girders, floor beams, and precast deck panels. The precast-concrete deck panels used a concrete mix that achieves a minimum 28-day concrete strength in excess of 9,000 psi. The panels extend from floor beam to floor beam and are made composite with the steel floor beams and edge girders using cast-in-place concrete closures. Similar to the approach-span deck panels, the main-span precast deck panels were analyzed and checked for transportation, lifting, and erection stresses. The transverse joints between deck panels were not heavily reinforced like those on the approach spans. The stay cables induce significant compression into most of the deck, which limits cracking, and the deck in the center portion of the center span was posttensioned longitudinally to limit cracking.

A project of this magnitude includes a variety of important structures other than the bridge itself. In addition to the two buildings and the shared-use path, these include an underpass and platform for the owner's maintenance personnel on the western shore of the river. The platform is approximately 1,345 ft long. It is 26 ft wide for the first 970 ft from the shore and widens to 57 ft wide for the last 375 ft, where the slip for the owner's maintenance water vessels is located. The platform primarily consists of precast-concrete bent-cap beams and a precastconcrete deck spanning the cap beams. The precast deck has a cast-in-place concrete topping as the final riding surface.

The project also required a structural health monitoring system that, to the best of the authors' knowledge, is the largest and most complex of its kind in the country. It includes more than 300 sensors that provide real-time feedback about pier and pylon tilt, stay-cable dynamics and loading, steel and concrete temperatures, expansion joint movement, deformation and settlement, and reinforcement corrosion rates. Most of the sensors are located on the cable-stayed bridges, but there are a limited number of sensors on the approach bridges as well.

The bridge is illuminated with light-emitting diode (LED) fixtures that are specifically designed to reduce glare and light pollution. The highly efficient system uses approximately 75 percent less energy when compared with traditional lighting technology. Aesthetic "uplighting" also accentuates the towers and stay cables on the main spans and approach-span piers. Operators can remotely schedule maintenance activities, monitor real-time energy usage trends, and program the lighting, choosing from more than 16 million color combinations.

The project has garnered numerous awards, including the American Council of Engineering Companies' Grand Award this year.

Jeffrey Han, P.E., M.ASCE, is a senior vice president and principal project manager for HDR and was the project manager and design director for the HDR design team on this project. Michael Martello, P.E., leads HDR's New York structures practice and was the global analysis task lead during design and the approach bridge project engineer during construction. Kenneth Wright, P.E., is a senior vice president of HDR and its northeast principal bridge engineer and served as the design manager for this project .

PROJECT CREDITS Owner New York State Thruway Authority, Albany, New York Design-builder Tappan Zee Constructors LLC, a consortium of Fluor Enterprises, American Bridge, Granite Construction, and Traylor Bros., Tarrytown, New York Lead designer HDR, New York City office Key design subconsultants COWI (formerly Buckland & Taylor), Vancouver, British Columbia, Canada (main span); URS/AECOM, New York City (foundations); and GZA GeoEnvironmental, Norwood, Massachusetts (geotechnical)

© 2019 AMERICAN SOCIETY OF CIVIL ENGINEERS ALL RIGHTS RESERVED

 



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