The original Tappan Zee Bridge, revolutionary in its day, was well past the end of its operational life. Its challenging site called for structural creativity, a modern cable-stayed design, and careful environmental precautions.


Location: South Nyack (Rockland County) and Tarrytown (Westchester County), New York
Client: New York State Thruway Authority, Albany, NY
Architects and Engineers: Tappan Zee Constructors, LLC, Tarrytown, NY
Lead Designer: HDR, Omaha, NE
Structural Engineer: HDR; COWI, Lyngby, Denmark
Geotechnical Engineers: GZA, Norwood, MA; URS, San Francisco, CA
General Contractor and Construction Manager: Tappan Zee Constructors, LLC,
Tarrytown, NY
Construction Manager: Tappan Zee Constructors, LLC, Tarrytown, NY
Structural Steel Fabricator: Hirschfeld, San Angelo, TX; High Steel, Lancaster, PA;
Canam, Saint-Georges, Québec
Structural Steel Erector: Tappan Zee Constructors, LLC, Tarrytown, NY
Miscellaneous Iron Fabricators: L&M, Bath, PA; Upstate Steel, Buffalo, NY; County Fabricators, Pleasantville, NY
Miscellaneous Iron Erectors: Tappan Zee Constructors, LLC, Tarrytown, NY
Architectural Metal Fabricator and Erector: UAP, New York, NY;
EW Howell, New York, NY

TRANSPORTATION PLANNERS WOULD NOT ORDINARILY, all other factors being equal, select the space between South Nyack and Tarrytown as the site for a bridge across the Hudson. It’s 3.1 miles across, the river’s second widest point, with a structurally challenging topography carved by glacial recession tens of thousands of years ago. Bedrock is some 220-270 feet below mean sea level at midriver; in a pre-glacial river channel near the western shore, the bedrock lies 700 feet below. There are, to put it mildly, easier places to build.

The chief reason the 1955-vintage Tappan Zee Bridge (TZB) was financial. It was the closest site to the city outside a 25-mile radius from the Statue of Liberty; by law, any bridge within that area would be within the Port Authority’s jurisdiction, meaning New York and New Jersey would split the toll revenue. Gov. Thomas Dewey preferred to direct that stream toward the newly created authority for the New York Thruway. Hence, drivers got a bridge that sent them over the water for three miles but kept the tolls in the state.

Engineered with ingenuity and frugality during a Korean War-related materials shortage, the TZB used a military structural technology known as Phoenix caissons, a series of eight buoyant concrete breakwaters set in riverbed rather than on rock, the brainchild of chief engineer Emil Praeger. The TZB was planned to last 50 years and was showing its age well before the 2007 collapse of Minnesota’s I-35W bridge focused national attention on decaying infrastructure. Originally designed to carry fewer than 40,000 vehicles per day, it was averaging 138,000 by the early 2000s (roughly the same capacity as the deadly I-35W bridge), and its nonredundant design meant that failure of any member could put the entire bridge in danger. Construction attorney Barry LePatner, author of the infrastructurehazards exposé Too Big to Fall (Foster Publishing, 2010), referred to the TZB as the “scary of scaries.” With its narrow lanes and no shoulders, by 2007 it also had over twice the average collision rate per vehicle mile as the rest of the Thruway system.

Its replacement, the twin-span Gov. Mario M. Cuomo Bridge, represents a different kind of innovation. It couldn’t have moved closer to the city—the site is effectively locked in by approaching roads and regional development since the fifties—but its design and construction reflect the dramatic progress in
the field and the evolution in civic priorities since the Eisenhower era. Each span has four lanes for general traffic, a breakdown/emergency shoulder, and a dedicated bus lane; the westbound (northern) span also gives pedestrians and cyclists a 12-foot shareduse path with six overlooks offering river views.

Like many striking bridges built in recent years (including, locally, the new Kosciuszko span featured in Metals in Construction’s Spring 19 issue), the Cuomo Bridge uses a cable-stayed design, once arresting, now nearly as familiar as it is economical. The project’s innovations are in realms beyond the striking aesthetics: ease and pace of construction, Intelligent Transportation Systems (ITS), structural health monitoring (SHM), and attention to long-term environmental effects. The site’s multiple challenges have once again been mothers of invention.

“The iconic Governor Mario M. Cuomo Bridge is a state-of-the-art transportation facility that will meet the needs of Hudson Valley residents and visitors for the next century and beyond,” comments project director Jamey Barbas of the Thruway Authority. “This landmark crossing symbolizes New York’s resolute commitment to transforming and modernizing its infrastructure.” Fast-tracked as a High Priority Project by the Obama administration, it has carried full bidirectional traffic since September 2018.

For a project of this scale, after issuing an RFP in 2012, the New York State Thruway Authority chose a design-build strategy and a multidisciplinary consortium, Tappan Zee Constructors (TZC), comprising engineering and construction firms Fluor, American Bridge, Granite Construction Northeast, and Traylor Brothers, along with design firms HDR, Buckland & Taylor (now part of COWI), URS (part of AECOM), and GZA. (The TZC project team collectively contributed some of the information for this article through the Thruway Authority in lieu of personal interviews.) It is one of the nation’s largest design-build transportation projects, marshaling this method’s efficiencies to overcome the site’s unique challenges.

The Hudson Valley’s geology called for extensive geotechnical studies, including more than six dozen soil borings of the riverbed, revealing layers of clay, silt, sand, and glacial till covering bedrock below the river, with a deep valley of clay under the western half. This led the designers to bypass buoyant caissons and select a structural system based on piles consisting of steel tubes filled with steel-reinforced concrete; the process used over 30,000 tons of rebar. Most piles rest on bedrock, while others (longer and with greater surface area) use the friction of the deep clay to create supportive tension, which TZC estimates will withstand at least 100 years of load-bearing.

The piles of the main span above the deep-water navigation channel are unified in pile caps, the largest of which is longer than a football field, consolidating the strength of scores of piles into a single structure. Smaller pile caps support the approach spans on either side of the main span. In 2013, TZC performed load testing with massive weights, up to 7 million pounds, the equivalent of about 2,000 cars, ensuring that the pile system had adequate carrying capacity before construction began. Lowering the huge caps precisely and in sync required a computer-guided jack system, factoring in the river’s tidal flow.

The eastbound span is 87 feet wide; the lane for self-powered users makes its westbound counterpart 96 feet wide. Eight 419-foot concrete towers stand at five-degree angles from vertical, leaning outward to create a distinctive aerial profile and bearing “192 stay cables that would stretch 14 miles if laid end-toend,” according to the TZC team. The cable-stayed area is 2,230 feet long, and the cables support a total of 74 million pounds of steel and concrete.

TZC used modular construction procedures, preparing major segments of the foundations, roadway, and superstructure safely off-site on land, including structural steel assemblies up to 410 feet long. The largest of the project’s cranes had a 328-foot lifting arm capable of raising loads up to 1,900 tons. TZC purchased this huge device, originally built for use on the San Francisco-Oakland Bay Bridge and named the Left Coast Lifter, and moved it 6,000 miles from California via the Panama Canal, renaming it I Lift NY en route before it reached New York Harbor in January 2014. Months of testing and customization prepared it for service on the Hudson that April, raising nearly 100,000 tons of structural steel (including 140 girder assemblies and four main span crossbeams) as well as precast concrete foundations, substructure, and 120 road deck panels. After the first span opened in August 2017, I Lift NY also saw action removing sections of the old TZB; its final operation in May 2019 was to remove the old east anchor span after its controlled demolition that January, part of the process of disassembling the obsolete bridge for recycling and reuse at other locations across New York State. TZC credits I Lift NY with shortening construction time from original estimates by months and saving millions of dollars on the project.

Today’s bridges take active roles in guiding traffic, not just carrying it. The Cuomo Bridge’s ITS, a complex of sensors and communication channels, monitors conditions on the twin spans and automatically informs Thruway Authority staff of disruptions. Information about collisions, closed lanes, winter pavement conditions, or other sources of trouble goes out to motorists through electronic signs on the bridge and landings, directing drivers away from hazards and reducing risky last-second lane shifts. The ITS also connects the Thruway Authority with law enforcement, first responders, and tow-truck operators. As a component of the authority’s wider traffic-control network, the ITS helps synchronize both preventive maintenance and repair work, reducing disruptions.

Information technology also generates useful data about the bridge itself. Over 300 sensors in the SHM system—a complex of inclinometers, ultrasonic distance censors, fiber optic gauges, 3D accelerometers, and GPS instruments – measure corrosion, temperatures, climatic conditions, vehicle weights and counts, cable strain, tower sway, and expansion joints’ reactions to load patterns and vibration. Gantry-based automatic tolling, by either EZPass or photo/mail systems, spares drivers a slowdown to fish for cash. The roadway lighting comprises dark-sky-compliant LED fixtures, cutting light pollution in the scenic Hudson Valley while saving an estimated 75 percent in energy costs over older lighting technology.

In a region with history of environmental damage and remediation (the bridge is downriver from General Electric’s dredging operations for polychlorinated biphenyls), the impact of construction on local ecosystems has been a key priority. Construction equipment met strict Environmental Protection Agency emissions standards, using ultra-low-sulfur diesel fuel and tailpipe particulate filters. To control underwater noise and vibration during pile driving, the crew created a bubble-curtain system, sliding aluminum rings over pilings and pumping pressurized air through the rings to create a cloud of bubbles that absorbed the energy of impact, reshaping pressure waves, lowering the noise level by more than 10 decibels, and deterring fish from swimming into the hazardous area. Dredging was timed to avoid interrupting spawning and migration seasons. The Thruway Authority consulted with scientists from the NY Harbor Foundation’s Billion Oyster Project and placed more than 400 oysterreef structures in a five-acre zone near the bridge to support restoration of this ecologically beneficial filter-feeding species. A nesting box atop one bridge tower has attracted peregrine falcons, another important species whose prey includes pigeons; since pigeon waste is acidic enough to corrode steel and concrete, the falcons’ deterrent presence is a win/ win for humans and birds.

One substantive critique of the bridge addresses a decision made on a political level, not by architects or engineers: one of the 21st century’s largest projects chiefly accommodates the dominant transportation mode of the 20th. Public transit on the bridge is limited to bus service; both a planned bus-rapid-transit line and the pedestrian/bike path have been delayed. Early studies, including 2011 Ove Arup reports on cost estimates and feasible alternatives prepared for the state Department
of Transportation, the Thruway Authority, and the Metropolitan Transit Authority/Metro-North, evaluated structural options that would include RocklandWestchester commuter rail; the February 2014 final recommendations by the bridge’s Mass Transit
Task Force notes that commuter and light rail were considered but “included as long-term recommendations.” TZC reports that the bridge “was built with the structural capacity to handle light/commuter rail in the future. The future rail line would be located between the two spans.” This isn’t the first or last time financial concerns led to a decision that delayed a more future-oriented design component. But the most intelligent transport systems in the long run, many environmentalists, urbanists, and economists maintain, might not involve automobiles at all. While Hudson Valley residents enjoy driving across their new bridge, they may also dream of the day it offers them other transportation options as well.

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