A Bridge for the Future

By Deborah Huso and Claire Vath

Project: The New NY Bridge
Location: Lower Hudson Valley, New York
Builder: Tappan Zee Constructors, Tarrytown, N.Y.
Owners: New York State Thruway Authority and the New York State Department of Transportation
Precasters: Coastal Precast Systems, Chesapeake, Va.; Unistress Corp., Pittsfield, Mass.; The Fort Miller Company, Schuylerville, N.Y.; Bayshore Concrete Products Corp., Cape Charles, Va.

An hour north of New York City, the Hudson River carves out a deep valley, winding through a landscape of towns and woods before narrowing into the Catskills. Spanning a 3-plus-mile stretch of the river above the city, the Tappan Zee Bridge’s graceful S-shape helps connect key areas of the lower Hudson Valley. The bridge’s west end begins in Rockland County’s South Nyack community and meets Westchester County’s Tarrytown on the eastern shore.

The bridge’s poetic moniker can be broken down into two components. “Tappan” derives from the Tappan Native American tribe who settled the area. “Zee” is the Dutch word for sea. What’s not so poetic, though, is the bridge’s crumbling infrastructure, which bears the load of 138,000 commuter vehicles each day. In 2018, the New NY Bridge will officially replace the Tappan Zee, which opened in 1955.

When complete, the new 3.1-mile, double-span, cable-stayed bridge will feature eight lanes, four breakdown lanes, a bicycle and pedestrian path, state-of-the-art traffic monitoring systems and accommodations for future commuter rail transit. The $3 billion bridge project is the largest in New York history, and the building team, Tappan Zee Constructors, promises a structure that will last for at least the next century with no major maintenance.

To achieve that long-lasting service life, engineers and builders have figured substantial precast concrete components into key elements of the new bridge design. These elements will help bear the load requirements on the heavily traversed road deck panels and support the bridge’s foundation.

Bridging a century

TZC, a consortium of construction, engineering and design firms, leads the bridge’s construction in partnership with the New York State Thruway Authority and the New York State Department of Transportation. Following geotechnical and environmental investigation of the subsoil and bedrock to determine the ideal location for the bridge’s pilings and an assessment of environmental impacts on marine life, construction officially began in August 2013.

Contractors sunk massive steel pilings into the lower Hudson River to form the bridge’s foundation and load-bearing capacity. They then placed precast pile caps on top, unifying the supportive strength of the individual piles at the river’s surface and creating a strong base. The bridge’s main span pile caps, or footings, run longer than a football field in order to support its main towers. The approach spans – which carry traffic from land to the main span – feature 70 precast concrete pile caps.

The approach span pile caps are approximately tennis-court-size concrete tubs. The bottoms of the tubs, manufactured off site by Bayshore Concrete Products Corp. of Cape Charles, Va., feature a series of holes slightly larger in diameter than the pilings. The holes match up to the piling pattern, and once delivered to the job site, barge-mounted cranes position each pile cap over the pilings. Workers carefully thread the holes onto the pilings as though fitting together a puzzle. The pile caps sit partially submerged 6-to-8 feet below the waterline.

Next, builders seal each pile cap with concrete around the holes and pump out any remaining water in the tub. Workers then lay a labyrinth of zinc-galvanized rebar throughout the tub. Zinc minimizes corrosion while the steel cage stands up to immense tension. Up to 750 cubic yards of standard concrete – poured from one of the project’s two on-site floating batch plants – fills the remaining tub space. The concrete strengthens the footing, allowing it to withstand incredible compression, transferring the loads from the bridge superstructure, columns and caps into the piles.

To stand up to marine conditions, the footings, which sit partially below water level, need to be surrounded by sheet pile. Alternatively, “you can use precast concrete, which stands up to the conditions,” said Joe Rose, vice president of sales for Coastal Precast Systems of Chesapeake, Va. “Using precast concrete footings saves on time and construction costs.”

CPS manufactured the column caps that sit atop the columns of the bridge’s approach spans. The column cap, Rose explained, helps support the steel beams that create the superstructure.

Overall, CPS is manufacturing 59 column caps, which run on each side of the bridge. The massive precast, prestressed structures weigh approximately 300 tons each and measure either 83 or 92 feet in length, depending on whether the unit is for the eastbound or westbound structure. Each tub-shaped piece is 13 feet tall and 10 feet, 6 inches wide, with 10-inch walls and a 1-foot-thick floor.

Intermediate diaphragms – divided, chamber-like precast structures that strengthen the tub from collapse – make up the interior. “The diaphragms support the dead load of the wet concrete when the tubs are in final position,” Rose said.

Galvanized rebar and strand then snake through the infrastructure. A monolithic pour, consisting of 150 cubic yards of concrete, fills up to where the beam superstructure bearing seats will be cast, finishing the tub.

CPS also manufactures the bridge’s seal slabs, precast sections that create a casting platform for the main span and support span towers. The 22 anchor seal slabs – made up of 11 pieces each – are 18 inches thick. The largest end sections measure 35 feet wide by 43 feet, 10 inches long, and weigh 145 tons. The tower seal slabs consist of 30 pieces of precast, with the longest running 38 feet by 29 feet and weighing 95 tons. Each slab is cast with a 7-foot-diameter opening to allow a 6-foot-diameter steel shaft to project through the seal slab floor.

Crossing the Hudson

The bridge’s driving surface is comprised of 5,960 precast panels on the approaches and 973 panels on the main span. TZC awarded Unistress Corp. of Pittsfield, Mass., a $70 million contract to produce the approach’s deck panels.

“(TZC) weighed several alternative design-build ideas, but when they looked at all alternatives, their engineers chose precast because of the critical construction time schedule, as well as the need for a long-lasting, quality product,” said Perri Petricca, president of Unistress.

Each precast panel runs 40 feet long and 12 feet wide. To construct the deck panels, galvanized reinforcing steel is pre-tied into cages. After the cages are placed into concrete forms, Unistress installs cast-iron scuppers, mechanical access boxes and isolation valves. Precast allows those extras to be added right into the concrete, eliminating the need for on-the-bridge installation.

Concrete then fills in the remainder of the panel. Steam curing accelerates the concrete curing process and improves panel durability. The process adds warm, moist air to the concrete, helping it reach an optimum temperature.

“Since precast is manufactured in a controlled casting environment, it’s easier to control the mix, placement and curing,” Petricca explained. “Any time you cure concrete in a moist environment, it adds strength and durability to the product. We also use a low water-cement ratio in combination with the steam curing to further create dense, highly durable concrete.”

Panels undergo quality control to ensure full compliance with plans and specifications. Once panels leave the plant, they’re delivered to the job site, where a crane hoists them into place atop steel girders.

Because the panels are prefabricated, precast decking installation is much quicker than traditional cast-in-place and reduces workers’ on-the-job hazards. “We are fabricating the panels concurrent with the construction of the piers and steel support beams, so once the steel is in place, the deck panels can be erected in a fraction of the time it would take to pour the deck in place … and without potential adverse weather impacts,” Petricca said.

Rose agreed with Petricca’s assessment. “Precast eliminates the exposure to weather-related delays on the job site,” he said. “Plus, the value is further enhanced because the plant-controlled casting provides a much higher quality of concrete.”

Reflecting on the bridge of tomorrow

While precast concrete offers increased durability for both traffic and weather and saves time, helping keep the $3 billion budget in check, a project of this caliber isn’t always so straightforward.

“Handling the massive pieces of concrete is challenging,” Rose said. “There’s immense physical pressure when pouring these volumes of concrete. The intricacies of specifications and quality control measures are massive.”

For Unistress’ contribution, Petricca had to beef up staff to handle the project’s demand. “It’s the largest project in Unistress history,” he said. “We hired 138 new people and invested more than $6 million to increase our production capacity.”

The bridge, built to last 100 years without major maintenance, is something Petricca said owes to the use of precast. “Precast concrete offers stronger, more durable bridge elements over the lifetime of the bridge, with far fewer cracking and deterioration issues,” he said. “The extensive use of precast concrete will certainly showcase its advantages in quality, durability, and installation, and should result in more projects designed with precast as the preferred solution.”

The lower Hudson is a frenetic place these days. Workers, barges and cranes come and go. When complete, the New NY Bridge will be the world’s widest and New York’s priciest bridge project to date.

Final work wraps up in 2018, when the remnants of the old Tappan Zee Bridge will be completely dismantled. And, at least for the next century, the New NY Bridge will stand in its place – a testament to the durability of precast concrete and the innovation of the workers who came together to make it possible.

Deborah Huso is a freelance writer specializing in construction, real estate, finance and agriculture.

Claire Vath is a freelance writer specializing in agriculture, construction, and home improvement.