Tappan Zee Bridge

Engineering & Construction

To fully appreciate the engineering significance of the Tappan Zee Bridge, consider the problems that had to be solved before construction could begin. From geology to geography to cost, all impacted construction and design. The final challenge the bridge faced – one that the engineering of that era could not eliminate – was the impact of time.

River So Wide

The bridge project was controversial from the start, and no aspect of its design was more contentious than its location at one of the widest points of the Hudson River. There was a simple reason for the choice: no river crossing could be built within the jurisdiction of the New York Port Authority, which extended along a radius stretching 25 miles north of the Statue of Liberty in New York Harbor. The proposed Tappan Zee Bridge would cross the river just 1,200 feet beyond that boundary – far enough north to comply with the law and far enough south to serve traffic-generating areas in Rockland County and connect with the growing New York State Thruway.

concrete boxes supported as much as 80% of the bridge’s dead load


River So Deep

The Tappan Zee Bridge earned its place on the National is best known and justly celebrated for its use of watertight floating concrete boxes that supported as much as 80 percent of the bridge’s dead load (i.e., the bridge itself and all its components) while alleviating stress on the stabilizing steel piles that extended to the bedrock of the river bed. The depth of water in the Hudson is not constant – with the depth of the river varying from approximately 40 feet under the main span to about 15 feet from the main span to near the Rockland shoreline.  

Bedrock, on the other hand, can be found 250 to 350 feet below the riverbed under the main span and more than 700 feet in the third of the river’s width near Rockland.  Above the rock, there is a mixed composition of organic silt and silty clay which forms the riverbed.

Design Change Lowers Costs

The use of buoyant caissons had a significant impact on the cost of the Tappan Zee Bridge by reducing the number of steel piles needed to support the structure. Even with the caissons, the bridge used more than two million linear feet of piles. Another significant cost-saving came from a re-engineered design for the superstructure of the bridge. The original plan called for a long-tied arch, which would have been aesthetically stunning but expensive – and difficult – to build and maintain. Indeed, when the design was sent out for bid, none came back. The solution was to leave every other aspect of the design unchanged but replace the arch with a cantilever truss that saved time and money and ultimately earned a place in the record books as one of the longest in the world.

tons of concrete needed to be removed


A Daunting Clean-up

After more than 60 years of service, the Tappan Zee Bridge offered one last engineering challenge that was not part of its original plan: New York State environmental regulations required that it be completely dismantled, piece by piece, from the river that had been its home. The task was daunting, involving the removal of as much as 302,000 tons of concrete, nearly 47,000 tons of steel, more than 13,000 timber piles and more.

Nearly all of the salvage was destined for reuse, including material repurposed to create six artificial reefs to promote fish habitat in Long Island Sound. New York State even provided portions of the Tappan Zee’s 13 feet wide by 50 feet long concrete deck panels to counties in the state and other interested parties for $1 apiece.