Tappan Zee Bridge

Key Design Features

Its curved approaches and latticed main span made the Tappan Zee Bridge a striking sight as it crossed the Hudson River. What was even more remarkable about this historic structure was how the geometry and design of the bridge elements were adapted to address dramatic changes in foundation conditions that existed at different points along the crossing.

Number of bridge sections


longest section

8,739 FEET

Five separate sections:

Since the 3.1-mile distance from shore to shore was far too wide for a single span, engineers divided the bridge into 5 separate sections:

The longest section was the 8,739-foot Causeway


1: Causeway

Starting in South Nyack, the longest section was the 8,739-foot causeway, a concrete deck supported by horizontal steel beams resting on reinforced concrete columns spaced every 50 feet. With 166 spans in total, the simple span construction was also comprised of a stringer and deck superstructure, as well as footings on timber piles

2: west deck truss

Moving east towards Westchester, the next section was the 1,750-foot west deck truss. Portions of this section rested on solid concrete circular caissons and some of the smaller buoyant rectangular caissons (measuring 110 feet by 56 feet by 40 feet) which were partially supported by piles driven into the bedrock far below. The truss spans also included deck slab and steel trusses, along with the concrete columns supported by the caissons.

3: main span

The main span section of the bridge, over the navigation channel, comprised two anchor spans (1,204 feet) and two cantilevers flanking a suspended center span (1,212 feet), with the two anchor spans resting on columns supported by mid-sized buoyant caissons measuring 126.5 feet by 77 feet by 40 feet. The cantilever sections rested on the largest buoyant caissons measuring 190 feet by 100 feet by 40 feet. All the caissons in this section were secured by steel piles driven to bedrock.


The 3,250-foot east deck truss began at the eastern end of the main span, about 2.4 miles into the river from the western shore of Rockland and rested on reinforced concrete columns and circular caissons to rock. Although the east deck truss is longer than the west (13 spans versus 7), engineers used similar materials and processes, with the exception of the rectangular caissons – which were not used here. Rock is quite high here.

The final section was the 300-foot long East Trestle


5: East Trestle

The final section, the 300-foot long east trestle, was a concrete deck on multiple steel beams and concrete columns that rested on the rock escarpment at the river’s eastern edge.

watertight concrete boxes


Bridge-builder Mastery

The extraordinary innovation of the bridge design can easily overshadow the skill and innovation used to build the bridge. Not unlike the Governor Mario M Cuomo Bridge, some of the 1955 Tappan Zee Bridge was prefabricated and transported to site for installation. For example, the rectangular caissons that supported the main span crossing of the navigation channel: eight watertight concrete boxes the size of small buildings — some 40 feet tall — and then move them 12 miles downriver. The ingenious approach meant more easily constructing the caissons in a deep basin along the shoreline in Havestraw, flooding them to make them float, easing them into the river and using tugboats to guide them to the bridge construction site. 

And beyond the caissons, engineering mastery was evident in the design adaptations to the profile, superstructure and foundations to address what was going on deep beneath the water. From non-uniform geometry to short span lengths and shallow foundations where rock was not reachable to steep grades and longer spans where rock was reachable, the bridge designers implemented forward-thinking approaches to tackle the most challenging of problems.

The Wizardry of Construction

A leading technical journal, the Engineering News-Record, even marveled at the wizardry displayed in constructing the bridge, from casting and curing to welding and splicing. The magazine also applauded some of the labor-saving ingenuity displayed: scaffolding built on a truck that could be quickly driven around the caissons to wherever it was needed; the use of everyday tools to complete essential tasks such as an ordinary paint sprayer to apply compound to walls, an auto mechanic’s pneumatic wrench to tighten bolts, and lengths of standard garden hose, punctured at intervals, to spray water on concrete during curing.

A Floating Pier

Although the Tappan Zee Bridge was the first permanent river crossing supported in part by buoyant caissons, the approach was first used on the Hudson a few miles downstream for Pier 57 on Manhattan’s Lower West Side. That project was also designed by Emil Praeger, who later refined the approach for the Tappan Zee’s watertight concrete boxes.