June 18, 2012

I’d like to continue the “New York notes” that I started in the last issue.

1. New York Bridges

The FiveBoro Tour crossed about five bridges. Most of the big bridges in New York are suspension bridges — the Brooklyn Bridge, the Verrazano Bridge, the George Washington Bridge, etc. Perhaps the first suspension bridges were rope bridges slung across ravines by the natives of Tibet or Peru. Such a bridge would tend to sag in the middle (“suspension” comes from the Latin pendere, to hang down). But what if the bridge’s deck, or roadway, were held up not by sagging horizontal ropes, but by a second set of ropes — by vertical ropes hanging down from the horizontal ropes? If the vertical ropes were all the same length, they would perpetuate the sag in the middle, but if the vertical ropes in the middle were shorter, they would correct the sag. This is the basic principle of a suspension bridge.

If the horizontal ropes/cables are suspended from tall towers, the bridge will be high above the water, allowing ships to pass beneath. The most conspicuous feature of suspension bridges are their tall towers (usually two in number), and the design of these towers allows the tourist to identify the bridge. The towers of the Brooklyn Bridge have two Gothic pointed arches (the Brooklyn Bridge was begun in 1870, when the Gothic style was popular).

Also characteristic of the Brooklyn Bridge are the massive, solid bases of its towers.

The Manhattan Bridge, begun in 1901, has one pointed arch in each of its towers:

The Verrazano Bridge, begun in 1959, has tall, slender towers, with one rounded arch:

The Verrazano Bridge has a span of about 1300 m, as compared to just 500 m for the Brooklyn Bridge. The Verrazano Bridge is so long that it’s affected by the curvature of the earth: “Because of the height of the towers (211 m) and their distance apart (1,298 m), the curvature of the Earth’s surface had to be taken into account when designing the bridge — the towers are 1 5/8 inches farther apart at their tops than at their bases.”1

The Triboro Bridge (East River section), begun in 1929, has slender towers with no arches at all:

Suspension bridges can handle long spans. For shorter spans, a “cable-stayed bridge” is sometimes used. In a cable-stayed bridge, the roadway is supported by cables tied directly to one or more towers; it doesn’t have the hanging/sagging character of a suspension bridge.2 An example of a cable-stayed bridge is Boston’s Bunker Hill Bridge, begun around 1995:

Here’s another example of a cable-stayed bridge:

the Langkawi Sky Bridge, a pedestrian bridge in Malaysia

For even shorter spans, a cantilever bridge might be used. A cantilever bridge isn’t supported by cables from above, but by piers from below.

The Queensboro Bridge, begun in 1901, is a cantilever bridge, perhaps because it doesn’t need to span the entire East River, like the Brooklyn Bridge and the Manhattan Bridge, it only needs to reach Roosevelt Island. Or to put it another way, Roosevelt Island makes it possible for the Queensboro Bridge to have more piers, and a shorter span. Or to put it still another way, if Roosevelt Island weren’t there, the Queensboro Bridge would probably be a suspension bridge, with fewer piers and a longer span.

The Queensboro Bridge circa 1908

Several bridges over the Harlem River are arch bridges, such as these three:

From nearest to farthest: the High Bridge, the Alexander Hamilton Bridge (part of I-95), and the Washington Bridge

On the High Bridge’s east side (Bronx side), the old Roman-style arch bridge, begun in 1837, can still be seen.

This old section was part of the Croton Aqueduct, which brought water from the Croton River to New York City, a distance of 41 miles. This aqueduct is now a trail/park. The Friends of the Croton Aqueduct organizes walks. A group called NYC H2O also organizes water-related walks.

While suspension bridges have a downward arc, a sag, arch bridges have an upward arc, a rise.

The Konitsa Bridge over the Aoos River in northern Greece, built in 1870. The roadway is just two meters wide.

Some of Europe’s old bridges are arch bridges, such as the Ponte Vecchio in Florence, built in 1345:

While the Ponte Vecchio has three arches (three spans), the Rialto Bridge in Venice (begun in 1588) has only one arch:

Boston’s Longfellow Bridge has eleven arches:

An arch bridge like the Longfellow Bridge has its roadway on top of its arches. On the other hand, a Through Arch Bridge has its roadway through the lower part of its arch, and much of its arch is above the roadway. An example of a Through Arch Bridge is New York’s Hell Gate Bridge:

The Hell Gate Bridge, looking southwest toward Manhattan. On the right is Wards Island, on the left is Astoria Park in Queens. Behind the Hell Gate Bridge is the Triboro Bridge (East River span).

Hell Gate Bridge was a model for the Sydney Harbor Bridge.

Perhaps the best introduction to bridges — their history and their engineering — is David Macaulay’s short documentary, Building Big: Bridges. The Building Big series includes documentaries on tunnels, skyscrapers, dams, and domes, as well as bridges. The documentaries are based on Macaulay’s book, Building Big.

Someone started a blog devoted to New York bridges. It’s called “The Bridges of NYC: An Urban Odyssey,” and the motto is “There are 2,027 bridges in NYC. I’m trying to find all of them. And then cross all of them.” The blog gets data from nycroads.com.

If you want to learn more about New York bridges, consider The Great Bridge, David McCullough’s book about the building of the Brooklyn Bridge. If you’re interested in New York skyscrapers, visit this website, and consider reading Skyscraper: The Making of a Building, by Karl Sabbagh.3 John Tauranac has written several books about New York and its skyscrapers. Charles Lockwood is the author of Bricks and Brownstone: The New York Row House, 1783-1929. The architecture of many New York buildings is discussed in The AIA Guide to New York City (AIA = American Institute of Architects). There’s even a website that attempts to describe every building on every street in the southern half of Manhattan (south of 59th Street). And there’s a website that attempts to draw every building in New York. (New York seems to attract obsessive types, people who want to put their arms around its immensity, who want to know it from top to bottom.) The AIA hosts a boat trip around Manhattan, with architecture commentary.

There’s a mnemonic device to help remember the names of New York bridges: BMW — Brooklyn, Manhattan, Williamsburg is the sequence of bridges, moving from south to north. There’s also a mnemonic device to help remember Manhattan streets: Ladies Prefer Men (when travelling west) — Lexington, Park, Madison.

© L. James Hammond 2012
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1. Wikipedia back
2. Since the Brooklyn Bridge has some cables running directly from towers to roadway, it’s technically a hybrid of Suspension Bridge and Cable-Stayed Bridge. back
3. This book was made into a documentary, but the documentary isn’t easy to find. According to Wikipedia, “The building of One Worldwide Plaza [on 8th Ave. and 49th Street] was documented in a Channel 4 / PBS mini-series and a companion book Skyscraper: The Making of a Building by Karl Sabbagh.” back