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FYIStructure: Defying gravity

Good structures survive the elements and stand against the forces of gravity -- compression, tension, and bending.

  1. Bridges<
  2. How buildings stand up
  3. Earlier structural methods

Bridges and Buildings

FYI Structure<

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Think back to the last time your rode a Ferris wheel. . .

With the wind on your face, and the squeals of delighted laughter all around you, you clutch the ridiculously thin security bar holding you into your seat. You are probably thinking about many things: The exhilarating thrill of the ride, the breathtaking birds-eye view, the questionable wisdom of eating a hot dog and candy apple just before getting on this crazy wheel!

Highly unlikely, though, is the possibility that you are considering bridge technology.

"Bridge technology?" You ask. "Like, bridge-across-a-river technology?"


In 1893, a Pittsburgh engineer applied bridge technology to construct a large, revolving wheel for the Chicago World's Fair. His name? George Ferris. . . and his invention was called, of course, the Ferris Wheel! The Ferris Wheel is like a steel bridge in that it gets structural strength and light weight from triangular frameworks called "trusses."

Susan Donley

The strength of trusses comes from the rigid shape of the triangle. Besides holding up bridges, trusses are often found holding up roofs, floors, walls--and Ferris wheels!

With its rivers, hills and valleys, Southwestern Pennsylvania has more bridges per square foot and per capita than any other city in the world. Arch bridges, girder bridges, foot bridges, train bridges – experts estimate the number of bridges in our region to be in the thousands! In fact, the only kind of bridge we don't have here in Southwestern Pennsylvania is a drawbridge. . . but then, we don't need one because our hills conveniently make all of our bridges high enough for boats to pass under!

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Bridge basics

Bridges are a tough structure to master. The tricky part is to make them strong and lightweight enough to span a distance without bending or buckling, yet flexible enough to handle traffic, wind, and temperature changes without shattering. For over 200 years, Pittsburgh bridges have met those challenges with a succession of progressively modern bridge designs.

Bridges are a great way to study structure because all of the structural elements are in plain view. The next time you're caught in a traffic jam, take a moment to figure out the science and mathematics of such engineering feats.

The simplest kind of bridge is a simple beam spanning two points. When gravity acts on a beam, it bends. A beam can be stiffened against bending by making it larger and stronger, as in the Veterans Bridge across the Allegheny, or more commonly to make the beam into a huge truss, which uses the lightweight triangle to resist bending. Traffic on truss bridges can drive over the truss or literally through the truss.

Veterans Bridge
Susan Donley

The Veteran's Bridge is one large beam made of ribs of smaller "I" shaped-beams. Hidden trusses between the ribs also help stiffen the bridge deck against bending.


Another way to counteract bending is to bolster it from underneath with an arch, as in the steel-arch 31st Street Bridge or the concrete-arch George Westinghouse Bridge over Turtle Creek.

George Westinghouse Bridge
Pittsburgh History & Landmarks
The concrete arches of the George Westinghouse Bridge work under compression, transferring the load of the deck and its traffic to the bridges piers and from there to the earth.


A third way to counteract bending is to actually hang or suspend the deck of the bridge from above from an arch or its opposite--a catenary. The 16th Street Bridge is suspended from an arch, while further down the Allegheny are three bridges suspended from catenaries.

16th St. Bridge
Susan Donley

The 16th Street Bridge is suspended from a steel arch. Notice the trusses stiffening the arch.


The artistic trio of bridges connecting the North Side to the Golden Triangle is known affectionately as "The Three Sisters." The 6th, 7th and 9th Street Bridges are chain suspension bridges and they are the only example in the world of three identical bridges positioned side-by-side-by-side.

9th, 7th, and 6th St. Bridges
Susan Donley

The identical 6th, 7th, and 9th Street Bridges, nicknamed "The Three Sisters," are catenary suspension bridges. Suspension bridges use the force of tension to counteract the bending forces on the deck of the bridge.


Many bridges combine techniques to handle their loads. The filigree of trusses within in the Smithfield Street Bridge stiffen the larger lenticular truss, whose arch above and catenary below function like the leaf spring on a car. Walk across this bridge to feel the spring action at work!

Susan Donley

The Smithfield Street Bridge's unique design combines the qualities of arch and suspension construction to create the limber lentricular truss. The bridge, built in 1883, is Pittsburgh's oldest.

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Bridge history

Rich history and geography lessons also are evident in a study of our bridges, which suggest the evolution of commerce and transportation in the region. In the 18th century, French and British forces fought to control the Point where the three rivers meet – since the army that controlled Pittsburgh would control the Ohio and Mississippi Rivers and from there the interior of the continent. In the 19th century, Pittsburgh saw the advent of manufactories and the dawn of the steamboat era. Soon the ferries connecting Pittsburgh with the cities of Birmingham (now the South Side) and Allegheny (now the North Side) could not handle traffic across the rivers and bridges became a necessity of urban development. (See Western PA History and Rivers and Valleys.)

Smithfield Street Bridge
Pittsburgh History & Landmarks

Pittsburgh's first bridges were wooden and covered to protect wood decks from rain and snow. This is the 43rd Street Bridge built in 1870 over the Allegheny, long after iron bridges were becoming common, and demolished in 1924.

In 1820, Pittsburgh got its first bridge at Smithfield Street, a covered wooden, eight-span structure that was destroyed by fire in 1845. By then, pioneering bridge engineers made this region home to America's first iron bridge in Brownsville, and the first steel cable suspension bridge.

Smithfield Street Bridge
Pittsburgh History & Landmarks

The Pittsburgh, Fort Wayne, Chicago Rail Road Bridge was an early iron bridge built across the Allegheny in 1857.


An engineer from Saxonburg, John Roebling was the first to formulate the mathematics of suspension bridges and manufacture the wire cable that made them possible. He perfected his technique on the 6th Street Bridge of 1859 and the Smithfield Street Bridge of 1846, both predecessors of today's spans of the same names. Roebling later went on to use his innovations to design and build New York's Brooklyn Bridge!

The first 6th Street Bridge was one of John Roebling's pioneering suspension designs.

Roebling's 6th St. and Smithfield St. Bridges
Pittsburgh History & Landmarks

New designs and a new abundance of structural steel made possible bridges that stretched across the rivers and valleys with only one or two spans. By the dawn of the 20th century, Pittsburgh's Golden Age of Bridges was giving us such masterpieces as "The Three Sisters," the 16th Street Bridge (both above), the 10th Street Bridge, the Liberty Bridge, and the Westinghouse Bridge (above).

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In fact, many other cities' bridges were built at a Western Pennsylvania factory called the American Bridge Company -- which gave it's name to the town of Ambridge.

Today, Pittsburgh is recognized around the world for its impressive collection of bridges. Throughout the city's history and continuing into the next Millennium, our bridges stand as the ultimate marriage of form and function, and serve as a fitting tribute to the region's proud steel heritage.

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Bridges and Buildings