The
Structure
That Never Sleeps
Special
messengers race into ancient Rome bearing the news of the
emperor’s victory in battle. All citizens—rich
and poor—eagerly look forward to the festivities that
will accompany the emperor’s return to the city. The
greatest of these is the emperor’s triumphal procession
as he reenters Rome. All Romans cheer and wave to their
leader as he passes beneath the mighty arches of the city
walls, followed by legions of Roman soldiers and citizens
of the conquered nation.
As tributes to especially important victories or as tributes to the greatness of the emperors themselves, special monumental arches called triumphal arches were also erected in Rome. The Arch of Titus was built in A.D. 81 to celebrate Emperor Titus’s capture of Jerusalem in A.D. 70, and the Arch of Constantine was built between A.D. 312 and 315 to celebrate Emperor Constantine’s victory over his rival and brother-in-law, Emperor Maxentius. These were free-standing arches. That is, they stood alone without being part of a wall and were placed in prominent positions so that they could be seen from very far away.
Why
did the Romans choose the arch as a symbol of triumph? It
may be because the arch represented the triumph of Roman
engineering over the most difficult structural problems.
The architects of ancient Egypt and Greece knew about the arch, but their architecture was mostly trabeated, based on vertical columns spanned by horizontal, flat beams. The Romans didn’t invent the arch; they adopted the form from the architecture of the Etruscans, the people who ruled Italy before the beginning of the Roman Republic in the sixth century B.C. But to their credit, the Romans perfected the techniques of arch construction, making Roman architecture different from anything that came before it. It was because they understood the techniques of arch construction that Roman architects would later be able to develop even more complicated structures, such as the vault and the dome.
Even though the Roman Empire eventually collapsed in the fifth century A.D., its architecture continued to have a huge influence in later centuries. Triumphal arches were built long after there were no more Roman emperors. Perhaps the most famous triumphal arch of all is the Arc de Triomphe in Paris, commissioned by Napoleon Bonaparte in 1806 to honor the Grand Army and completed in 1836.
In
the United States, the Washington Arch was built in New
York City’s Washington Square to celebrate the one-hundred-year
anniversary of the inauguration of George Washington as
President of the United States. Actually, there were two
Washington arches. The first one, built in 1889, was a temporary
arch made of wood and plaster, and was intended to be taken
down when the celebration ended. New Yorkers, however, liked
the arch so much that it was soon decided to build a permanent
one in marble. This one was dedicated in 1895.
Today, arches are found everywhere. Small arches can be used as doors, and large ones as bridges. Arches can be built of brick, stone, concrete, or steel. They don’t have to be round, either, but can be made in lots of special shapes. In addition to their usefulness, arches still have the power to capture our imaginations.
One
such arch stands on the banks of the Mississippi River in
St. Louis, Missouri. The “Gateway to the West”
was designed in the late 1940s by architect Eero Saarinen
to honor Thomas Jefferson and the expansion of the United
States to the west. Topped off in 1965 and dedicated in
1968, the Gateway Arch has the form of a catenary curve.
(If you hold each end of a chain and let it droop between
your hands, you have created a catenary curve.) As high
as a sixty-three-story building, the carbon-steel arch is
covered in gleaming stainless steel. A special kind of tram
carries visitors to the top.
Of all the ways that architects have invented of going from “here” to “there,” the sweeping curves of the arch may be the most beautiful. Now that you know about arches, look around you. You’re sure to find these lovely shapes in your town or city, too.
How
an Arch Is Built
An arch
is made of wedge-shaped stones called voussoirs.
The central stone of the arch is called a keystone,
which takes its name from the key role it plays in the structure.
Neither half of the arch can support itself, so in order
to stay upright, both halves must lean against the keystone.
To build an arch, a temporary structure called a centering is constructed. The voussoirs rest on the centering until the keystone is wedged into place. At that point, the centering can be dismantled. Leonardo da Vinci described it this way: “An arch consists of two weaknesses, which, leaning on each other, become a strength.”
An arch can carry more weight than a straight beam can carry across the same span, so it’s a more powerful structure, but it is also more complicated to support. A beam resting on two posts is a simple structure: the beam presses down and the posts resist by pressing up. The arch, the more complex structure, presses down, at its top and presses out at its sides, so it requires constant bracing in two directions. Maybe it’s because of this that an Eastern proverb says “the arch never sleeps.”
