Arch bridges are one of the oldest types of bridges and have great natural
strength. Instead of pushing straight down, the weight of an arch bridge is
carried outward along the curve of the arch to the supports at each end. These
supports, called the abutments, carry the load and keep the ends of the bridge
from spreading out.
Try It!How do the abutments support an arch bridge?
Cut a strip of cardboard that's about one inch by 11 inches. Gently bend the
strip so that it has a curve. Position the cardboard on a table so that it
resembles an arch. Press down on the center of the arch. What happens to the
ends of the cardboard?
Next, place a stack of books at each end of the arch. Press again. Now what
happens? Notice how the stacks of books act as abutments, keeping the ends of
the arch from spreading apart.
When supporting its own weight and the weight of crossing traffic, every part
of the arch is under compression. For this reason, arch bridges must be made of
materials that are strong under compression.
The Pont du Gard aqueduct
The Romans used stones. One of the most famous examples of their handiwork is
the Pont du Gard aqueduct near Nîmes, France. Built before the birth of
Christ, the bridge is held together by mortar only in its top tier; the stones
in the rest of the structure stay together by the sheer force of their own
The wooden rainbow bridge depicted in this painting is no longer built in China.
The ancient Chinese used an elegant variation of the arch to build the unique Rainbow
Bridge, somewhat of a mystery to engineers today. This unusual arch bridge consists
of short pieces of timber woven under and over cross-beams and then lashed together
with bamboo straps. The bridge's graceful arch allowed bustling commercial river
traffic to pass underneath it.
The New River bridge
Today materials like steel and pre-stressed concrete have made it possible to
build longer and more elegant arches, including a spectacular 1,700-foot span in
New River Gorge, West Virginia. (More typically, arch bridges span less than 800 feet.)
Centering under construction
Constructing an arch bridge can be tricky, since the structure is completely
unstable until the two spans meet in the middle. One technique is to build
elaborate scaffolding, or "centering," below the spans to support them until
they meet. A newer method supports the spans using cables anchored to the
ground on either side of the bridge. In situations where there is an active
water- or roadway below, this method allows contractors to build without
Arch construction using cable supports
One of the most revolutionary arch bridges in recent years is the Natchez Trace
Bridge in Franklin, Tennessee, which was opened to traffic in 1994. It's the
first American arch bridge to be constructed from segments of pre-cast
concrete, a highly economical material. Two graceful arches support the
roadway above. Usually arch bridges employ vertical supports called
"spandrels" to distribute the weight of the roadway to the arch below, but the
Natchez Trace Bridge was designed without spandrels to create a more open and
aesthetically pleasing appearance. As a result, most of the live load
is resting on the crowns of the two arches, which have been slightly flattened
to better carry it. Already the winner of many awards, the bridge is expected
to influence bridge design for years to come.