Arch Bridge
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?
The Romans used stones. One of the most famous examples of their
handiwork is the Pont du Gard aqueduct near Nīmes, 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 weight.
Today materials like steel and pre-stressed concrete have made it
possible to build longer and more elegant arches, including a spectacular
1700 foot span in New River Gorge, West Virginia. (More typically, modern
arch bridges span between 200-800 feet.)
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 road way below, this method allows contractors to build
without disrupting traffic.
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. |
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.
