| Bridges are built to allow a continuous flow of highway
and railway traffic across water lying in their paths. But engineers
cannot forget that river traffic, too, is essential to our economy.
The role of 1
is important. To keep these vessels moving freely, bridges are built
high enough, when possible, to let them pass underneath. Sometimes,
however, channels must accommodate very tall ships. It may be uneconomical
to build a tall enough bridge. The 2
would be too high. To save money, engineers build movable
bridges. |
|
1a) wind
c) weight
e) experience
2a) levels
c) standards
e) deck |
b) boats
d) wires
b) cost
d) waves |
|
|
|
|
| In the swing bridge, the middle part pivots or swings
open. When the bridge is closed, this section joins the two ends
of the bridge, blocking tall vessels. But this section 3
. When swung open, it is perpendicular to the ends of the
bridge, creating two free channels for river traffic. With swing
bridges, channel width is limited by the bridge's piers. The largest
swing bridge provides only a 75-meter channel. Such channels are
sometimes too 4 .
In such cases, a bascule bridge may be built. |
|
3a) stands
c) wears
e) supports
4a) narrow
c) long
e) straight |
b) floods
d) turns
b) rough
d) deep |
| |
|
|
|
| Bascule bridges are drawbridges with two arms that
swing upward. They provide an opening as wide as the span. They
are also versatile. These bridges are not limited to being fully
opened or fully closed. They can be 5
in many ways. They can be fixed at different angles to accommodate
different vessels. |
|
5a) crossed
c) lighted
e) positioned |
b) approached
d) planned |
| |
|
|
|
| In vertical lift bridges, the center remains
horizontal. Towers at both ends allow the center to be lifted like
an elevator. One interesting variation of this kind of bridge was
built during World War II. A lift bridge was desired, but there
were wartime shortages of the steel and machinery needed for the
towers. It was hard to find enough 6 .
An ingenious engineer designed the bridge so that it did not have
to be raised above traffic. Instead it was 7 .
It could be submerged seven meters below the surface of the river.
Ships sailed over it. |
|
6a) work
c) time
e) space
7a) burned
c) secured
e) lowered |
b) material
d) power
b) emptied
d) shared |
