Bridge is a very important human invention. It is essential
to our daily life and it facilitates a convenient transport.
Mankind has learned how to cross rivers and valleys by bridges
since the ancient time. Early bridges were made by trees,
ropes or rocks, etc. The main function of a bridge is to improve
our transportation to facilitate the travel. A bridge is like
an aerial road that let our traffic be capable of striding
across natural obstacles such as straits, rivers, valleys…
In some big cities, bridges are used frequently as a crossover
to reduce traffic jam and raise transportation efficiency.
The construction of a bridge focuses on bearing
the weight (or gravity) of vehicles and humans on it. The
load acts directly on the desk and then passes to the base.
How can the load be passed to the ground efficiently? In the
following, we are going to introduce different designs of
bridges under different conditions.
Arch bridge is one of most ancient types of
bridge design. Arcuate architectures were common in medieval
time. By examining the arch bridge structure, we find that
the vertical force acted on the desk would pass to the supporting
points on both sides of the bridge through the designed arch-shaped
curve. The following experiment simulates an arch bridge under
pressure:
Hold
the ends of a long ruler with two hands and then put a book
on the middle of the ruler. You will notice that the ruler
deforms obviously. Then hold the ends of the long ruler again
but haunch it up to simulate an arch bridge. Put a book on
the middle of the ruler again. You will observe that the ruler
has no obvious deformation.
As a result, the capacity on bearing the vertical
force acting on a structure would increase if the structure
is in an arch shape. The famous Sydney Harbour Bridge in Australia
is a typical example of arch bridge structure. Think carefully
what happens when an arch bridge is under pressure! You may
discover that the entire arch bridge structure is under pressure.
Therefore, materials that can bear high pressure, such as
natural rocks, are the best materials for building arch bridges.
Many old bridges are arch in shape for increasing the spans
between the bridge piers. Anji Bridge, built at Zhaozhou in
China in the seventh century, was once the bridge with the
longest arch span in the world. This record had been kept
for more than six hundred years until the thirteenth century.
(Sydney Harbour Bridge)
(Anji Bridge)
Continuous bridge is another common type of bridges. Continuous
bridge is usually built over shallow water areas or places
where large ships cannot pass through. The Island Eastern
Corridor in Hong Kong is a well-known example of continuous
bridge design. If you observe its structure carefully, you
may discover that the bridge desk was supported by many bridge
piers.
(The Island Eastern Corridor)
Cable-stayed bridge also is one of the common types of bridges.
Its special structural feature is that there are pylon/bridge
towers built on the bridge pier with cables on both sides
of tower. To construct a cable-Stayed Bridge, we have to build
a pylon/bridge tower on the bridge pier, and then suspend
cables from both sides of the pylon/bridge tower to the bridge
desk. The load acting on the bridge is passed to the pylon/bridge
tower through the cables and then to the ground through the
pylon/bridge tower, so as to reduce the deformation extent
of the bridge and the stress on the bridge. Steel cables with
high tensile intensity are chosen as they have to sustain
strong tension over a long period of time. For examples, Kap
Shui Mun Bridge and Ting Kau Bridge in Hong Kong are designed
in cable-stayed style using steel for making the bridge cables.
The length of the cable-stayed bridge is usually between 160
and 900 meters.
[Remarks: Ting
Kau Bridge is 1177m long. 900m is NOT the limit for cable-stayed
bridges. From the economic point of view, the cable-stayed
bridge is usually most competitive when the span is between
160 to 900m (but even this range is just a rough estimate).
Depending on the local geographical condition as well as aesthetic
considerations, the cable-stayed bridge can be the choice
even when the span is over 900 m.]
(Kap Shui Mun Bridge)
(Ting Kau Bridge)
The last kind of bridge we like to introduce is the suspension
bridge. This type of bridge design is commonly used for striding
across straits. The structural features of the suspension
bridge are similar to those of the cable-stayed bridge: both
designs are making use of the pylon/bridge tower. The bridge
piers are connected to the bridge desk with a huge suspending
cable and vertical steel cables. To construct a suspension
bridge, we have to first build pylon/bridge towers on both
sides of the bridge pier. Then the main suspending cables
are hung on the towers. Then connections are made between
the main cables and the bridge desk with vertical steel cables.
The ends of the main cables are anchored to the ground by
concrete to hold against the great tension acting on it. Golden
Gate Bridge, located at San Francisco in California, is a
famous suspension bridge. The recently constructed Tsing Ma
Bridge, which connects between Tsing Yi and Lantau Island
in Hong Kong, is also designed in suspension style with a
surprisingly large span. In general, it is a very good bridge
design in practice as the span of the suspension bridge can
reach to more than 2000 metres.
(Golden Gate Bridge)
(Tsing Ma Bridge)
We have summarized a lot of information on bridges
in this article. Is it helpful to you? Try to pay attention
to the bridges around you in daily life and observe their
structural features to see whether it matches what you have
learnt in the above passages.
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