- Why are steel and stone bridges
arched in opposite directions, one upward and one downward?
This is because stones and steel are made up of very different
materials. Stones can withstand being pressed, but it is
relatively easy to pull them apart. For this reason, stones
are pressed together in an arched bridge. This makes stone
bridges strong enough for people, cargoes and vehicles to
cross.
Stone bridge
Steel is just the opposite. It is strong when under tension,
i.e. when being pulled. The structure of a steel bridge is
such that the steel cables are all arched upward so that they are
constantly under tension. For this reason, people rarely
build a steel bridge that is arched
downward.
Tsing Ma Bridge
(Photo: Courtesy of H.Y. Chiu)
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- Is this why concrete has to be
pre-stressed?
Yes. Concrete is like stones – it is strong when
pressed.
Pre-stressed concrete is concrete with steel cables inside.
These cables are under tension, i.e. being pulled, before the concrete
hardens. The cables then want to contract, but they
cannot. This way, the cables keep the concrete constantly
compressed, while they themselves are still under tension.
The opposite properties of concrete and steel are thus combined to make
a building material which is stronger than reinforced concrete and
cheaper than steel alone.
Pre-stressed concrete was developed in the 1950s and is recognized as
one of the greatest advances in construction of the 20th
century. It is widely used in many buildings.
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