PUSHES AND PULLS
Bridges have to be very strong. They have to be able to hold
up their weight and the weight of the cars,trucks,and people
going across it. A bridge also has to be stable and not move
during high winds and heavy rains. A change in weather can
make the temperature very hot or very cold and the materials
that make up a bridge can't break or crack.
Engineers have to consider many things when they build a
bridge:how far it will have to cross,what load it will carry, what
size will it be, and how much it will weigh. The foundation of a
bridge may stand in water. Engineers use a special device
called a cofferdam that allows them to pump water out and
replace it with concrete this way, the foundation will be sturdy
Bridges can be built with many different materials but the
most common are wood,steel,and concrete. Each material has
pluses and minuses. Engineers have to consider the tension
and compression of each material and how well it will stand up
at different pressures.
Wood is very cheap and doesn't weigh alot. It is also very
strong but it decays quickly. It also swells in the rain and can
burn in a fire. There aren't many covered bridges any more. It
was practical for settlers and pioneers to put covers over the
bridges because the rain and snow would rot the wood. People
like to take photos because the bridges are so pretty and old.
Concrete is another material used to build bridges. It doesn't
cost much and it is fire and weatherproof. It is easy to shape to
any size. But concrete can crack easily when the weather
changes from hot to cold then back again.
Steel is yet another material used to build bridges. Steel is
one of the strongest materials used to build things but it can
rust. Steel cables are used in suspension bridges because they
make strong cables and cables are used to support the roads.
Many engineers use a material called reinforced concrete. It is
concrete that has metal bars inside. It is inexpensive and is fire
and weatherproof. It can also be changed to any shape and is
extremely strong. It can crack through.
A simple machine help people do work easier. Simple
machines are used in many different ways to build bridges. On
our field trip to a drawbridge, we saw simple machines in action.
Engineers used wheels,gears,ramps, and levers to assemble the
Wheels are almost the most important thing ever invented. A
wheel is round and is used to make things move. When a
wheel has a handle on each side it can be turned. We saw a
special wheel called a gear used in many ways on the
drawbridge. Gears have special slots called teeth. Gears work
together in pairs and one gear is always bigger than the other.
The teeth of one gear fit together with the teeth of the other
gears teeth. One gear has a handle and can be turned to move
the bridge manually! Can you imagine how strong you would
have to be to do this?
Ramps are also used in bridges. A ramp is a slope that looks
like a triangle turned on its side. A wedge is a small ramp.
Engineers use abutments to support the end of a bridge.
Abutments are a type of wedge that jam into the ground. Think
of how a doorstep works.
Another type of simple machine we saw was a lever. A lever
is a bar that balances things and move big loads. Think of a
see-saw. Levers use a pivot in the middle of the bar to turn or
pull drawbridges up so that boats can pass.