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Physical Science: Bridge
Building
Goals:
For thousands of years, people have been obsessed with taking the shortcut
over valleys and rivers instead of going around them. To do that, we use
bridges. In this module, you will have the chance to become a bridge
engineer and design and build your own. You will also learn about some of
the different types of bridges and how to decide which one to use in
different situations.
Bridge Building: Teacher Notes
Download: (Abobe Acrobat Reader required--download free
reader)
Print the Bridge Building Lab Manual
Supplies Needed:
The experiment is designed to be done in teams of 2-3. Each team needs this
lab manual, a bridge building kit, and a book or other weight.
Procedures: The Beam Bridge
Bridges are characterized by how they support themselves. The simplest type
of bridge is the beam bridge. This type of bridge has a single horizontal
beam across two supports. You have probably made this bridge a number of
times yourself. Any time you have placed a log across a creek, or have
created a balance beam by placing a long piece of wood on two bricks, you
have created a beam bridge.
Figure 1: Beam
Bridge
Beam bridges are fairly common. An example in Figure 1.
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Beam bridges are also common for highways going over other highways or over
small creeks.
Try building one of these yourself using the kit. The first thing you will
need to do is build a bunch of units that act as the basic building blocks
we will use.
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Figure 2: Basic Building Block for Bridge
Experiments
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As you saw in Figure 6, a truss bridge can have a lot of triangles in it.
Why is this? Build the following two objects in Figure 8. Each is a
four-sided box, but the one on the right (8-B) has a cross piece which
really makes it two triangles.
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Gently push the upper-left and the lower-right corners towards each other.
Whoa! That's enough. Not too hard now. Which object is better at holding
its shape? Why do you think this is true? The andwer is in the next
paragraph, but don't peek until you have thought about this for a moment.
The two triangles are better at holding their shape. The cross member
adds a lot of strength. In engineering terms, the box with the cross member
is more rigid. The reason is that 3 lengths uniquely define a triangle. If
I tell you what the 3 lengths are, you can tell me what the triangle has to
look like. There are not 2 answers, just 1. (In a geometry class, you will
call this "side-side-side".) This is not the case with a 4-sided
parallelogram. It can adopt many shapes depending on the angle between 2
sides.
So, a truss bridge is really just a beam bridge that has been made way more
rigid by the addition of more layers and possibly strengthened even more
with triangles. Add triangles to your truss bridge and see how much stiffer
it is. Refer to Figure 9 as necessary.
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Figure 9: Strengthening the
Truss Bridge
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Procedures: Suspension Bridges
Suppose we want to build a bridge across a wide space, but don't want to
use as much steel as all those triangles would require. After all, steel is
heavy and expensive. What else could we do to support our bridge and keep
it from crashing down to the water and rocks below?
I know, how about we hire a fleet of helicopters to suspend the bridge
with strong ropes as shown in Figure 10?
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Figure
10: Suspending a Bridge from Helicopters 
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What's that you say? "You can't be serious!"?
You are right, I'm not. If you thought steel was expensive, try a fleet of
helicopters! The idea of suspending the bridge is good, but this particular
implementation is goofy. OK, let's put a cross piece across two tall towers
and suspend the bridge from that, as shown in Figure 11.
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Figure
11: Suspending a Bridge from a Crosspiece
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This looks much better, except the crosspiece is essentially a beam bridge
by itself and will sag enormously. Well, as long as we know the thing doing
the suspending will sag anyway, why not make it cheaper and lighter by
making it into a sagging cable and suspending the bridge from that, as in
Figure 12?
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Figure
12: Suspending a Bridge with Large Cables
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This approach is called a suspension bridge. But if it
were to be built exactly as showin in Figure 12, the two towers would try
to fold in on each other, as in Figure 13:
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Figure
13: A Small Problem...
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So, suspension bridges also have an extra set of cables
on the outside to help keep the towers from collapsing inwards as shown
here, in Figure 14:
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One of the tricks to building suspension bridges is how
large you can make the distance between the two towers. Because of advances
in engineering, bridge designers have been able to make that distance
larger and larger.
The graph on the right, Figure 15, shows how that distance has increased as
time goes on and engineering techniques improve. It came from a great
article in the December 1997 Scientific American called "Building the
Biggest."
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Figure
15: Increasing Center Span Lengths on Suspension Bridges
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Some of the most beautiful and dramatic bridges in the world are suspension
bridges. The Golden Gate bridge in San
Francisco is probably the best-known.
In essence, the strings or cables transfer the load from the middle of
the bridge span to the towers. The towers need to be that much stronger, of
course, but you need less of them.
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Figure 16: The Golden Gate Bridge in San Francisco
is a Suspension Bridge
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Procedures: Arch Bridges
Suppose we want to build a bridge across a wide space, but don't want to
string cables for a suspension bridge. Are there any other approaches? Yes,
in fact, there are. One of these is an arch bridge (Figure 17):
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Figure 17: An Arch
Bridge
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Suggested Home Activities:
Look around at the bridges in your life. You will find them spanning
rivers, valleys, other highways, etc. What kind of design was used? Can you
figure out why? Were the designers worried about cost? Were they worried
about not having to use too many supports?
Web Connections:
Check out these interesting web sites that have to do with bridges:
- Bridges - Photographs, diagrams and
explanations of the basic types of bridges, links to many other
web-sites about bridges and downloadable computer programs about bridge
behavior and bridge models. http://www.brantacan.co.uk/artofbridges.htm
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