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How can math help me understand my world?
What factors influence bridge strength?
Project Directors use good judgment to
make suggests to guide carpenters in
ways to help their team understand
how to strengthen their bridge.
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Bridges can be designed in different ways to
withstand certain loads and potentially
destructive forces.
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START EARLY, PLAN THOROUGHLY, AND WORK STEADILY. Do
not put your work off to the night before it is due. Toothpick bridges
need time to dry property.
* Try different constructions. Build as many versions of your
prototype bridge as possible. There is a list of suggested bridges .
Keep track of time. You may have to let go of an idea so that your final
design can be thoroughly tested. Final building day is next Friday,
January 14th.
The spacing between the intersections of toothpicks greatly affects the
strength of your toothpick bridge. Test each toothpick BEFORE you
use it. When using flat toothpicks to build your bridge, roll the
toothpick between your fingers first to see if it can stand up to a little
pressure without breaking. If using round toothpicks, apply a little
pressure to bend it . Good glue joints make for good strength.
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Your bridge’s strength is also dependent on its vertical height.
However, this is not to say “the higher the better” as there are
limits.
Consider the strengths and weaknesses of a single toothpick
under these three forces: COMPRESSION, TENSION, AND
SHEER.
Watch out for “stress risers”. Before breaking your bridge will
give and bend under the load of your test weights. Remember
what happened to the Tacoma Narrows Bridge collapse of 1940?
What caused it to collapse?
•If you have made some parts of your toothpick bridge
so strong and hard that it will not give (be flexible),
your toothpick bridge will fail in another area sooner
than if the whole bridge were able to adjust to the
forces acting on it.
•To reduce drying time, do initial gluing with SMALL
drops of glue (applied with an extra toothpick). You
can go back later and reinforce these small drops with
more glue after everything dries.
•GOOD LUCK AND HAVE FUN.
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Compression: Compression is a pushing (compressing) force. The shorter a piece of wood
is, the more compression it can hold. The longer a piece of wood is, the less
compression it can hold. When you compress a long stick of wood you will notice that
it starts to bend. When a piece of wood breaks because of compression, we say it failed
from buckling.
Typically the top chord of a bridge, including model bridges, will be in compression.
Different truss designs spread out the force so that various internal parts will be in
compression as well.
Tension:
Tension is a pulling force. Wood has the ability to resist a lot of tension. It
would be hard to break a popsicle stick if you held both ends and pulled
apart.
Tension may be applied parallel to the grain of the wood, but should be
avoided perpendicular to the grain. Wood is very strong in tension
parallel to the grain, but much weaker in tension perpendicular to the
grain.
Also, unlike in compression, the ability of wood to resist tension does not
change with its length. A shorter piece of wood should hold the same
amount of tension as a longer piece.
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Torsion: Torsion is a twisting force. When you wring out a
cloth, you are applying torsion to the cloth. If you take a
stick pretzel, twist one end, and hold the other end still, it
will break very easily. If you do that with a baseball bat, it
will not break. However, if you take a piece of licorice and
apply torsion to it, the licorice will twist around several
times before it breaks. Each of these materials has a different
way of responding to torsion. Bridge designers must watch
for torsion and try to reduce it as much as possible.
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Shear: Shear is a interesting force. It happens when
there are two opposing forces acting on the same point.
If you hold a piece of wood with both hands next
to each other, and push up with one hand and
down with the other, you are applying shear to
that piece of wood. Shear usually occurs
horizontally, and not vertically.
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Weight distribution
Truss system
Bracing via support spacing
Piers or towers
TRIANGLES…TRIANGLES…TRIANGLES
Triangulation
Strength
Length of bridge
Weight of bridge
Forces of compression
Forces from tension
Bridge stability
Quadrilaterals used
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As Project Director you are responsible for producing a crew of workers
that understand the dynamics of bridge construction and the forces that
act on those bridges.
As part of your job you are to find a creative way to have your workers
understand and be able to explain the basic concepts behind bridge
design.
You are to go to the Warehouse & purchase your materials packet.
Observe what will be required of your employees to learn. You may
decide on any method of teaching that you feel will promote the best
quality of learning of these concepts. You can be as creative as you feel is
necessary.
If you need more materials or resources to achieve your goal, be specific
when asking for the items you think you will need.
NOTE: There will be a team competition on the day after your last
building day. It is critical that every member of your team do well. You
may discuss your challenge with your construction crew and all of you
can determine the method that best suits you in order to learn the
material provided. Understand that this is not a memorization task. It is
about understanding the underlying concepts of bridges.