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After
Katherine's team won 3rd place in Destination Imagination's Bridge Building contest
in Knoxville, I became
interested in understanding the engineering behind the bridge.
It turns out that a basic knowledge of algebra and physics was enough
to understand the forces acting on Katherine's queen post bridge and
to figure out some general engineering principles.
External Force Analysis
Here, we
look at the bridge as though it were an single, solid object and
determine how force is transmitted from the top edge to the bottom
corners.
Internal Force Analysis
Here, we
will look at the bridge's interior structure and determine how much tension and
compression force is transmitted through each member.
Material Analysis
Here, we
use Force Analysis to see how we can use balsa
effectively.
Buckling Analysis
This is
complicated. I wasn't able to directly apply Buckling Equations
to the Queen Post bridge. However, some
general rules are apparent by looking at the basic equations, and I
discuss them here.
I'd appreciate any feedback on how to do a better job on this section.
Side Note:
Automatic Force Calculation
If you
don't want to calculate External and Internal Truss Bridge forces
manually, a wonderful resource for automatic calculation is the
Johns Hopkins
Bridge Designer. Interestingly, this program cannot analyze
a Queen Post Bridge! This is because the program requires
that bridges have the following relationship.
Members
+ 3 = 2 * Nodes
A Queen
Post Bridge has 8 Members and 6 Nodes, so doesn't obey this
relationship. You can however insert a "dummy" member to
overcome this problem (below). Note that this member has zero
force on it. By inserting the dummy member, I was able to double
check my own force calculations.
Other
Resources
A great resource we used
to explore bridge building was
here. Also, in addition to our
video, which shows the detail of creating a balsa wood
structure,
PBS has a nice overview video on real bridges that is good
for elementary school kids.
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