The Bridge Buster
Some Construction Pictures
- Goal:
- To make a structure, i.e. a bridge, that can support the
greatest
mass with the lowest mass. The design is up to you. As you can see
the bridge beam, and the mousetrap car chassis are the same part.
The girder has more stiffeners, and the lateral girders are inset.
I wanted to see when and where the paper on the girders started
to tear. The answer was much more complex than I had imagined.
The paper tears near the lateral gussets (the black flanges next
to the core girders) but not near the center where the deflection is
most noticeable. I will continue to experiment and I invite you to as well.
The rules sheet can be downloaded here.- This
construction technique proved instructive. The foam core
board is much more elastic than I would have imagined. I
"painted" this structure with thick tempera and saw where the
paper began to tear. Strength increases, and
the amount
of twisting flexure, leading to a tear, was diminished as the
number of vertical stiffeners increased. So did the mass of the
bridge.
- Gorilla glue (any
polyurethane "foaming" glue) is the best at fastening this type of
material. There seems to be an intimate bond between the foam board and the "foam"
glue that is superior to hot glue.
That having been said:
- The
gorilla glue can be a minor pain to work with. It sets very
slowly (4 hours), and needs a water molecule to start the chemical
reaction. It can be very messy to work with. A LITTLE goes
a long way.
- The gorilla glue must be heavily clamped, and the
joints must fit snugly. The hot glue filled in the gaps, not the
ape . . .
- The learning curve was steep as this is a new (to me) adhesive.
- The structure must span at least 60 cm, but can be no longer
than 76cm. There is no height requirement. The total cost should
be kept low. A new piece of foam core board 50.8cm x 76.2cm costs about $1.50 That comes to about 3.9 cents per cm2 Remember, the lowest mass, strongest (load bearing)
bridge
will win. The bridges will be tested to failure with a strain gauge
calibrated in N. The winning bridge will have the greatest numerical value Force/Mass (N/kg). For example:
- a 0.040 kg bridge breaks at 40N; N/kg = 1000.
- A 0.039 kg bridge breaks at 40 N; N/kg = 1002.5 <---The winner!
- There will be a prize, as in money1
Materials:
- Four pieces of foam core poster board 6 cm x 76 cm.2
- Balsa or basswood. Toothpicks, tongue depressors (Popsicle sticks) or other small pieces of wood3
- Hot glue; the low temperature kind works very well.4
- Gorilla Glue, Tacky glue or white "Elmer's" glue.5
- String, fishing line, or thread. No metal.
Footnotes:
1If
the past is a reliable predictor of what is to come, then ten dollars
is probably close to the amount.
2The
black colored board is the easiest to work with. I am not sure
why. Please be aware it is no stronger than the white colored board.
The separation strength with gorilla glue is demonstrably higher
than the white board, the paper tears just the same.
3These
are wonderful materials, strong and light, but I would never use them in any large quantity.
They are very expensive, and cost is always a
factor. This
is not to say you should not use wood. If your goal is to produce
a bridge that can support a lot of weight, and
don't mind spending
some money, then feel free to use the materials that are costly. Foam
board in not as strong, but it's almost free if you recycle one of
your other projects.
4The
high temperature glue also works well. The burn unit is always
open . . .
5Use
the cheap stuff. Don't squander your parents money on fancy glues.
The fact is white glue works best on wood fiber, and what is the paper
on the foam board? Wood fiber!
If you are glueing the foam part of the board to the paper then consider the ape. It is VERY expensive.
File translated from
TEX
by
TTHgold,
version 3.70.
On 14 Nov 2007, 10:52