This web page has some pointers but is intended to make people think
about what what factors are important in building their popsicle bridge.
Things to know before making the bridge:
How is the bridge to be loaded?
Whether the weight is applied gradually, in a nice linear fashion,
or abruptly can make a large difference in design strategy. The bridge
shown on the previous page was fortunate enough to be crushed by a tensile
- tester running in reverse. If the loads were applied by shaky, strained
human hands, the bridge would not have withstood as much weight. The
loads on any bridge should applied as gradually, uniformly and vertically
as possible. As the bridge is loaded, there will be an initial "settling
in" stage where the bridge will make snapping / cracking sounds. This
sound comes from focused stresses breaking weaker glue joints and possibly
the crushing of smaller members of the bridge. Bowing and movement
of the bridge during this initial stage will probably be visible. After
this, the bridge is extremely rigid and the next deflection and snapping
you will probably hear will be as it is crushed.
Where is the load focused?
Bridges that are being crushed at their center undergo much greater
stresses than ones that have the loads focused more towards the bridge supports.
Try to build members on the bridge to distribute the weight and bring
it away from the center of the bridge, if possible.
Where is the bridge being supported?
In the designs for a bridge, draw a line from where the load is applied
to the place where the bridge is being supported, only moving the pencil
along the bridge members. This is the path the stresses will take.
Try to make this path as short as possible and as reinforced as possible.
What materials can and cannot be used (glues, paints, types
of sticks, etc.)?
Use the best materials possible. Testing the materials that you
get are pretty critical: glue some sticks together with different glues
and then try to pull them/bend them/twist them - which glue holds up the
best. Try other similar experiments.
Have the bridge done with plenty of time before the competition. The
glues/paints and wood need to dry THOROUGHLY if it is expected
to do well.
- Don't have long members along the stress path. These bow
very easily. If there are any, reinforce them.
- Don't have air gaps in the beams. This is wasted space
and becomes a stress riser where the member will break.
- Cut the popsicle stick ends to 45 degrees or more when gluing
- Don't have any members just 1 stick thick - gluing them side
to side is essential.
- Design the bridge so that the members but against one another
- build the bridge members and make it stand on its own without any
glue. After several hundred pounds, glue doesn't really do to much.
- Test the glue to see which it handles better: tensile, compressive,
or shear forces and build around it. Reinforce any members that
are not using the best method.
- Have leveling pads that you can sand down to make sure that the
load is applied at all points equally and that the top and the bottom of
the bridge are parallel.
- Test the glue and determine if it glues sticks together best
when much or little force is used
- build some fixtures to make sure that the members do not move
when they are drying
- Use only the strongest and straightest popsicle sticks
- let the bridge dry in a warm area (on the fridge/in a sunny window)
for a couple days to ensure a good glue set.
- Seal the pores of the wood with a watery-glue mixture to further
strengthen it (if allowed)
- Look for real bridges/radio towers/pedestrian foot bridges and
mimic parts of the design
- Realize that building a bridge will take a lot of time, not just
your Thanksgiving vacation.