Popsicle Bridge Building Techniques

This web page has some pointers but is intended to make people think about what what factors are important in building their popsicle bridge.

CAD mock-up of bridge FEA analysis of bridge

The pictures above are of a CAD model that was drawn of the bridge, long after it was crushed.  A Finite Element Analysis (FEA) was used on the model to determine where the maximum stresses and strains would be located at. The brightest areas of the picture show where the bridge was most likely to fail. (the deflection is not shown to scale)

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.

Time frame?
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.