This is a STEM challenge I set my first year engineering students every year. I have found this challenge a brilliant tool for teaching students how structures work and how they can be used in real life as it makes learning fun and intuitive. I have also found that by making learning rewarding and competitive, students are more willing to push themselves and achieve greater things.

The rules:

Each team has are given 150 Popsicle sticks and a hot glue gun
Teams should be groups of 2-4
The bridge must span a distance of 30 cm
The edges of the bridge can't overlap each side of the table more than 5 cm
The bridge can't be wider than a single Popsicle stick
Popsicle sticks may not be layered unless at a joint or on the load bearing bar (the bar the weights will be hung from)
The students can use all the sticks, however the bridge will be judged on its strength to weight ratio so a heavier bridge might result in a lower score.
The students should be set a time limit of 2 hours not including the lesson before where they should be taught about structures in engineering.
The team with the best strength to weight ratio wins the prize

Supplies

Each team will require:

150 Popsicle sticks (these can be found cheaply in bulk on amazon)
A glue gun and glue (if you don't have enough for every team a gluing desk can be set up)
paper and pencils for planning
weights (multiple 100 g weights work best)
string to hang the weights from

Step 1: Teaching the Basics and Designing

To avoid complete anarchy, before allowing them to start I usually spend one lesson where I teach them them the basics of trusses.

I show them 6 famous examples of truss structures:

The Howe truss
The Pratt truss
The Warren truss
The Quadrangular Warren truss
The Baltimore truss
The camel back truss

(I have included a selection in the diagram above)

For each truss design I explain the function of each bar and explain to them how bars are either under tension or compression by highlighting the components under tension/compression in different colours (see diagram above).

Once I feel that the students have grasped the concept of a basic truss and they have seen existing designs the next step is sketching truss designs. The teams are each given Popsicle sticks to arrange on a page (as shown above), and paper to sketch ideas on. It is important that the students begin to consider the restraints given to them so that they can incorporate them in their design (it might be useful for them to draw them on the page.) The students should also consider where the weight will be suspended. If the students are smart they will realise that the Popsicle stick is far stronger in tension and design their bridges to utilise this. It is also important that the students incorporate stiffeners and diagonals in all axis to prevent collapse under pressure.

Step 2: Construction

Now that all the teams have a design they can begin construction. As stated before the students can only use hot glue and Popsicle sticks, and the sticks can only overlap at a joint. They should all get the same amount of time (2 hours is more than enough if they work efficiently together.)

The groups should begin to construct two identical trusses, using their sketches as a guide.
Next they should connect the two trusses with transverse bars and if they are clever diagonals to create added strength.
Finally they should add stiffeners to the corners of the bridge to prevent collapse.

Once the students have finished they should add the load bearing bar. This is the part the weights will be hung from, and is the only place where multiple sticks can be glued together.

Step 3: Testing and Judging

Once all the groups have completed their designs they should be weighed to test how much material was used. Next they should be placed across two tables 30 cm apart and carefully loaded with weights from the load bearing bar until they reach breaking point. Finally the strength to weight ratio should be found using the equation:

SWR=weight lifted / weight of structure

The team with the best strength to weight ratio are the winners (I usually give them chocolate!)

In the event of a draw the judge (you) must decide which team has the best: