In Straws We Trussed - Page 8

THE PHYSICAL CHALLENGE

Now it is time to apply what we've learned to an actual physical structure! We will take what we know about tension and compression to build our own models using straws, pins, and cups to hold as many marbles as possible at as great a height as possible. The challenge is to build the structure with a limited number of parts. As explained below, team points can be attained in two ways, based on the height of the structure AND the number of marbles that the cups built into the structure can hold - the team that gets the most points wins!

Use the pins to connect two straws together, as shown in the figure above. Be careful of the pins, since we do not want anyone injured in this competition!

The Rules Each team will be given two styrofoam cups, 50 drinking straws, 100 straight pins, and thimbles (for the middle school workshop use coffee stirrers and pipe cleaners) with which to build their structure. Scoring is as follows: Measure the height of structure from the table surface to the base of the lowest cup in the structure (any unit of measurement can be used, as long as all competitors use the same measurement scale). Add marbles one at a time until the structure either falls over or breaks or until a cup touches the ground. Count the number of marbles which the structure can hold. Stop counting when both cups are full. Multiply the height and the number of marbles to attain the total number of points for that team.

While designing your model, it is important to identify the possible ways that the structure will fail and work to mimimize them while you are building. For example:

• Pay attention to the shapes in the structure. Remember that four sided figures are not stable and try to tick to triangles during construction.
• Remember the rules for balance - recall the rules for center of mass and the location of the base of the structure to make sure the structure doesn't topple over (see the lesson Takes Two to Topple)
• Identify which members are in tension and which are in compression. Straws in compression will tend to bend or snap in a process known as buckling (see figure below). Buckling is much more likely to occur when the compressed straws are long, so in building the structure, try to make sure that all straws in compression are short.
• Buckling is also very likely to occur if 'bending' straws are used. It is highly recommended that any straws in compression not have a bend in them.

Figure: Example of buckling

Let the challenge begin! Each team will have 45 minutes to design and construct a model (click here for an example of a winning model!) which will hold marbles, adhering to the rules and guidelines above. Then there will be 15-30 minutes to test each structure by loading it with marbles. Use what you have learned in this lesson and be sure to have fun!

Explorations In Engineering Homepage

Introduction Introduction to Truss Applet Tension and Compression in A Truss Reverse Load Horizontal Loading Experimenting with Different Shapes The Computational Challenge The Physical Challenge