Modeling Loads on Structures Using "Human Molecules”

Engineers consider many forces when planning and designing materials and structures, including tension, compression, shear, bending and torsion.

After this activity, students should be able to:

• Identify the five fundamental loads: compression, tension, shear, bending and torsion.
• Explain what is meant by something being elastic and non-elastic (inelastic).
• Describe characteristics of molecular bonds.

Have you ever seen a collapsed building or a collapsed bridge? What happened? Why did it fail?

Materials and structures can sometimes fail when subjected to large enough loads. Each different type of load can cause its own type of failure. Stresses, fatigue and failure can sometimes be seen with your eyes by looking at the surface of the member or structure. However, the movement of tiny individual molecules is where the real failure begins.

Modeling Loads on Structures Using "Human Molecules"

For this activity, each person represents a molecule of steel inside a steel bar and their arms represent the internal bonding forces that hold molecules together. A molecule is the smallest piece of steel that can exist with the chemical and physical properties of steel – billions of molecules link together in lines to make a piece of steel.

1. Form two lines of 10 people each, lining up side by side, facing each other (see Figure 1). These two lines represent a structural element. Each person must use his/her left hand to hold hands with the person whom they are facing in the other line. Then, each person locks his/her right arm around the arm of the person on his/her right.Figure 1: Forces on the human molecules.
2. Four other students act as an applied load. The applied load can either be in tension (pulling) or compression (pushing). If students are pulling, a positive force is applied (indicated by an arrow pointed out to the right, as seen above). If students are pushing, a negative force is applied (arrow pointed inwards to the left). Position one student at each end of both lines, and have them pull with equal force (if possible). Have the students pay attention to what they are feeling while the molecules are being pushed and pulled. Next, form the same lines again, but have the four people applying the loads push equally on each line end. The job of the molecules is to try to maintain their original formation, like a solid, non-elastic object.

Group Discussion

1. What type of load did you model this time?
2. What did it feel like to be a molecule inside the material?Figure 2: Human molecule in shear.
3. Now have the "applied load" students pull one line of molecules to the left, and the other line of molecules to the right (as shown in Figure 2).

Evaluate students for participation in group discussions and answering questions.