Place your hands together, palms facing each other. Now rub your hands back and forth. Your hands are “shearing” past one another. Look at the support beams of a bridge the next time you are near one. These beams experience a shear force every time the wind blows. Can you think of other items in your daily life that experience shear force?
For example, when you step on wet sand at the beach, some of the sand beneath your foot is “shearing” past the sand outside of your footprint that stayed in place.
When you use a pair of scissors, the two blades of the scissors “shear” past one another. This is where the name for a garden shear comes from. When you work with a garden shear to trim bushes or plants, the two blades of the garden shear are literally “shearing” past each other.
Shear is a force that causes a material to deform due to parallel internal surfaces sliding past one another. This internal sliding is caused by an imposed stress. When the wind blows very strongly, the metal inside a beam in a bridge may want to try and “slide” past itself in a “shearing” motion. The material a bridge or building is made of is very important in designing against shear forces. Steel beams can withstand much stronger shear forces than wood beams can.
In wood framing in house construction, shear force experienced by nails is avoided by making sure that support is wood on wood. In steel structures rivets only are used. This is because of the material properties of steel. Steel is a much stronger material than wood, therefore an engineer can design a steel bridge in a much different manner than they can design a wood bridge. The strength of steel allows a bridge or building to take shapes and forms that could not be used in a wood bridge of building. If a steel or wood beam broke because of shear force, what would the broken part of the beam look like? It is very important for metal or wood bridges to be able to bend, twist, and stretch a little. If the material in a bridge or building could not bend, twist or stretch a little, it would be too brittle to support loads or withstand shear and bending forces.
Take a craft stick and place both your hands around it as if you were grabbing a crossbar or bicycle handles. Now push one hand away from you while pulling your other hand towards you. Observe what happens. What is shear force causing to happen inside the wood of the craft stick? Why would a bridge support break if the shear force it experiences gets too large?
Repeat this activity using a strip of tinfoil instead of the craft stick. Observe what happens to the strip of tin foil. Why is the wood stick so much stronger than the tin foil? Try this activity using a carrot. How does the carrot compare to the craft stick? Now try this activity using a celery stick. Do the long fibers in the celery help make it stronger? Do you think that the long fibers in celery help protect it from shear forces? Try the activity with soft bread. What happens? Try the activity with toasted bread. Is this different than with soft bread? Why do you think this happens?
Next Generation Science Education Standards
Scientific and Engineering Practices:
-Asking questions (for science) and defining problems (for engineering)
-Planning and carrying out investigations
-Cause and effect
-Structure and function
- Try this activity with cooked spaghetti. Now try the activity with uncooked spaghetti. How do the spaghetti noodles hold up when they are wet compared with when they are dry? Do you think wood bridges would act differently when they are wet compared with when they are dry?
- Try this activity with soft clay. Now try it with cooked ceramic. How does firing the clay into ceramic affect its material properties? Can ceramic withstand more shear force than soft clay?
- How can using stronger materials help work against shear forces? Why do you think steel is stronger than wood? Why will a steel bridge resist larger shear forces than a wood bridge will? How could metal be used to strengthen and support a wood bridge, and where on the bridge would you place metal plating to make it stronger and more resistant to shear forces?