This lesson will introduce the concepts of compression, tension, and loads. Students will perform an activity to explore how materials react to compression or tension.
Lecture
A force is a push or pull in any direction in which a change is produced. A force can be weight (mass x gravity), wind, air resistance, or even a moving vehicle exerting its weight on the road below. Before we begin to study the design of bridges it is important to understand the forces that are applied to the bridge and that forces never act alone. As stated in Newton's Third Law of Motion, it is not possible for a single force to occur because every action has an equal and opposite reaction. Engineers must consider this law of motion when they design a bridge because the bridge will react to wind, moving vehicles, or even earthquakes although it may not be noticeable to the human eye.
When forces are applied to a structure and added together, the sum of the forces is defined as a load. Two types of load engineers must consider when designing bridges are known as dead and live loads. A dead load, also known as a static load, includes the weight of the bridge or stationary objects on the bridge. A live load, also known as a dynamic load, includes objects in motion as well as natural forces such as the wind (which is typically a lateral or horizontal load) or an earthquake. Live loads are the most difficult to design for because they are always changing. It is crucial that engineers account for all types of loads when designing a bridge because the magnitude of loads will affect the material selection and possibly the type of bridge engineers choose to build. To accommodate for variations of dynamic loads, engineers design for maximum live loads using specialized software which can generate many calculations before building material is even selected.
As stated above, for all forces acting on a bridge, there must be a counter force pushing or pulling in the opposite direction. These forces are defined as either compression or tension forces. A force of tension will lengthen or pull on a material while a compression force will squeeze or push a material together. (Avaikian, 482) Compression and tension forces can act parallel or at an angle to a bridge member's (a member is a segment of a bridge) axis. When two equal and opposite forces in tension or compression (but not both) act parallel or at an angle to the axis of a member, the forces applied are defined as axial forces. Figure 1a clearly demonstrates a pair of compression forces (axial forces) applied to the axis of a member. Figure 1b demonstrates a pair of compression forces applied at some angle to the axis of a member. A pair of equal and opposite tension forces applied to the axis of a member can be seen in
Figure 2a.
Not all members of a structure experience solely tension or compression. Many forces are applied perpendicular to the axis of a member and some forces may not be applied directly across from each other. (Avaikian, 482) In this case, tension and compression forces are both present and are considered non-axial forces. When two forces are not directly opposite each other, engineers must calculate a resultant force (sum of two forces that act at angles to each other) utilizing vector resolution. Refer to Salvadori's
Why Buildings Stand Up
(pages 82-84) for an example of how to solve for a resultant using a 30°-60°-90° triangle.
Activity
The purpose of the following activity is to help students visualize what happens to a material when it is under tension or compression. They should be able to explain what is happening inside the material.
Materials
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1 container of Play-Doe or Silly Puddy
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1 Rubber band
Procedure (Part I)
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1. Break students into groups of four.
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2. Have each group take 1 container of Play-Doe and 1 rubber band.
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3. Take the Play-Doe and shape it into a cube.
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4. Push down on the Play-Doe
Assessment
Explain what you see and what you think happens to the molecules inside the Play-Doe.
Procedure (Part II)
Select one rubber band and pull with both hands
Assessment
Explain what you observe and what you think happens to the molecules inside the rubber band.
Post Activity Discussion
Students should discover that when they compress the Play-Doe, it becomes shorter and the molecules press together more tightly. When students pull on the rubber band, they should discover that the rubber band gets longer. After this activity students should understand that when a bridge's member is in compression, it will become shorter and when a bridge's member is in tension, it will become longer.
