Fire Department Incident Safety Officer Practice Test

What type of force is applied to the bottom of a beam under compression at the top?

1. Compression

2. Torsion

3. Shear

4. Tension

The correct answer is: Tension

When a beam is subjected to forces, the way those forces are applied determines the type of stress that occurs within the beam. In the case of a beam under compression at the top, a force is indeed applied downward at the top of the beam, pushing the materials together which creates a compressive force. To understand why tension is the correct answer, consider how forces act within the beam: the compression at the top of the beam causes the bottom of the beam to experience forces attempting to elongate or stretch it. This results in an internal force that is oriented in an opposite direction to the compressive force—tension. Hence, while the top is experiencing compression, causing a decrease in length, the bottom experiences tension, as the material tries to resist the compressive forces from above. This interaction between compression and tension illustrates how forces distribute throughout a structural element under load. Seeing the dynamics of this interaction clearly defines the reasoning behind why tension at the bottom of the beam is the response to compressive forces at the top.