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Difficulty: MediumTypes of Stress and Strain

A simple wall-mounted jib crane is used to lift a heavy engine block. The crane consists of a rigid horizontal beam attached to a vertical wall at one end, and a diagonal support cable connecting the outer end of the beam to a point higher up on the wall. The engine block hangs directly from the outer end of the horizontal beam. Based on this configuration, which of the following correctly identifies the primary type of stress experienced by the diagonal cable and the horizontal beam?

  1. The cable is in tension; the beam is in compression.Answer
  2. B
    The cable is in compression; the beam is in tension.
  3. C
    Both the cable and the beam are in tension.
  4. D
    The cable is in tension; the beam is in shear.

Answer

The diagonal cable experiences tension, and the horizontal beam experiences compression.
The heavy load pulls down on the outer connection point. The diagonal cable resists this by pulling up and in toward the wall, putting the cable in tension. To counteract the inward pull of the cable, the horizontal beam is squeezed against the wall, putting the beam in compression.

Step-by-Step Solution

1
Analyze the forces acting on the outer end of the horizontal beam.
The suspended load creates a downward force at the connection point.
Identifying the primary external force is necessary to determine internal member stresses.
2
Determine the stress on the diagonal support cable.
To prevent the beam from falling, the cable must pull upward and backward toward the wall. This stretching force means the cable is in tension.
Cables can only support pulling forces (tension); they would buckle under pushing forces.
3
Determine the stress on the horizontal beam.
Because the cable is pulling backward toward the wall, the horizontal beam must push outward to keep the system in equilibrium. This squeezing force means the beam is in compression.
Structural members that are squeezed between opposing forces undergo compressive stress.

Key Concept

Tension and Compression in Structural Members
Estimated Time:45s
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Topics

Types of Stress and Strain

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