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Difficulty: MediumStructural Components

A standard derrick crane lifts a heavy shipping container. The crane consists of a vertical rigid mast anchored to a ship's deck, a rigid steel boom extending outward and upward from the base of the mast, and a horizontal steel cable connecting the top of the mast to the upper tip of the boom. The container hangs vertically from the upper tip of the boom. Under the downward weight of the container, what primary internal structural forces are experienced by the horizontal steel cable and the rigid steel boom, respectively?

  1. The horizontal cable experiences tension, and the rigid boom experiences compression.Answer
  2. B
    The horizontal cable experiences compression, and the rigid boom experiences tension.
  3. C
    Both the horizontal cable and the rigid boom experience tension.
  4. D
    Both the horizontal cable and the rigid boom experience compression.

Answer

The horizontal cable experiences tension, and the rigid boom experiences compression.
The correct answer accurately identifies that cables can only stretch (putting them in tension) to keep the structure from collapsing, while the rigid boom acts as a column holding up the weight, which squeezes it (putting it in compression).

Step-by-Step Solution

1
Analyze the forces acting at the tip of the boom.
Three forces meet at the tip: the downward weight of the container, the horizontal force from the cable, and the angled force from the boom.
Identifying the junction point allows for force balancing (equilibrium).
2
Determine the state of the horizontal steel cable.
The cable prevents the boom from falling outward and downward. To hold the boom back, the cable must pull inward, meaning it is being stretched.
A stretched structural member is experiencing tension.
3
Determine the state of the rigid steel boom.
The boom pushes upward and outward against the downward pull of the load and the inward pull of the cable. The weight pushes down into the length of the boom.
A structural member that is being pushed or squeezed from its ends is experiencing compression.

Key Concept

Tension and Compression in Structural Members
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Topics

Structural Components

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