Construction Cranes
A tower crane balances its load using a counterweight on the opposite side of the tower. This idea investigates how the moments balance, and why the maximum safe load falls as the load moves further from the column.
Introduction
A tower crane carries its load on a horizontal jib, with a counterweight on the short arm on the opposite side of the tower. For the crane to stay balanced, the moment produced by the load must be matched by the moment from the counterweight, taken about the tower's centre. Because moment equals force times distance, a load can be heavier close to the tower than far out, which is why every crane publishes a load chart showing maximum load against radius. As an extension, the same setup can be analysed for the tension in the crane's structural cables.
Guiding Questions
- A tower crane balances its load with a counterweight. Where do the moments balance?
- For a real crane's dimensions, calculate the counterweight needed for different loads and jib positions.
- Why does the maximum safe load depend on how far out it hangs? Draw the load-radius curve.
- Compare your curve with a manufacturer's load chart. How close is the simple model?