A tower crane is so tall that it cannot arrive at a construction site as one complete machine. It is built in stages, starting with a concrete foundation and steel mast sections that create a rigid vertical tower. A mobile crane does the early heavy lifting by placing the mast, slewing unit, operator cab, jib, counter-jib, counterweights, and hook system.
This process matters because safe assembly depends on balance, load limits, and careful control of forces.
Key Facts
- Torque = force x lever arm, so a load farther from the mast creates more turning effect.
- For a balanced crane, clockwise torque must be matched by counterclockwise torque.
- Weight force is W = mg, where m is mass and g is about 9.8 m/s^2.
- A mobile crane must keep the lifted load within its rated capacity at the working radius.
- The tower crane base transfers large vertical forces and overturning moments into the concrete foundation.
- During self-climbing, hydraulic jacks lift the upper crane assembly so a new mast section can be inserted below it.
Vocabulary
- Tower crane
- A tall fixed crane with a vertical mast and horizontal jib used to lift materials on large construction sites.
- Mobile crane
- A movable crane, often mounted on a truck or crawler base, used to lift heavy parts into position during assembly.
- Mast section
- A steel tower segment that is bolted to other sections to increase the height of a tower crane.
- Jib
- The long horizontal arm of a crane that carries the trolley and hook used to lift loads.
- Counterweight
- A heavy mass placed opposite the load to help balance torque and reduce the risk of tipping.
Common Mistakes to Avoid
- Ignoring the working radius is wrong because the same load becomes more dangerous when it is lifted farther from the crane's center of rotation.
- Assuming the tower crane builds itself from the ground is wrong because a mobile crane first assembles the base, mast, slewing unit, jib, and counter-jib before self-climbing can begin.
- Adding counterweights before the required supporting parts are secured is wrong because the crane must be assembled in a stable sequence to avoid unbalanced torque.
- Treating the concrete foundation as just a platform is wrong because it must resist vertical loads, side forces, and overturning moments from the crane.
Practice Questions
- 1 A mobile crane lifts a 6000 kg mast section. What is the weight force of the mast section in newtons? Use g = 9.8 m/s^2.
- 2 A tower crane lifts a 2500 kg load at a radius of 30 m from the mast. Calculate the torque about the mast due to the load. Use g = 9.8 m/s^2.
- 3 Explain why a tower crane needs counterweights and why the order of assembly matters before the crane begins self-climbing.