Understanding Construction Cranes: A Comprehensive Guide
A diverse spectrum of crane types is deployed on construction projects to facilitate the lifting and movement of materials and other building elements from one location to another. The range of cranes available spans from a basic rope pulley to a towering tower crane affixed to the pinnacle of a skyscraper. The selection of a crane for a particular site is contingent upon several influencing factors, including:
- The nature of the loads requiring elevation.
- The necessary vertical height and horizontal distances to be spanned.
- The required radius of swing.
- The duration of the lifting operations.
- The utilisation factor (the ratio of time spent in use versus the total potential operational time).
- The degree of required mobility.
- The type of plant that will operate in conjunction with the crane, such as concreting plant.
Broadly, cranes can be categorized as:
- Mobile
- Static
- Tower
Mobile Cranes
Mobile cranes possess the ability to be relocated within a site, affording them considerable flexibility. They can be configured as mobile wheeled, truck-mounted, track-mounted, or gantry-based.
Mobile Wheeled Cranes
Mobile wheeled cranes are mounted on a wheeled chassis equipped with stabilisers that can be deployed during lifting to prevent movement. Generally, the crane is controlled and driven by an operator situated inside the cab. A slewing ring beneath the cab permits a full 360-degree turning circle, and the boom extends upwards, supported by suspension ropes.
- Lifting capacity: Typically ranges from 3 – 50 tonnes, with approximately 10 tonnes being the most common capacity.
- Speed limits: Applicable depending on the type of vehicle and road conditions.
Mobile wheeled cranes are frequently utilized for moving moderately heavy loads, equipment, and other plant within goods yards or storage areas.
Truck-mounted Cranes
Truck-mounted cranes are affixed to a truck or lorry specifically adapted to support an increased load. The lorry can be driven from a front cab like a conventional vehicle, in addition to having auxiliary controls for a lattice mast or telescopic boom that extends in sections. Fly jib attachments can be used to augment lifting height.
- Lifting capacity: Can range from 5 – 2,000 tonnes, with 35 – 100 tonnes being the most commonly utilized range.
- Speed limits: Applicable depending on the type of vehicle and road conditions.
Given their high transportability and minimal site preparation time, this type of crane is commonly employed for short hire periods.
Track-mounted Cranes
Track-mounted cranes are mounted on a diesel-powered crawler unit along with a lattice mast that can accommodate a fly jib attachment with additional lifting ropes to allow for enhanced site coverage. They can move at low speed around sites but necessitate transportation to and rigging on the site.
- Lifting capacity: Typically ranges from 5 – 30 tonnes.
- Average speed: Usually does not exceed 2 km/h.
The flexibility of this crane is its primary advantage, as it can be adapted to act as a dragline with the addition of a winch drum at the front. The tracks enable its operational capability on poor ground conditions.
Gantry Crane
Also known as portal cranes, gantry cranes typically consist of two A-shaped frames connected by a lattice cross member which straddles the work area. The lifting gear is suspended from the horizontal cross member and can traverse along it on rails. On small gantry cranes, the A-frame is wheel-mounted, whereas larger cranes are mounted on powered bogies that run on rail tracks. Larger cranes tend to have the lifting gear mounted with a driving cab on the cross member.
- Lifting capacity: Small gantry cranes have a 10-tonne capacity, but larger versions can lift up to 100 tonnes.
This form of crane is commonly used for repetitive work on low to medium-rise developments, or in stock yards to move equipment and materials.
Also refer to Gantry.
Static Cranes
Despite possessing a similar appearance to traditional tower cranes, static cranes differentiate themselves in that the lattice mast is mounted on the slewing ring, implying that the entire tower rotates from one fixed position.
They are equipped with a luffing jib which can be raised through 90 degrees from horizontal, featuring a trolley and hoist block that can move along the length of the jib.
- Lifting capacity: These will vary depending on the length of the jib, the angle to which the jib is raised, and the position of the hoist block along its length.
Static cranes tend to be used for low capacity lifting and are advantageous in that they generally permit a closer approach to the face of a building.
Tower Cranes
Tower cranes are a familiar sight on most construction projects. They are typically assembled and erected on-site with either a horizontal or luffing jib. Common features of tower cranes include:
- Lattice sectionalized tower.
- Jib (typically 25 m long) fitted with precast concrete counterweight blocks.
- Access ladder housed within the tower or attached to an outside face.
- Stay rods and support frame.
- Cab at the top of the tower where an operator sits.
- Slewing ring which allows for rotation of the jib.
- Trolley and hoist block which can move along the length of the jib.
Tower crane operators are capable of observing most of the lifting operations from the cab, although a banksman (signal person) is required at ground level both for overseeing the loading of the crane and for issuing signals and guidance to the operator.
Tower cranes come in a number of different forms:
- Self-supporting static.
- Supporting static.
- Travelling or rail-mounted.
- Climbing.
Self-supporting Static Tower Crane
The tower (typically 30 m tall) is anchored at ground level using mass or reinforced concrete bases. Piles may be necessary if ground conditions are poor. Lifting capacities range from 2.5 tonnes with the trolley at the minimum radius and 1 tonne at the maximum radius at the end of the jib. This type of tower crane is most appropriate on confined sites where there is insufficient room for a travelling crane.
Supporting Static Tower Crane
This is similar to a self-supporting tower crane but is employed where high lifts are required. To gain additional stability, the tower is tied at suitable intervals to the face of the structure, from a minimum distance of 2 metres. This will induce additional stresses in the structure which must be accommodated in the design, likely incurring additional costs.
Travelling or Rail-mounted Tower Crane
On sites that cannot accommodate static cranes, a travelling or rail-mounted tower crane may be the most suitable option. The tower is supported at the base by precast concrete ballast blocks placed evenly on both sides. This is then mounted on heavy-wheeled bogies that traverse along a rail track which is laid on sleepers and ballast. Traversing corners is achievable by constructing radius rails or turntables. It is crucial that the track is not placed on a gradient exceeding 1 in 200, and that it is meticulously monitored and maintained, as any imperfections or slight movement could render the tower unstable.
The typical lifting capacities are 1 tonne at the maximum radius and 4 tonnes at the minimum radius.
Climbing Tower Crane
Climbing tower cranes are suitable for structures that are exceptionally tall. The tower is erected within the structure and raised as the structure itself gains height during the construction process.
Typically, the crane is fixed to a base and raised two storeys at a time or after floors have been cast and cured. After the first four floors are in position, self-adjusting wedges and collars are fixed to the floor around the tower to transfer its load to the floors, thereby contributing to stability. Once work is complete, the crane is dismantled in sections. The decision to use a climbing tower crane must be made carefully, since frames or collars will need to be designed to suit the structure in question, and the structure must be capable of supporting the required loads.
You can read about the various regulations and safety considerations that apply to the use of cranes on construction sites here.