There are many types and sizes of tower cranes. Therefore, there have to be many types of tower crane bases to suit. Here we’ll cover what they are.
The different types of tower crane bases are:
- Gravity bases
- Piled-foundation bases
- Ballasted cruciform bases
- Grillages
Gravity base
A gravity base is a concrete base that houses crane anchors. These anchors are what the first tower sections of the tower crane fix to. The forces of the crane travel down through the mast into the anchors. This force travels to each corner of the base.
Because of this, gravity bases don’t require pile foundations. But the ground underneath has to be checked first to see if it’s suitable.
The fixing anchors get lifted inside the pre-dug base hole, then levelled up by adjusting screws on all four corners. Once the steel reinforcement is put in around the anchors, a lightning conductor gets attached to the steel and anchor to form an earth connection, in case the crane gets struck by lightning.
Then concrete gets poured inside to form the finished structure.
Pile foundation base
A pile foundation crane base is a cubed concrete base that has concrete piles underneath it for extra support. A pile goes under each corner as standard, but depending on the size of the base, other piles can also be incorporated. Some bases can have up to 20 piles in them.
The four crane anchors tie into the tops of piles so that the weight of the crane is supported, not only by the cubed foundation, but the piles themselves.
Ballasted cruciform base
Cruciform crane bases are bases that are formed of ballast weights that sit on the bottom frame of a tower section. The base of this frame is made up of horizontal beams in an X shape that gets built piece by piece on site before the ballast weights are added. This works on a compression basis, rather than tension.
The legs of the frame that the ballast weights rest on can sit on various forms of bases. Here are some examples:
- Single piles, each connected by beams.
- Concrete pads.
- Manhole rings that get put into the ground and filled with concrete, which transfer the load onto more suitable ground below.
- Grillages that connect to the top of the building’s core.
Grillages
Grillages are used:
- To place tower cranes on top of the core being built.
- When renovating existing structures.
- When excavating deep below the crane base level.
On top of cores
The designs for placing grillage bases on top of cores are driven by crane deflection (the leaning and movement of the crane). Also, the types of anchorage used to fix these to the core and crane are very important. When a core is being used for this purpose, it will have to undergo more rigorous checks than usual.
Renovating existing structures
When there is an existing building that’s being renovated, sometimes the only option is to position the crane on columns that are already on the building. The columns and the surrounding building would have to undergo rigorous checks to ensure the structure is safe enough to withstand the weight and movement of the crane.
Inside deep excavations
Some buildings have large basements or underground carparks, etc. To build these underground facilities, lots of ground needs to be dug out. But there still needs to be somewhere to place the tower cranes within that area.
- In this case, piles will be poured prior to the ground being excavated. Then, steel columns will be placed on top of those piles. These columns will support the grillage, which in turn, supports the cranes.
- The ground can then be dug out to the required level under and around the crane.
What are sprung and pinned piles?
Pinned piles are where there is a fixed connection between the pile and pile cap. When this occurs, there can be an opposing compression tension moment, depending on what way the crane faces.
In the image, the cranes jib is facing north. This puts weight (compression) on the piles underneath the jib. And it pulls on the piles south of the jib (tension).
Sprung piles allow the compression and tension pressure to be distributed evenly between all the piles.
Base design stage
The design of a tower crane base is dependent on many factors. A few examples are:
- The type and size of the tower crane.
- Ground conditions.
- Existing structure of which the base will stand on.
- Local wind speeds. For example, the wind speeds in the South of England tend to be much lower than in the Scottish Highlands.
When these factors have been established and assessed, the design and build process can begin.
The client receives the foundation loads (the force created by the weight of the crane and its movements, in kN). They will then pass these details onto an engineering base designer to design a base capable of withstanding the loads on a drawing.
A company will then build the base in the required location, as per the designer’s specifications.
Tower crane bases are classed as temporary works. So there are certain procedures to meet stipulations that come under BS 5975 Code of practice for temporary works procedures and the permissible stress design of falsework.
One of the stipulations is that checks be done to the temporary works designs. So, when the base is made, a Temporary Works Coordinator (TWC) will then check it.
If it’s been built to the designer’s specifications, without faults, this engineer will sign it off. Only then can the tower crane be erected onto the base.
All four corners of the top of the base must be level to ensure is perfectly vertical when erected.
To finish
When you think of how aeroplanes can stay in the sky, it’s amazing. These huge, heavy machines seeming to defy the laws of physics.
It’s a similar thought with tower cranes. These tall metal sticks with in excess of 250t of crane sitting on top of them, with some able to lift up to 350t of weight into the air. It’s a wonder how they stay upright. But surprisingly, tower cranes topple over much less than other types of cranes.
When they do go over, it generally isn’t due to overlifting or ground conditions like with those other cranes. It’s usually because of wind conditions. Like not leaving the crane out of service so that it can blow around freely rather than resisting against the wind.
Because of the height, weight, and lifting abilities of tower cranes, their bases tend to have more designing and engineering than those of other cranes.
So when you look up in the city sky and see those tall machines building things, take a minute to think about the design and engineering that goes into, not only making them lift heavy objects, but keeping them upright.
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