Tower cranes

How does Tower cranes work, step by step?

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   Step 1: Fix the crane position with the logistics plan

   Crane position is one of the first decisions on a vertical project and one of the hardest to reverse. The team works backwards from what has to be lifted, where it can be delivered and where it can be landed. The chosen position has to cover the working area, keep the load path clear of occupied areas and public routes, sit clear of the permanent works or in a hole that can be left until last, and be reachable by the vehicles that will erect it. On congested urban projects the crane usually ends up dictating the gate, the unloading bay and the whole delivery sequence rather than the other way round.

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   Step 2: Resolve oversailing and the neighbours

   A tower crane jib sweeps a large circle, and much of that circle is often over land the project does not control. Oversailing agreements with neighbouring owners are negotiated in advance, and where they cannot be obtained the crane has to be arranged so it does not need them. That is the point at which a luffing jib crane earns its cost, because it can bring its jib in rather than parking it over somebody else's roof. Railways, live highways, waterways and airports each bring their own approvals and their own lead times, and those approvals are secured before the machine is ordered.

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   Step 3: Design the base and the ground beneath it

   The base is a designed structure. It may be a cast concrete block, a set of pad foundations, a grillage on piles or a fixing frame cast into the permanent works, and the choice depends on the machine, the ground and how long the crane will stand. The temporary works designer sets the arrangement and the structural engineer confirms that anything the permanent works has to carry can carry it. The ground investigation feeds this directly. Where the crane stands on made ground or close to a basement excavation the design becomes a genuine engineering problem rather than a formality, and it is signed off in writing before anything is delivered.

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   Step 4: Design the ties into the permanent structure

   A free-standing tower crane can only reach a certain height before it has to be tied back to the building. The tie arrangement is designed by the temporary works designer, checked against the frame that will actually exist at the time each tie is installed, and coordinated with the structural engineer because the frame has to be strong enough at that stage of construction. This is a frequent source of programme conflict on concrete frames, where the crane wants a tie at a level the frame has not yet gained strength to provide. It is resolved on paper in advance, not by argument on the day.

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   Step 5: Erect the crane under a planned operation

   Erection is a major lifting operation in its own right, carried out by the supplier's specialist teams using a mobile crane, over a road closure or a possession where one is needed, and planned by an appointed person. It usually takes several days and it needs a large clear area for the mobile crane, the trailers and the components. Weather governs it, and the plan carries the decision points about when to stop. The project's job is to give the erection team the space, the ground, the exclusion and the time the plan asks for, and to schedule it where it does the least damage to the programme.

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   Step 6: Climb the crane as the building rises

   The crane keeps pace with the structure. An external crane is climbed by inserting further mast sections through a climbing frame at or near the base; an internal crane is jacked up through the building and re-supported on the floors below. Both are specialist operations by trained erection crews under a lifting plan, and both need the structure around them to be complete and strong enough at that point. Climbs are planned into the programme early because each one takes the crane out of production for a day or more, and a climb that slips takes the frame with it.

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   Step 7: Run, inspect and eventually dismantle

   In service the crane is operated by licensed operators, directed by trained slingers and signallers, and every lift is planned by the appointed person, with the non-routine and heavier lifts planned individually. The machine is subject to a regime of daily checks, periodic inspections and thorough examinations by competent people, and the records are kept on site. Dismantling is planned with the same rigour as erection, and needs the same clear area - which is why the space for it is protected in the logistics plan rather than being built over as soon as the frame tops out.