Mobile cranes

How does Mobile cranes work, step by step?

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   Step 1: Define the lift before choosing the machine

   The appointed person starts from the load, not the crane. What is being lifted, what it weighs, where it comes from, where it lands, what stands in the way and what the crane can get to are established first. Only then is a machine and a position selected. Reversing that order is where mobile crane work goes wrong: a crane hired because it was available, then asked to stand somewhere it cannot reach from, is the classic route to an improvised lift. On most projects the lift plan is prepared well ahead so that the ground works, the access and the deliveries can all be arranged around it.

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   Step 2: Design the crane pad and the ground beneath

   The crane transmits its loads to the ground through a small number of support points, so the ground has to be assessed and, on most projects, engineered. The temporary works designer specifies the pad build-up, the mats or spreaders and any stand-off from excavations, basements, drainage runs, culverts, vaults and buried services. Made ground, recently backfilled trenches and the edge of an open dig are the recurring problems, and a crane setting up over an unrecorded basement or a shallow chamber is a familiar cause of collapse. The pad is inspected and signed off before the crane arrives.

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   Step 3: Plan the access and the route on to the pad

   A mobile crane is a large road vehicle before it is a crane. The route to site, the turning circles, the width restrictions, the overhead cables, the bridge and manhole crossings and the gate all have to work for the crane and for the trailers carrying counterweight and jib sections. Larger machines need permits and escorted movements arranged in advance. Once on site the crane needs a route to its standing position that will carry it, and a place to be assembled if it does not arrive in one piece.

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   Step 4: Set up, level and establish the exclusion

   Setting up is a controlled operation supervised by the crane supervisor. The machine is positioned, its supports are deployed on to the prepared mats, and it is levelled. The area around and beneath the load path is cleared and controlled, and access is stopped where a load could pass over people. The size of the exclusion, the arrangement of the supports and the positions people may stand in all come from the lift plan. Nothing is lifted until the set-up matches the plan, and where it cannot, the lift stops and the plan is revisited rather than adjusted on the spot.

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   Step 5: Rig, lift and land under the plan

   The load is prepared and attached by trained slingers using inspected accessories, guided by tag lines where the load needs controlling, and directed by a signaller in contact with the operator. The lift follows the sequence in the plan: take the weight, check, lift clear, slew, land and release. Where the load leaves the operator's line of sight the signalling arrangement covers it. Where two cranes are used together the operation becomes a distinctly more complex one that the appointed person plans in its own right rather than treating as two lifts happening at once.

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   Step 6: Manage weather, light and the surrounding project

   Wind is the constraint that stops mobile crane work most often, particularly with large light loads such as panels, and the plan sets out who monitors it and who calls the halt. Poor light, ice and heavy rain all bear on the decision. The wider project matters too: adjacent trades are stood down from the exclusion zone, deliveries are held, and where the crane oversails a live road, a railway or a neighbour, the closure or possession has to be in place for the whole operation and not just the lift itself.

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   Step 7: Derig, release the area and clear the ground

   When the lifting is finished the crane is derigged and driven off, the mats and spreaders are removed, and the pad is either handed back to the project or maintained if the crane is returning. Any damage to the ground, to buried services or to the surfacing is recorded and repaired. Because the pad often sits where the permanent works will eventually go, the sequence has to allow for lifting it out again, and that is planned rather than discovered.