Ground Bearing Under a Track-Mounted Self-Erector
A 6-tonne-capacity self-erecting crane is to be sited on a compacted hardcore pad behind a new warehouse. The pad sits over a backfilled drainage trench 1.2 m deep running diagonally beneath one corner of the proposed footprint. The crane's four outrigger floats are 600 mm square, and the manufacturer's ground pressure data gives 95 kN per outrigger under full load, rising to 130 kN during the climb sequence. Site investigation reports loose fill with an allowable bearing pressure of 60 kN/m² in the trench zone and 150 kN/m² elsewhere. The site manager wants to spread the load with 1.0 m square timber pads. The crane must be climbed to its full free-standing height before the steel frame is erected, and the erection sequence cannot be delayed.
1. Calculate the bearing pressure under the proposed 1.0 m timber pads and determine whether the siting is acceptable as proposed.
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Pad area = 1.0 × 1.0 = 1.0 m². Worst case is the climb phase at 130 kN per outrigger, so pressure = 130 / 1.0 = 130 kN/m².
Compare with the ground: 130 kN/m² exceeds the 60 kN/m² allowable in the trench zone by more than double. It is within the 150 kN/m² elsewhere, but the trench passes under one corner, so that outrigger is the governing case.
The proposal fails. Timber pads do not change the total load - they only spread it, and 1.0 m² is not enough over the fill. Simply enlarging the pad is also risky because the load would still be transferred into the loose backfill at depth. The correct judgement is to relocate the crane so no outrigger, and no part of the slew or counterweight envelope, is within a recognised zone of influence of the trench (typically a 45° load spread from the base of the excavation), or to have the trench properly engineered - e.g. excavated and replaced with compacted granular material and verified by plate bearing test. Any revised arrangement must be re-checked against the manufacturer's ground pressure data for the climb case, not just the static lift case.
2. The site manager argues that because the crane is only lifting light steel sections, the climb-phase figure can be ignored. Assess this reasoning.
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The reasoning is wrong. The outrigger reactions during climbing/jacking are not governed by the weight of the payload - they are governed by the crane's own geometry and the way the mast, jib and counterweight loads redistribute as the machine is raised. Manufacturers publish a separate, often higher, maximum outrigger load for the climbing configuration precisely because the load path changes.
The load chart for lifting tells you what the crane can pick up; the ground pressure data tells you what the ground must carry. They are different design cases, and the ground must satisfy the worst credible one. A light lift does not reduce the reaction at the outrigger during the climb.
Practical judgement: use the maximum outrigger load from the manufacturer's data for every phase the crane will pass through on site, including erection, climbing, and dismantling, and design the foundation for the highest. If the manufacturer's data is unavailable or ambiguous, the siting should not proceed until it is obtained in writing.