Outrigger Setback on a Surcharged Pad
A 60 t slew crane is set up on a compacted gravel hardstand beside a new warehouse to lift a 9.2 t packaged air-handling unit onto a roof curb 22 m away. The crane is configured with 18 m of boom, a 12 m fly jib at 15 degrees, and full outriggers. The load chart for this configuration permits 10.5 t at the required radius. The south-west outrigger sits over a patch of ground where a stormwater trench was recently backfilled; the supervisor assures you it was compacted. You notice the pad under that outrigger has sunk 40 mm since levelling and a hairline crack has opened in the gravel crust. Wind is 12 km/h gusting to 25 km/h from the north-west. The roof crew is waiting and the client wants the lift done before a 15:00 concrete pour blocks the access road.
1. What is the immediate decision about proceeding with this lift, and what evidence would you need before the crane is operated?
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Do not proceed. The 40 mm settlement and cracking indicate the bearing capacity under the south-west outrigger is inadequate for the imposed load, and the crane is only 1.3 t (about 12%) inside the chart capacity. Any further settlement on that corner shifts load onto the diagonally opposite outrigger and can induce a tipping moment the chart does not cover.
Stop the lift, lower and secure the load, and keep personnel clear of the crane's tipping arc. The supervisor's verbal assurance is not evidence. Required evidence: the backfill compaction records and any geotechnical report for that patch; the crane's actual outrigger loadings from the manufacturer's load chart or load cell readings; and a competent person's assessment of the pad's bearing capacity.
If capacity cannot be verified, the crane must be relocated or the outrigger founded on a properly designed mat or grillage sized for the calculated point load plus a factor of safety. Only after the pad is proven and the crane re-levelled with the bubble in the centre should the lift be replanned.
2. The client pressures you to lift now and 'just watch the outrigger'. Explain why observation alone is not a control, and what you would say to the client.
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Observation is not a control because settlement can be sudden and progressive: a pad can hold momentarily and then punch through as the load comes onto that corner. The point at which you would see movement is already past the point of no return for a tipping crane, and the operator's view of the outrigger is obstructed while concentrating on the load and the roof crew. Human reaction time is far too slow to arrest a tipping event.
The hierarchy of control requires eliminating the hazard or engineering it out, not monitoring it. The correct control is to relocate the crane, enlarge the pad, or reduce the radius and load to give a genuine margin.
To the client I would state plainly: the ground is not proven, the crane is within 12% of chart capacity, and I am not permitted to operate where the foundation is unverified. I would offer alternatives: a different setup position, a smaller lift plan with a reduced radius, or a short delay while the pad is assessed. I would document the decision and the reasons in the lift plan and notify the site supervisor.
3. How would the gusting north-west wind affect this configuration, and what wind-related limits must be checked?
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The crane presents a large sail area through the 12 m fly jib and the packaged unit, and gusts from the north-west will act broadside to the boom at some slew angles. This increases the load's swinging tendency and the side load on the boom, and can cause the load to drift out to a larger radius than planned, eating into the 1.3 t margin.
I must check the manufacturer's maximum permissible wind speed for this boom and jib configuration, the load's sail area limits in the chart notes, and the rigging's ability to resist rotation. The gust figure, not the average, is the governing value.
Controls: use tag lines to control rotation, orient the load to minimise sail area, restrict slewing speed, and stop if gusts exceed the manufacturer's limit or the load becomes uncontrollable. The roof crew must not be positioned to catch a swinging load. The 15:00 deadline is irrelevant to these limits.