Formwork Bundle at 44 m Radius in Rising Wind
At 14:20 on a 22-storey concrete project, a hammerhead tower crane with a 55 m jib is scheduled to land a bundle of aluminium wall-form panels on level 14. The bundle weighs 2.4 t, measures 6.0 m by 2.4 m, and the hook radius is 44 m. The capacity chart shows 3.6 t at 44 m. The ground anemometer reads 8 m/s, while the jib-mounted anemometer reads 13 m/s with gusts to 17 m/s. The site rule stops lifts at 15 m/s average at the jib. The superintendent argues that the panels are only two-thirds of chart capacity and asks for 'one slow pick' during a lull. Tag lines are rigged, the load is still on the street, and the crew is waiting. The operator must decide whether to lift and what alternative to propose.
1. Which wind reading and capacity assumptions should control the go/no-go decision? Show your reasoning.
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Controlling wind reading: the jib-mounted anemometer, not the ground gauge. At 13 m/s average with 17 m/s gusts, the jib is already at or above the site's 15 m/s average stop when the gust is considered; the ground reading of 8 m/s is not representative at 44 m radius and at jib height. Capacity: 3.6 t at 44 m is a structural and rigging limit under the manufacturer's stated conditions, not permission to work outside the wind limit. The 2.4 t load is about 67% of chart capacity, leaving 1.2 t, but the bundle presents about 14.4 m2 of sail area. A gust can add side load and swing, and the chart does not necessarily account for that sail area at 17 m/s. The manufacturer's manual and the site lift plan are the controlling documents. If either sets a lower working wind for high-sail loads, that lower value applies. Decision: no lift now. Land the load, park the trolley as specified in the manual, and place the crane in its out-of-service condition if the wind trend continues. Tell the superintendent the lift is on hold until the jib anemometer shows a sustained reading below the limiting value, with margin for gusts.
2. A lull drops the jib anemometer to 12 m/s. The superintendent asks you to 'take it now.' What must be true before you lift, and what controls would you put in place?
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A single momentary drop is not a sustained change. Before lifting I would require a sustained reading below the controlling limit for a defined period, confirmation that the trend and forecast are not showing another gust front, and a check that the load has not moved or become snagged. The lift plan should be revised to reduce the sail area, for example by breaking the bundle into single panels, because a 6.0 m by 2.4 m bundle acts as a sail even at 12 m/s. The rigging must include tag lines long enough for the crew to control rotation from outside the drop zone. The lift path must be clear of people, and the crane must be stopped and the load landed at the nearest safe location if the jib anemometer reaches the limiting gust value. I would maintain continuous radio contact with the signaler and a spotter, keep the jib anemometer under observation, and record the wind readings and the decision in the lift log. If the superintendent insists on taking the load during an unverified lull, I would use stop-work authority; the chart margin does not override the wind limit.