Tower Crane Concrete Pour as a Squall Line Approaches
A 60 m tower crane with a 55 m jib is working on a residential site in Kowloon. The jib over-sails a public footpath that is still open and a temporary works area. The operator is pouring concrete with a 1.2 m³ bucket at 32 m radius. The load, including bucket, is 3.4 t; the crane's rated capacity at that radius is 6.0 t. The jib-top anemometer shows 11 m/s steady wind, gusting 17 m/s. The manufacturer's maximum operating wind speed is 20 m/s, but the site lift plan sets a lower stop limit for loads with large sail area. The concrete bucket has about 2.2 m² of exposed sail area. The radio reports a squall line arriving in about 10 minutes with gusts to 25 m/s. The banksman asks the operator to finish one more bucket because the load is only 4 m above the hoarding and the concrete is already mixed.
1. What should the operator do in the next two minutes, and why is finishing the bucket unacceptable?
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Stop the pour immediately. The operator should signal the banksman, stop hoisting and slewing toward the public footpath, and land the bucket in a pre-agreed lay-down area inside the site, away from the hoarding, footpath and workers. If the bucket cannot be landed safely at once, it should be lowered to a controlled exclusion zone on the ground, not left suspended. Taglines should be used to control rotation. Once the load is landed, the crane should be placed in the manufacturer's out-of-service configuration: trolley at the specified radius, hook raised, slewing brake set or released exactly as the manual requires, and power isolated. The public footpath should be closed or protected before any further movement. Finishing the bucket is unacceptable because the gust forecast of 25 m/s exceeds the crane's operating limit and the bucket's 2.2 m² sail area will catch the squall, increasing swing, radius and dynamic load. The load is only 57% of rated capacity in still air, but wind load and the open footpath make the consequence severe. The operator has authority and duty to stop.
2. The site agent says the anemometer is below 20 m/s, so the lift is still legal. How should the operator respond?
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The operator should explain that 20 m/s is not a legal permission to keep lifting. It is the manufacturer's maximum operating wind speed, and the site lift plan can impose a lower stop limit. The anemometer is at the jib top and reads instantaneous wind; a gusting 17 m/s already shows turbulence, and the forecast 25 m/s is above the limit. The 20 m/s figure does not account for the bucket's large sail area, the load's pendulum effect, or the public footpath below. The correct comparison is between the actual and forecast wind at the crane, the manufacturer's manual, and the lift plan's stop criteria. If the anemometer is damaged or doubted, a handheld anemometer should be used from a safe position. The operator should not accept a verbal instruction to continue; the stop should be recorded and the lift plan reviewed before restarting. The decision should be escalated to the lifting supervisor or site safety officer if the agent insists.
3. What conditions must be met before the crane returns to service after the squall?
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The crane should not restart merely because the wind has dropped. First, wait until the wind is below the site's restart limit and is steady, not between gusts. Then a competent person should inspect the jib, wire ropes, sheaves, hook block, slew ring and brake, limit switches, anemometer and foundation or anchor bolts for damage or displacement. Any loose panels, formwork or debris blown against the jib must be removed safely. The out-of-service configuration should be checked against the manual. The operator should carry out a no-load functional test of hoist, slew and trolley, then a small test lift within the exclusion zone. The lifting supervisor should authorise restart in writing, and the event, wind readings and inspections should be recorded. If the manufacturer requires a post-high-wind inspection, that must be completed before production lifting resumes.