Wind Gust During a Glass Panel Lift
You are operating a luffing-jib tower crane on a city centre site, lifting a 1.2 t glass curtain-wall panel with a large sail area. The manufacturer's manual states the maximum in-service wind speed is 20 m/s at the jib, and the crane is fitted with an anemometer that logs peak gusts. The panel is rigged with two vacuum lifters and tag lines. At 60 m radius and 40 m height, the panel is halfway to the landing point when the anemometer shows a steady 14 m/s with a gust to 21 m/s in the last 10 seconds. The rigger on the roof signals you to keep coming. The load is currently over a protected walkway that has been closed to pedestrians but is inside the site hoarding.
1. What should you do in the next 30 seconds, and why does the steady-versus-gust distinction matter?
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The gust has exceeded the manufacturer's 20 m/s in-service limit, so the lift must be stopped. The steady 14 m/s is below the limit but the gust peak is the governing figure because the sail area of the panel means wind force scales with the square of wind speed - a 21 m/s gust produces roughly (21/14)² ≈ 2.25 times the force of the steady wind on the same panel. That force acts at 60 m radius, creating a large overturning moment and also swinging the panel unpredictably.
Action: bring the load down under control to the nearest safe landing area, not over the walkway. Signal the rigger that you are aborting rather than continuing. Do not lower the load by free-falling or by rapid slewing; use slow, controlled motions. Once the load is landed and unrigged, park the crane per the manufacturer's out-of-service procedure (jib free to weathervane for a luffing crane, or as specified). Record the gust and the abort in the log. The rigger's signal to continue does not override the manufacturer's limit or your duty to stop.
2. The site manager later says the walkway closure means the load could have been kept aloft until the gust passed. Evaluate that reasoning.
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The walkway closure only addresses one consequence - injury to people below - and does not address the actual hazard. The governing issue is that the crane has exceeded its design in-service wind limit, so structural and stability margins are no longer assured. Holding a high-sail-area load aloft in a gust increases the dynamic load on the jib, the slew ring and the mast, and a sudden gust can cause the load to swing into the mast or neighbouring structures. The correct control is to remove the load from the crane and take the crane out of service, not to rely on an exclusion zone. The exclusion zone is a mitigation for a falling load, not a justification for continuing an out-of-limit lift.