Wind Load and the Double-Stack at the Waterfront Slot
You are asked to lift a 40 ft container from ground level onto a stack that is already two high, making the target the third tier. The load chart for your machine at the required boom/extension geometry and the measured radius of 9.5 m shows a rated capacity of 8.2 t for a spreader-only configuration. The container's CSC plate and weighbridge ticket show a gross mass of 6.4 t. Wind is gusting from the seaward side; the site anemometer reads a steady 12 m/s with gusts to 17 m/s, and the manufacturer's operating manual states that lifting operations must cease when wind speed exceeds 15 m/s, and that sail-area effects on empty or lightly loaded containers must be considered below that figure. A spotter reports the container 'feels light' on the first test lift and that it is swaying noticeably.
1. Using the figures given, assess whether this lift should proceed as planned, and show your reasoning.
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Static capacity check: The load is 6.4 t against a rated capacity of 8.2 t at 9.5 m. That is a utilisation of 6.4 / 8.2 = 78%, leaving 1.8 t margin - acceptable on paper, but only on paper.
Dynamic and wind derating: A reach stacker's rated capacity assumes still air and controlled dynamics. The manual's 15 m/s limit is a hard stop; gusts of 17 m/s already exceed it, so the lift is outside the manufacturer's permitted envelope regardless of the load margin. The 12 m/s steady reading is not the governing number - gusts govern, because a gust loads the container's sail area suddenly and the machine's boom and spreader act as a pendulum.
Sail area: A 40 ft container presents roughly 12 m x 2.6 m ≈ 31 m² of side area. Even at 6.4 t the container is light relative to its volume, so wind force is a significant fraction of the load and will cause sway and side load on the boom - exactly the symptom the spotter reported.
Conclusion: Do not proceed. Stop the lift, lower and set the container down, and wait for sustained and gust wind speeds to fall within the manual's limits with margin. Reassess the lift plan (consider a lower tier, a different slot sheltered from the wind, or rescheduling) before resuming.
2. The supervisor says 'the ticket says 6.4 t, so just go slower and it'll be fine.' Explain why 'going slower' does not resolve this situation.
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Slowing the hoist addresses dynamic amplification from acceleration and braking - it does not address wind. The wind force on the container is external and continuous; it does not reduce because the machine moves slowly. In fact, a slow lift increases the time the container is suspended and exposed to gust loading, and a slow, high, swaying lift gives the operator less ability to correct a swing before the container reaches the third-tier guides.
There is also a control problem at height: placing onto a third tier means the container must be held steady above the target while it is lowered into the corner castings. In gusty conditions the sway is unpredictable and the operator may be tempted to 'catch' the swing with boom movements, which induces side loads the boom and spreader were not designed for.
The ticket mass is also only as good as the weighing; a 'light' feel on the test lift plus visible sway is a signal to stop and verify, not to press on. The correct response is to halt, secure the load, and resume only when conditions are within the manual's limits.