The Overweight Container and the Tired Slings
You are the slinger for a vessel discharge at a small quay. The crane's safe working load (SWL) at the working radius is 12 tonnes. A 20-foot container is presented for lifting; the manifest says 9.4 tonnes gross, but the container's CSC safety approval plate is faded and you cannot read the maximum gross mass. You plan a four-leg chain sling, each leg rated 5 tonnes in a single-leg vertical hitch, at an included angle of 90 degrees between opposite legs. The crane driver reports the load cell showing 11.8 tonnes as the container just clears the deck, and one chain leg shows a twisted link near the master link. The wind is gusting to 18 mph and the container has a slight list.
1. Determine whether the lift is within the crane's capacity and whether the four-leg sling assembly is adequate at the planned configuration. Show your calculation.
Read the worked answer
Crane capacity: The load cell reads 11.8 t against an SWL of 12 t at radius - that is 98% of capacity with no margin for dynamic effects from gusting wind or a listing load. This alone should stop the lift pending reassessment.
Sling capacity at 90° included angle: For a symmetrical four-leg sling, only two legs are assumed to share the load in the worst case (the standard conservative assumption). At 90° included angle between opposite legs, each leg's share is increased by 1/cos(45°) ≈ 1.414.
- Load per assumed load-bearing pair = 11.8 / 2 = 5.9 t
- Vertical component per leg = 5.9 / 2 = 2.95 t... but the correct treatment: total vertical load 11.8 t shared by two legs at 45° from vertical gives leg tension = (11.8/2) / cos45° = 5.9 / 0.707 = 8.34 t per leg.
That exceeds the 5 t single-leg rating. Even using all four legs symmetrically (leg tension = (11.8/4)/cos45° = 4.17 t), the twisted link and the unverified container mass mean you cannot rely on even sharing. The sling is inadequate.
2. List the actions you must take before any further lift, and explain the legal basis for refusing to continue.
Read the worked answer
Actions, in order:
- Stop and lower - signal the driver to land the container; do not allow it to travel over people or the quay edge.
- Establish the true mass - obtain a verified weight (weighbridge ticket, verified VGM documentation) rather than relying on the manifest. The CSC plate must be legible; a container without a legible plate is not fit for lifting.
- Reject the damaged sling - a twisted link near the master link is damage; tag it out of service and quarantine it. Do not 'straighten' it.
- Re-plan the lift - either reduce radius to raise SWL, use a spreader beam to bring legs nearer vertical (reducing the angle factor), or select higher-rated slings with certified SWLs.
- Re-check wind - 18 mph gusts with a listing load add dynamic load; consider waiting for calmer conditions or using tag lines.
Legal basis: Under the Lifting Operations and Lifting Equipment Regulations (LOLER) the lift must be planned by a competent person, appropriately supervised, and carried out safely; lifting equipment must be of adequate strength and marked with its SWL. Under the Provision and Use of Work Equipment Regulations (PUWER) equipment must be maintained in efficient working order. You have a duty to stop unsafe work - refusing to continue is the correct, legally supported action, not insubordination.