Two-Leg Chain Sling at 90 Degrees on a Sharp Precast Ring
A 6.2 t precast concrete manhole ring is being unloaded from a flatbed at a new sewer works. The rigger plans a two-leg chain sling in a straight-leg hitch, with the legs at an included angle of 90°. Each chain leg is marked WLL 4.0 t for a straight vertical lift. The ring has sharp arrises and no edge protectors are on site. One chain has a twisted link and a bent coupling link; the inspection tag is missing. The crane has a 25 t capacity at the planned 7 m radius. The landing area is level compacted sand, but four workers are standing inside the planned swing area to 'steady the ring'. The wind is 18 km/h. The rigger says, 'Two legs at 4 t each means 8 t, so 6.2 t is fine.'
1. Calculate the tension in each sling leg and decide whether the two-leg chain sling is adequate. Show your method.
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For a symmetrical two-leg sling, each leg carries half the vertical load, but the tension is greater because the legs are angled. With an included angle of 90°, each leg is 45° from vertical.
Vertical share per leg = 6.2 t ÷ 2 = 3.1 t. Leg tension = 3.1 t ÷ cos 45° = 3.1 ÷ 0.707 = 4.38 t.
Each leg is marked WLL 4.0 t for a straight vertical lift, and that WLL is a limit on tension in the leg. At 4.38 t, each leg is overloaded by about 9.5%. The rigger's '4 t + 4 t = 8 t' reasoning is wrong because the WLLs are not simply added when the legs are angled; the angle increases leg tension. Another way to see it: the vertical capacity of the assembly is 2 × 4.0 × cos 45° = 5.66 t, which is less than 6.2 t.
The sling is therefore not adequate. The lift must be replanned with a higher-capacity sling, a shorter leg configuration that reduces the included angle, or a different hitch approved by a competent rigging authority. The bent link and missing tag also make this sling unserviceable regardless of the calculation.
2. List the rigging defects and the controls you would require before the lift.
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Defects: the chain has a twisted link and a bent coupling link; the inspection tag is missing; there are no edge protectors for sharp concrete arrises; and the planned angle overloads the legs. A twisted or bent link can indicate damage or overloading, and without a tag the sling's history and WLL cannot be verified. Sharp edges can score or deform chain links and accelerate failure.
Controls: remove the damaged sling from service and tag it. Use a certified sling with a readable identification tag and a current inspection record. Recalculate the arrangement so no leg exceeds its WLL at the actual angle. Fit softeners or edge protectors at every arris. Check the master link, shackles and hooks for rating, deformation and latch operation. Confirm the 6.2 t weight and the centre of gravity; use a trial lift of about 100–200 mm to check balance and rigging. Keep the load path clear, use tag lines from outside the fall zone, and barricade the swing area. The crane capacity at 7 m radius must also be checked as gross load, including rigging.
3. What landing and exclusion controls are needed, given that workers are standing in the swing area?
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The workers must be cleared from the swing area and the landing zone before the lift starts. 'Steadying' a suspended load by hand is not acceptable; tag lines can control rotation from a safe distance. The exclusion zone should cover the full swing radius plus the load's potential travel if it swings or slips, and it should be physically barricaded, not just marked with cones.
A nominated signaller should control the landing. The load should be lowered slowly onto level, compacted sand with timber or steel bearers to spread the load and prevent the ring rolling. No one puts hands or feet under the ring. The rigger should check the ring is stable before the sling is slackened, and the crane operator should keep the load under control until the signal to release is given. If the ring can roll, chock it immediately. The wind at 18 km/h is moderate, but the ring has a large surface area; if it starts to swing, lower it and use tag lines rather than people. The lift plan should state the exclusion zone and the signaller's position.