Two-Leg Bridle Lift with an Offset Centre of Gravity
A steel skid weighing 8,600 kg is 4.8 m long and 2.2 m wide. It has two lifting lugs on the centreline, 3.6 m apart. The centre of gravity is 1.2 m from the left lug and 2.4 m from the right lug. A two-leg bridle is proposed with the hook point 2.4 m above the lug line and directly above the centre of gravity. The left sling leg is 2.7 m long and the right leg is 3.4 m. The rigger has two 3/4-inch screw-pin shackles with a working load limit of 4.75 t each and two single-leg wire-rope slings rated at 6.0 t vertical. The left lug is rated 8 t and the right lug 5 t. The load must be landed level on a concrete pad; a tag line is available.
1. Calculate the vertical load carried by each lug and the tension in each sling leg. State whether the shackles and slings are adequate.
Read the worked answer
Left vertical share = 8,600 × 2.4/3.6 = 5,733 kg. Right share = 8,600 × 1.2/3.6 = 2,867 kg. Left leg angle: sin θ = 2.4/2.7 = 0.889, so tension = 5,733/0.889 = 6,450 kg. Right leg: sin θ = 2.4/3.4 = 0.706, tension = 2,867/0.706 = 4,061 kg. The left 3/4-inch shackle at 4.75 t is overloaded (6.45 t), and the left sling at 6.0 t vertical rating is also overloaded. The right leg is within rating but should match the left for a symmetrical rigging set. Lug loads are acceptable: left 5.73 t < 8 t; right 2.87 t < 5 t.
2. The rigger proposes using a four-leg sling because “four legs share the load better.” Explain why this is not automatically safer and what you would do.
Read the worked answer
A four-leg sling does not automatically share load equally. On a rigid load, only two or three legs may be effective; the rest hang slack. If the legs are not equalized, the CG still controls the share. Adding legs increases weight and can create unexpected side loads. Use a calculated two-leg bridle, an equalizing link, or a spreader beam; verify the CG and lug capacities; do not rely on “more legs” to fix an overloaded leg.
3. What rigging changes would you require before the lift?
Read the worked answer
Replace the left shackle with at least an 8.5 t WLL shackle and use slings rated for the calculated tension with the required design factor, or use a spreader beam so the legs are vertical and the tension equals the vertical share. Confirm the master link and hook latch. Attach tag lines. Lift a few centimetres, check level and rigging, then land on the pad. If the hook cannot be placed above the CG, recalculate because the load will tilt and the angles will change.