Two-Leg Bridle at a Shallow Angle
An operator must lift a 6,000 lb steel tank with two lifting lugs 10 ft apart. A rigger plans a two-leg bridle using two 1/2-inch wire rope slings, each with a vertical-rated capacity of 5,000 lb and a 30-degree-from-horizontal leg angle. The tank's center of gravity is 1 ft toward one lug. The rigger says each sling only carries 3,000 lb because there are two legs and the load is 6,000 lb, so the slings are at 60% capacity. A tag line will be attached, and one lug is slightly worn. The landing area is on a concrete pad with a 12-inch drop-off on one side. The signalperson is on the far side of the tank and cannot see the operator. The slings are not protected at the lug edges.
1. Calculate the approximate sling tension for a centered load at the planned angle and evaluate the rigger's conclusion.
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For a symmetric two-leg bridle, each leg tension is W divided by (2 times the sine of the angle from horizontal). At 30 degrees, sine 30 is 0.5, so each leg tension is 6,000 / (2 x 0.5) = 6,000 lb. The rigger's 3,000 lb figure is only the vertical share of the load; it ignores the horizontal component that the shallow angle creates. Each sling would be at 6,000 lb tension against a 5,000 lb vertical rating, so the bridle is overloaded. The rigger must change the plan: use a spreader beam so the legs can be more vertical, increase the leg angle, use higher-capacity slings, or reduce the load. A 45-degree leg angle would reduce each leg to about 4,243 lb, but that still must be checked against the sling rating, hardware, and lug capacity. The worn lug and unprotected edges must also be addressed before lifting.
2. How does the 1 ft center-of-gravity offset affect the lift, and what should the rigger do?
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With the center of gravity 1 ft toward one lug, the load will not hang level and the two legs will not share the load equally. The lug nearer the center of gravity will carry more tension, so the already overloaded sling condition is worse on that side. The rigger should not attach the slings and rely on the tag line to level the tank. The rigger needs to adjust the pick points or sling lengths so the hook is over the center of gravity, use a spreader or equalizing arrangement designed for the offset, or have the lift engineered. A trial lift just off the ground can show the tilt, but it does not prove the tension is acceptable. The worn lug should be inspected and rated for the load, and sharp edges need softeners or edge protection. If the rigger cannot determine the load distribution, the lift should stop.
3. What communication and landing problems must be corrected before the lift?
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The signalperson cannot see the operator, so the operator has no reliable signal for a load that may tilt or rotate. A designated signalperson must be repositioned to see both the load and the operator, or a relay signalperson with clear communication must be used. The tag line handler needs a safe position away from the drop-off and out from under the load. The 12-inch drop-off at the landing pad requires an exclusion zone and a plan to keep the load from being pulled over the edge. If the signal is lost, the operator must stop. The rigger should also confirm that the landing area can support the tank and that no one is between the load and the drop-off. These controls must be in place before the trial lift, not added after the load is airborne.