Estimating Load Weight from a Drawing
A rigger is asked to lift a steel plate assembly. The drawing shows a 4 ft x 8 ft x 1/2-inch steel plate with two 6-inch x 6-inch x 1/2-inch stiffener plates welded to it. The drawing does not list a weight. The rigger knows steel weighs about 490 lb per cubic foot. A coworker says the plate 'looks like about 500 lb' and the crane's capacity at the planned radius is 3,000 lb. The rigger also needs to add the weight of a 200 lb spreader beam and 40 lb of shackles and slings. The lift will use a four-leg sling with a 60-degree angle from horizontal.
1. Calculate the estimated weight of the steel assembly and the total load to be lifted.
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Main plate volume = 4 ft x 8 ft x (0.5/12 ft) = 32 x 0.04167 = 1.333 cubic ft. Weight = 1.333 x 490 = 653 lb. Each stiffener = 0.5 ft x 0.5 ft x (0.5/12 ft) = 0.01042 cubic ft; weight = 5.1 lb each, about 10 lb for two. Total steel = about 663 lb. Add spreader beam 200 lb and hardware 40 lb = about 903 lb total. The coworker's 500 lb estimate is low by roughly 160 lb of steel alone and ignores the rigging. The rigger should use the calculated weight, not the visual estimate.
2. How does the four-leg sling angle affect the tension, and is the crane capacity adequate?
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With a 60-degree angle from horizontal, the vertical component of each leg is sin(60) = 0.866 of its tension. If the load were shared equally among four legs, each leg's vertical share is 903/4 = 226 lb, and tension = 226/0.866 = 261 lb per leg. However, a four-leg sling rarely shares equally; the standard practice is to assume only two legs share the load unless the rigging is designed to equalize. Using two legs, each vertical share is 451 lb, tension = 451/0.866 = 521 lb per leg. The crane capacity at radius is 3,000 lb, and the total load is about 903 lb, so capacity is adequate with margin. The rigger should still confirm the crane's capacity at the actual radius and account for the spreader beam's effect on the sling angle.
3. What should the rigger do about the missing weight on the drawing?
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The rigger should calculate the weight from dimensions and material density, document the calculation, and if there is any doubt, ask the fabricator or engineer for the actual weight. The rigger should not rely on a coworker's visual estimate. The calculated weight and the rigging weight should be recorded on the lift plan, and the crane capacity should be checked against the total. If the drawing's dimensions are unclear, the rigger should stop and get clarification before lifting.