Load estimation and sling angle
You are asked to lift a steel tank of unknown weight. The tank is 2.2 m diameter, 3.0 m long, with 8 mm shell and ends, and has a nameplate that is painted over. It is to be lifted with a two-leg sling from two top lugs 2.4 m apart, with the sling legs at 45 degrees to the horizontal. The dogger estimates "about 3 tonnes". A crane chart shows 4.5 t at the required radius. You have a 5 t shackle and 6 t slings.
1. Estimate the tank mass from its dimensions and explain how the sling angle changes the load in each leg.
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Estimate the steel volume. Shell: circumference pi x 2.2 = 6.91 m, times length 3.0 m = 20.7 m2, times 8 mm = 0.166 m3. Two ends: each pi x 1.1^2 = 3.80 m2, times 0.008 = 0.0304 m3 each, so 0.061 m3. Total about 0.227 m3. Steel at 7.85 t/m3 gives about 1.78 t for the steel alone.
That is the shell only. Add nozzles, internal baffles, saddles, lugs and any residual contents. A tank of this size could easily carry several hundred kilograms of fittings, and if it held water or slurry, residual contents could dominate. The estimate of 1.78 t is a floor, not a weight.
Now the sling. Two legs at 45 degrees share the vertical load, but each leg carries more than half. Leg tension = (W/2) / sin 45 = 0.707 W per leg. At W = 3 t, each leg sees about 2.12 t. Flatter angles make this worse: at 30 degrees, each leg sees W/2 / sin 30 = W, so a 3 t load puts 3 t in each leg.
The 6 t slings and 5 t shackle are adequate for the estimated load, but the estimate is the weak point. Do not lift on a painted-over nameplate and a guess: weigh the tank or obtain the manufacturer's mass, and add the rigging weight to the load.
2. The chart allows 4.5 t at the radius. If the true mass turns out to be 3.8 t, what changes and what would you check?
Read the worked answer
At 3.8 t against 4.5 t, the margin is 0.7 t, or about 16 percent. That is thin once rigging, wind and radius growth are considered, and it is close enough that the lift needs a written plan and a check of the worst-case radius.
Recalculate leg tension: each leg at 45 degrees carries 0.707 x 3.8 = 2.69 t, still within 6 t slings. Check the lugs and shackles against that tension and against the included angle at the lug. Check the spreader or sling arrangement so the legs do not crush the tank shell.
Check the chart at the actual radius after slewing, not just the starting radius. If the radius grows, capacity drops and the 0.7 t margin can vanish. Add rigging weight to the 3.8 t and recheck. Confirm the ground and outrigger loading for the heavier figure. If any check fails, do not lift; reduce radius, use a larger crane, or reduce the load.