Sling angle and leg tension

R 03 mHow it works

θ = angle from horizontal   (from vertical: θ = 90 − φ)
from the rigging: θ = asin(√(L² − s²) ÷ L)
n = legs (1 or 2); 2 for a rigid load on 3 or 4 legs; otherwise at most 3
factor = 1 ÷ sin θ
T = load × factor ÷ n

The angle. Riggers quote sling angles two ways, so the tool takes either. Angle from horizontal (θ) is the one IHSA's tables use; angle from vertical (φ) is the one WorkSafeBC's table uses, and the two add up to 90°. If you know the rigging instead, enter the leg length L and the horizontal distance s from below the hook to the pick point. Two quick checks from IHSA: with a two-leg bridle the angle is 60° when the distance between pick points equals the leg length, and 45° when the legs form a right angle at the hook¹.

How many legs carry the load. One or two legs share the load evenly. A three- or four-leg sling is different. IHSA advises that a four-leg sling lifting a rigid load may carry it on only two legs, so it should be rated as a two-leg sling¹. WorkSafeBC limits the WLL of a sling with more than 3 legs to the WLL of any 3 legs². The tool uses two legs for a rigid load on three or four legs, and at most three otherwise.

Tension. Each leg carries its share of the load divided by the sine of its angle from horizontal. The multiplier is the load angle factor: 1.155 at 60°, 1.414 at 45° and 2.000 at 30°¹. Those are the same numbers as IHSA's sling-angle figure, which shows each leg of a two-leg sling lifting 1,000 lb carrying 500 lb at 90°, 577 lb at 60°, 707 lb at 45° and 1,000 lb at 30°¹. Enter 1,000 kg on two legs at 45° and the tool returns 707 kg per leg.

Checks against the sling's WLL. If you enter the WLL marked on the sling, the tool makes two comparisons. First, the tension in each leg against the marked WLL: in WorkSafeBC's words, the load carried by any single leg of a bridle sling must not be greater than the WLL of the leg². Second, if the lift is in British Columbia, the reduced WLL from Table 15-3 against each leg's share of the load. Table 15-3 is how the regulation accounts for the angle, so the reduced figure is set against the share, not the tension; setting it against the tension would count the angle twice. Both checks must pass.

R 06 mBritish Columbia: Table 15-3

When a sling lifts at any angle from the vertical, WorkSafeBC requires the WLL to be reduced by a qualified person or the manufacturer, or according to Table 15-3². The table works on the angle between the sling leg and the vertical:

WorkSafeBC does not permit sling legs more than 60 degrees from vertical, which is under 30 degrees from horizontal, unless they are part of an engineered lift². A leg that hangs straight down has no angle, so the tool applies no reduction to it.

R 09 mWhy flat slings are dangerous

The load angle factor climbs slowly at first and then very fast. From 90° to 60° it rises by about 15%; from 45° to 30° it rises from 1.414 to 2.000¹. Below 30° a small error in judging the angle makes a large difference to the tension in each leg. That is why IHSA advises keeping sling angles above 45 degrees from horizontal whenever possible and calls any angle below 30 degrees extremely hazardous, partly because a 5 degree estimating error then changes leg load sharply¹. The tool flags both thresholds.

The fix is usually longer slings, which raise the angle for the same pick points, or a spreader or lifting beam, which takes the horizontal component out of the slings. Either changes the rigging weight, so put the new figure into the size estimator too.