Sling angles and rigging for boom truck lifts

R 03 mWhy sling angle matters

A sling leg pulls along its own line. Only the vertical part of that pull holds the load up, so as a leg leans toward horizontal it has to pull harder to do the same work. The load does not change; the tension in the rigging does.

IHSA's sling-angle figure shows it for a two-leg sling lifting 1,000 lb: each leg carries 500 lb when the legs hang vertical (90° from horizontal), 577 lb at 60°, 707 lb at 45° and 1,000 lb at 30°¹. At 30°, each leg carries the whole load.

IHSA advises keeping sling angles above 45° from horizontal whenever possible, and calls any angle below 30° extremely hazardous, partly because a 5 degree estimating error then changes the leg load sharply¹. The arithmetic bears it out. Estimate 60° when the leg is really at 55°, and the tension is about 6% more than you worked out. Estimate 30° when it is really 25°, and it is about 18% more.

Flat legs cause a second problem. The horizontal part of each leg's pull squeezes the load inward, and the flatter the legs, the harder the squeeze. A long, light load can bow under it. A spreader bar takes that squeeze instead of the load.

R 06 mThe tension table

Here is IHSA's figure as published, in pounds, for a two-leg sling and a 1,000 lb load¹, with the load angle factor beside it:

The tension in each leg of an evenly loaded bridle is the load divided by the number of legs times the sine of the angle from horizontal, which gives load angle factors of 1.155 at 60°, 1.414 at 45° and 2.000 at 30°¹. The factor has no units, so the table reads the same in kilograms: 1,000 kg on two legs at 45° puts about 707 kg in each.

Example in kilograms

A 2,400 kg precast panel on a two-leg bridle (example figures). Each leg's share is 1,200 kg:

• At 60° from horizontal: 1,200 × 1.155 = about 1,386 kg per leg.
• At 45°: 1,200 × 1.414 = about 1,697 kg per leg.
• At 30°: 1,200 × 2.000 = 2,400 kg per leg, the full weight of the panel in each.

Pick slings whose working load limit (WLL) covers the tension at the angle you will actually get. If the angle on site is flatter than planned, re-run the number before the lift.

R 09 mFrom horizontal or from vertical: read the rule carefully

Sling angles are quoted two ways, and the same number means opposite things. IHSA measures from horizontal, where 60° is a steep, comfortable leg. WorkSafeBC's Table 15-3 measures from vertical, where 60° is a flat leg at the edge of what BC allows without an engineered lift². The two always add up to 90°.

Two field checks from IHSA need no protractor. On a two-leg bridle, the angle is 60° when the distance between the pick points equals the leg length, and 45° when the legs form a right angle at the hook¹.

For anything else, measure the leg length and the horizontal distance from below the hook to the pick point. The sine of the angle from horizontal is √(leg² − distance²) ÷ leg. Example: 2.0 m legs on pick points 3.0 m apart give a distance of 1.5 m, so √(4.00 − 2.25) ÷ 2.0 = 0.66, about 41° from horizontal, below IHSA's 45°, with a factor of about 1.51. Legs 3.0 m long on the same points bring it to 60°.

R 12 mBC's working load limit reductions

When a sling lifts at an angle from vertical, WorkSafeBC requires its WLL to be reduced, either as a qualified person or the manufacturer determines, or by Table 15-3:

The same Part says the load carried by any single leg of a bridle sling must not exceed that leg's WLL, that the design factors must be maintained, and that a sling assembly's WLL must not exceed the WLL of any of its components (ss.15.33 and 15.34)². That makes two checks, and each counts the angle once:

1. Tension: the tension in each leg, from the factor above, against the WLL marked on the leg.
2. Table 15-3: the marked WLL, reduced for the angle, against the leg's share of the load.

Table 15-3 is how the regulation allows for the angle, so its reduced WLL is set against the share, not the tension; setting it against the tension would count the angle twice. Where it is unclear, s.15.34 lets a qualified person or the manufacturer set the reduction.

Example (example figures): two legs each marked 2,000 kg, hanging 40° from vertical (50° from horizontal), lifting 2,600 kg. Each leg's share is 1,300 kg, inside 70% of 2,000 = 1,400 kg. The tension is 1,300 ÷ 0.766 = about 1,697 kg, inside 2,000 kg. Both pass. At 3,000 kg, the tension is about 1,958 kg and still passes, but the 1,500 kg share is over 1,400 kg: in BC, that lift needs bigger slings.

R 15 mMulti-leg slings: how many legs really carry the load

Adding legs does not add capacity the way it looks on paper. A rigid load, such as a steel frame, a precast panel or an equipment skid, cannot flex to share its weight evenly, and slightly different leg lengths decide which legs take it. 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 legs, under s.15.33(2)².

Example (example figures): a 4,000 kg rigid skid on a four-leg bridle at 60° from horizontal.

• Counted as four legs: 4,000 ÷ 4 × 1.155 = about 1,155 kg per leg.
• Counted as two legs, as IHSA advises: 4,000 ÷ 2 × 1.155 = about 2,310 kg per leg.

The honest number is double the optimistic one. Choose legs rated for it.

Off-centre loads change the picture again. When the centre of gravity is not centred between the pick points, the legs nearer it carry more, and the even-bridle formula understates the worst leg. Find the centre of gravity before choosing pick points, from the drawing, the manufacturer or a trial lift a few centimetres off the ground, and get a qualified rigger's or engineer's input for anything awkward.

R 18 mDesign factors for wire rope and chain

WorkSafeBC sets minimum design factors of 5 for wire rope slings and 4 for alloy steel chain slings, in Table 15-1². Alberta writes the same idea as a share of breaking strength: rigging may not be loaded beyond 20% of the ultimate breaking strength of its weakest part, 25% where the manufacturer has fatigue-rated it to EN 1677-1, and 10% when a worker is raised or lowered (s.292)³.

A design factor is the ratio between the force that breaks a sling and its working load limit. A wire rope sling marked 2,000 kg with a design factor of 5 should not break below about 10,000 kg (example). That margin is already inside the marked WLL, so the WLL is the number you work to. It is not spare capacity to borrow on a heavy pick.

Markings and inspection

• BC: wire rope slings with swaged or poured sockets or pressed fittings must be permanently identified with their WLL and the angle it is based on; alloy steel chain slings with size, grade and WLL (ss.15.42 and 15.46)².
• Alberta: the maximum load rating must be legibly marked on the rigging, or available at the site if marking is not practicable; rigging is inspected before each period of continuous use in a shift; and sharp edges on loads are guarded (ss.293, 294 and 296)³.

A sling with no legible rating has an unknown WLL. Set it aside.

R 21 mRigging is part of the load

Everything between the hook and the load counts against the truck's chart. IHSA states that a hoist's rated load does not account for hook blocks, hooks, slings or equalizer beams, so their combined weight is added to the load¹. U.S. OSHA defines the load to include the load block, ropes, slings and shackles, a non-Canadian reference⁴. Queensland's mobile crane code, a non-Canadian reference, works the example: 20 t of chart capacity with 1 t of lifting gear leaves 19 t for the load⁵.

Alberta sets standards for the gear itself: slings made on or after July 1, 2009 must meet ASME B30.9-2006, other below-the-hook lifting devices ASME B30.20-2006, and a spreader bar may carry its rating on a capacity data sheet tied to it by a unique number (s.297)³.

Rigging also costs height. Every metre of sling and hook block is a metre of tip height the boom must find. Shorter slings save height but flatten the angle and raise the tension; a spreader bar buys back the angle at the cost of its own weight. Settle the rigging before the truck is sized. See how to read a load chart and the size estimator, which takes rigging weight and length as inputs.

R 24 mUsing the sling calculator

The sling angle calculator does the arithmetic on this page.

• Inputs: the load in kg, the number of legs, the angle from horizontal or vertical (or the leg length and the horizontal distance from below the hook to the pick point), whether the load is rigid, the WLL marked on each leg, and whether the lift is in BC.
• Outputs: the tension in each loaded leg, the load angle factor, the legs assumed to carry the load (two for a rigid load on three or four legs, at most three otherwise), the Table 15-3 reduction, and both WLL checks.
• Warnings: below 45° from horizontal, IHSA's advice to stay above it; below 30°, IHSA's extremely hazardous warning and BC's rule against legs more than 60° from vertical outside an engineered lift¹².

It assumes an evenly loaded, symmetric bridle. It does not handle an off-centre load, unequal legs, choker or basket hitch ratings, or edge protection; those come from the sling's marking, the manufacturer's tables and a qualified rigger.

When you request a boom truck, send the load weight, the rigging you plan to use and the headroom at the set point. A supplier quotes the job; this page explains what goes into the quote. With your consent, your request goes to one company that rents, sells or finances boom trucks in your area, and that company may pay BoomTrucks.ca for the referral.