Advanced Rigging (RA) common mistakes
Advanced Rigging (RA) · Understand the traps and how to avoid them.
Computing a sling's tension from the load weight alone and assigning the full weight to each leg, instead of dividing by the number of legs and applying the vertical/horizontal angle factor (tension = load share / sin of angle from horizontal).
Candidates memorize 'divide by legs' and forget that as sling angle from horizontal drops, leg tension rises sharply; they also mix up angle from horizontal with angle from vertical, inverting the sine and cosine.
What to do instead
Always sketch the hitch, resolve the vertical load share per leg, then divide by sin(angle from horizontal) for tension. Sanity-check: at 30 degrees from horizontal, tension is double the vertical share; at 60 degrees it is about 1.15 times. Confirm the angle definition before plugging numbers in.
Treating a two-leg bridle as sharing load equally regardless of geometry, or assuming a four-leg sling carries equal load in all four legs, when actual share depends on leg stiffness and the center of gravity position.
Textbook 'divide by legs' rules are over-applied; candidates do not account for asymmetric pick points, off-center CoG, or the fact that a rigid four-leg system is statically indeterminate and rarely shares evenly.
What to do instead
For asymmetric or indeterminate hitches, design to the conservative case (two legs carrying the load for a four-leg sling) unless a qualified engineer states otherwise. Locate the CoG, keep the hook over it, and use equalizing hardware or a spreader to force predictable sharing.
Selecting shackles, hooks, and slings by 'size' or by the marked working load limit without checking the configuration: side-loading a shackle, loading a hook at the tip, or using a sling in a choker when its rating assumes vertical.
Hardware ratings are configuration-specific, but candidates read a single WLL number and assume it applies in every orientation. Markings on hooks and shackles are small and easy to misread.
What to do instead
Match the hardware to the hitch: use bolt-type or properly oriented shackles for side loads, keep hook loads in the saddle/bowl, and apply the correct efficiency (choker, basket, vertical) to sling ratings. Verify the specific WLL for the actual loading direction, not just the tag.
Ignoring the reduction in capacity from sling angle, D/d ratio, edge protection, and choke efficiency when calculating a sling's usable capacity, treating the tag rating as the capacity in every hitch.
The tag shows a base rating; efficiency factors are learned separately and candidates fail to chain them together, or they forget that sharp edges and small D/d ratios reduce capacity.
What to do instead
Build a habit of applying every modifier in sequence: base rating, then hitch efficiency, then angle factor, then edge/D-d derating, and compare the final number to the required tension. Use softeners or edge protectors and record the derated capacity on the lift plan.
Reading a load chart for the boom length and radius but forgetting to apply the deductions for jib, auxiliary head, stowed attachments, or to check the chart's structural versus stability limit and the correct outrigger configuration.
Charts are dense and multi-page; candidates grab the first plausible cell and skip footnotes, capacity deductions, and the configuration matching the actual setup.
What to do instead
Before reading a cell, confirm machine configuration (outrigger spread, counterweight, jib stowed/deployed), then apply all footnotes and deductions. Take the lower of structural and stability limits and record the governing limit on the lift plan.
Signaling with improvised or ambiguous hand signals, or relying on radio without a dedicated signalperson and clear stop authority, so the operator acts on a misunderstood command.
Candidates know the standard signals loosely and improvise when line of sight is blocked; they also fail to establish who has stop authority and how communication failures are handled.
What to do instead
Use only the standard hand signals, designate one signalperson with a clear line of sight or relay, and brief the crew that anyone may give an emergency stop. Agree on radio channel, backup signals, and a lost-communication stop procedure before the lift.
Judging wire rope or synthetic sling condition by appearance alone, missing internal broken wires, valley breaks, core damage, or heat/chemical damage hidden under the jacket, and failing to apply removal criteria by count and location.
Candidates inspect the outer surface and stop there; removal criteria depend on distribution of broken wires (valley vs. crown, one lay vs. sections), diameter reduction, and type-specific limits that are easy to confuse.
What to do instead
Inspect the full length including terminations, use a wire rope gauge for diameter loss, count broken wires over one lay and check for valley breaks and core protrusion. For synthetic slings, check for cuts, abrasion, UV glazing, and internal damage by feel and by the tag's removal criteria. Remove when any criterion is met, not only when obviously failed.
Confusing a knot with a splice for rated capacity, or using a knot in a synthetic rope for a critical lift without accounting for the large strength loss, and misjudging which splice type retains rated efficiency.
Knots are convenient and candidates assume a 'good knot' retains most strength; splice efficiency varies by type and rope construction, and the difference is not intuitive.
What to do instead
Treat knots as non-rated for critical lifts and apply the appropriate efficiency loss (knots typically retain well under half of rope strength). Use the splice type specified for the rope and application, verify it was made by a qualified splicer, and never assume a splice retains 100 percent unless the manufacturer states it.
Planning a jack-and-slide or roller move without accounting for friction, slope, and the shifting center of gravity, and without a controlled lowering/blocking plan for the transition points.
Candidates focus on the jack capacity and ignore the horizontal force needed to move the load, the effect of rollers on stability, and what happens when the load leaves the rollers or crosses a soft spot.
What to do instead
Calculate the push/pull force from load weight times the rolling or sliding friction coefficient, add slope allowance, verify ground bearing capacity along the entire path, and plan blocking and controlled transitions at every stage. Keep the CoG low and monitored, and use a spotter and tag lines.
Writing a lift plan that lists equipment but omits the hazard analysis, CoG, rigging diagram, exclusion zone, environmental limits, and contingency, then treating a critical lift as routine.
Plans are often copied from templates and filled with equipment lists; candidates underestimate what makes a lift 'critical' and skip the engineering review and hold points.
What to do instead
Build the plan around the load: CoG and rigging diagram, calculated tensions and capacities, ground conditions, wind/weather limits, exclusion zone, communication plan, sequence with hold points, and contingencies. Get the required engineering review and pre-lift meeting before execution.