Failure mechanism
Crane and lifting failure during erection
A crane fails a lift when the load is heavier, further out or less favourably rigged than the load chart allows, or when the ground under it cannot carry the reaction. Most lifting failures are decided before the load leaves the ground, at a radius measured too early, with the rigging weight left out of the sum, or with an outrigger standing on fill. Once the load is in the air there is very little margin left to recover the error.
How this failure works
A crane is rated at a radius, and the capacity falls steeply as the radius grows: the published maximum is available only at a boom configuration and working radius that most site picks never approach. The radius that governs is the one measured under load, with the boom deflected and the outriggers settled into whatever is beneath them, not the one measured before the lift with the machine empty. The load that governs is the gross load — the member, the spreader beam or lifting frame, the slings, shackles, hooks and any attached rigging, plus whatever allowance the chart itself carries. Working from the net weight of the steel against the chart figure is an arithmetic that leaves out everything else the crane is holding at the same time.
An outrigger is a point load on the ground, and it is frequently several times the weight being lifted. The machine rests on three or four small pads, and the reaction under one pad includes the crane itself plus the moment from the load on the hook. That pressure then spreads into the ground at roughly 45 degrees, so the soil that has to carry it extends far wider and deeper than the pad. Fill, broken ground, a buried drain, an old cellar, or a slab over a void will not carry it, and none of that is visible from the surface where the rigging crew is standing. Spreader mats work by spreading the pressure, which is why they are sized from the calculated reaction rather than picked out of the mat stack.
Sling tension is a function of the angle between the legs and not of the weight alone. As the included angle opens, tension in each leg rises steeply for the same vertical load, and at a shallow enough angle the legs carry far more than the load itself. Two-crane lifts add a second problem, because the share each crane takes is not determinate from the geometry of the load: it depends on the stiffness of the two rigging arrangements at that instant, and it changes every time either crane booms, slews or travels. A two-crane lift therefore needs one person in charge, a load-sharing check that says what each crane is expected to take, and no movement that the check did not anticipate. All of this sits on top of the dynamic amplification from the pick itself — the suction and release as the load breaks from the ground, the swing to vertical, and any snatch — each of which can exceed the static load by a margin a chart without a dynamic factor does not allow.
The exclusion zone is not a formality, and it is larger than the footprint of the load. It covers the ground beneath the load, the ground into which the load would swing if the ground gave way, and the ground into which the crane itself would go if it tipped. Tag lines are there to stop a long or slender member rotating about the hook; used to drag a load into position they become a way of applying an unintended horizontal force to a crane with almost no capacity left for it. The gear itself is load-bearing equipment with a marked capacity, an inspection regime and a record. A hook without a working safety catch, a sling with a cut jacket exposing the core, or a shackle with a distorted pin is a change to the lifting system that nobody calculated and nobody signed for.
Illustration
Conceptual illustration of a pick showing the working radius measured to the load, the gross load at the hook, and the outrigger reaction spreading into the ground at 45 degrees.
Typical causes
- Gross load underestimated, with the rigging, spreader beam and tackle left out of the total.
- Capacity read at the wrong radius, or the radius increasing beyond the chart as the boom deflects and the load is slewed out.
- Outrigger reaction exceeding what the ground could carry, on fill, voids, buried services or a suspended slab.
- Rigging arranged at an included angle so steep that sling tension far exceeds the load being lifted.
- Two-crane lift carried out with no load-sharing check and no single person directing both machines.
- Shock or snatch loading as the load breaks from the ground, or a dynamic effect not allowed for in the plan.
- Lifting gear unserviceable, unmarked or out of inspection, or a rigging arrangement improvised at the pick.
Warning signs
- Outrigger pads settling, mats sinking, or fresh cracking and heave in the ground beside a pad.
- The crane visibly out of level, or an outrigger beam carrying no load against its pad.
- One sling leg slack while its partner is tight, in a multi-leg or two-crane arrangement.
- A load rotating or swinging on the hook despite the tag lines, or a tag line being pulled rather than tended.
- The load drifting away from the standing radius while it is walked or boomed out, with no corresponding check of the chart.
- Cut or crushed slings, distorted shackles, a bent lifting beam lug, or a hook with no safety catch.
Inspection methods
- 01Read the lift plan for the specific crane, radius and rigging arrangement in use, rather than against a generic chart carried on the machine.
- 02Measure the working radius at the position where the load will actually hang, and confirm the capacity at that radius.
- 03Establish the gross load by calculation from the drawing and the actual section, with the rigging added rather than forgotten.
- 04Assess the ground under each outrigger — bearing, buried services, voids, and properly sized mats — before the machine is levelled.
- 05Inspect lifting accessories before each use: marked capacity, inspection tag and date, no cuts or distortion, hooks with functioning catches.
- 06Watch the operation itself against the plan: exclusion zones held, tag lines tended, one banksman in charge, movement inside the assessed zone only.
Preventive measures
- Prepare a lift plan for every pick from the working radius, the gross load and the ground bearing, and confirm it before the crane is mobilised.
- Prove the ground under each outrigger rather than assuming it, and size the mats from the calculated reaction.
- Calculate sling tension at the actual included angle, and use a spreader or lifting beam instead of opening the legs.
- Treat a two-crane lift as a design exercise: a load-sharing check, one supervisor, a written sequence and defined stop conditions.
- Allow for dynamic effects in the plan, and take the slack out of the rigging gently instead of snatching the load off the ground.
- Inspect lifting gear on a defined regime with records, and remove unserviceable items from service rather than from the lift.
- Set and enforce the exclusion zone for every pick, and brief the tag line crew on what the load is expected to do.
Relevant standards
The codes and specifications a design is normally judged against. Naming them is not a claim that AKEN certifies, inspects or approves against them — the design authority for a project does that.
- IS 807
- IS 13834
- BS 7121
- IS 800
Related AKEN services
Scope note
This article explains a failure mechanism for engineering education. It is general guidance, not a design check, an inspection report or a certification for any particular structure. Assessments of an actual structure must be carried out by the responsible design authority against the project’s own specifications.