When a load comes down, three broadly different things can have gone wrong. The machine failed, at a boom section, a pin, a weld, a brake or a rope. The machine was operated outside the envelope its chart defines. Or the rigging between hook and load was inadequate for the tension it actually saw. Each produces different physical evidence, is governed by a different body of standards, and resolves to a different set of records. Treating them as interchangeable explanations for one outcome is the quickest route to an opinion that does not survive examination.
Three families of failure, one appearance
The visible result of all three is similar: a dropped load, a collapsed boom, or a machine on its side. Reasoning backward from the outcome is unreliable, because the same end state is reachable from any of them. The analysis instead asks what each family should have left behind, then tests the evidence against those expectations.
The families also interact. A machine worked near its structural limit for years is more likely to fail from a defect a lightly used one would tolerate.
What a mechanical failure leaves on the fracture surface
Metallurgical examination separates a component that broke because it was overloaded from one that broke because it was already cracked. An overload separation shows gross plastic deformation with a dimpled ductile surface, or in some materials a fast brittle fracture with chevron markings pointing back toward an origin.
Fatigue looks nothing like that. A smooth, beach-marked region grows from an initiation site, typically a weld toe, a bolt hole, a section change or a corrosion pit, and ends in a comparatively small final overload zone. Electron microscopy resolves striations and locates the origin.
Conditions that predate the lift
Weld discontinuities such as lack of fusion, undercut or porosity, an undocumented field repair, a substituted or under-strength pin, and corrosion inside a lattice chord or telescopic section are all conditions the lift did not create. So is a slew-ring bolt group that lost preload over years.
Establishing that such a condition existed beforehand turns on the fracture evidence, the fabrication and repair history, and whether the condition was of a type and location the applicable inspection regime should reasonably have found.
Inspection regime and service history
The ASME B30 volumes and the OSHA crane standards both structure inspection into frequent and periodic intervals, with additional requirements after modification, repair, or a period out of service. The records those requirements generate are the baseline against which any claim of a hidden defect is measured.
Duty matters as well. Classification schemes such as ISO 4301 group cranes by load spectrum and number of operating cycles, and equipment used well above the class it was specified for accumulates fatigue damage on a schedule nobody planned.
Operating outside the envelope
Envelope failures divide into stability and structure. A stability event rotates the machine about a tipping axis, usually with the structure intact until impact, and sends the analysis to the setup: outrigger extension, level, counterweight and ground. A structural overload buckles or fractures the boom while the machine remains supported.
Both are reconstructed from configuration rather than wreckage alone — boom length, angle and radius as found, counterweight fitted, the quadrant the load was in, and net capacity after deductions.
Two-blocking and the operational aids
Two-blocking, in which the load block is driven up into the boom-tip sheave assembly with the hoist still powered, is neither a structural failure nor an overload in the ordinary sense. The rope parts or a component fails at a load the chart never contemplated, and the damage pattern at the boom tip is distinctive.
Anti-two-block devices and rated capacity limiters are addressed in the ASME B30 volumes and in the safety-device and operational-aid provisions of OSHA's construction crane standard. Whether one was fitted, functional and not bypassed is routine, and a jumpered switch is usually apparent.
Rigging: the tension is not the load
Tension in a sling leg is not the load divided by the number of legs. As the angle between leg and horizontal decreases, tension rises sharply; at thirty degrees each leg carries twice the vertical share it supports. Unequal leg lengths and a centre of gravity that is not where it was assumed concentrate it further.
ASME B30.9 covers slings and ASME B30.26 covers rigging hardware, with rated loads that assume specified angles and loading directions. Side-loaded shackles, synthetic slings choked over an unprotected sharp edge, and hardware loaded across a pin fall outside the basis of the published rating.
Hardware condition and substitution
Synthetic slings degrade from abrasion, cuts, ultraviolet exposure and chemical contact, often on the bearing surface where nobody looks. Chain and wire rope slings show wear, stretch and broken wires; hooks show throat opening and latch damage. The B30 volumes define removal criteria for each.
Substitution is its own category: unrated hardware, a field-fabricated lifting lug, a shackle carrying a pin from a different shackle, or a below-the-hook device used outside its design purpose.
Sequence, and where the attribution is challenged
Nearly every collapse produces secondary damage resembling primary failure, so the central discipline is establishing order. A boom that struck the ground shows fractures a boom that buckled under load does not, and a sling cut during recovery is not a sling that parted under tension.
The challenges are predictable: that configuration was assumed rather than measured, that a fracture was interpreted without ruling out impact, that indicator data was accepted without calibration evidence, or that a sling was condemned on appearance without applying the removal criteria.
This article is general technical orientation, not a failure analysis, an engineering opinion, or advice on any specific matter. Determining the cause of a particular incident requires hands-on examination by a credentialed expert.