A rated capacity chart is usually read as a number: the crane can lift so many tons. It is closer to a contract with conditions attached. Every figure is valid only for a stated boom length, radius, counterweight and outrigger position, and only while the machine sits level on ground firm enough to hold the reactions it generates. Failures that look like overload are frequently configuration failures or ground failures instead, and the three lead in very different directions.
A chart is a set of conditions, not a number
Load charts are published as a matrix, and entries change with boom length and radius together. A capacity valid at short radius on fully extended outriggers may be a fraction of that at longer radius, on partially retracted outriggers, or over the side rather than the rear. The chart also states its assumptions: a level setup, a load applied vertically and gradually, wind below a stated limit, a specific counterweight arrangement.
Those are not fine print. They are the boundary of the engineering the chart represents.
Configuration is what varies, not the load
The weight of the load is often the best-documented fact in the file, appearing on a shipping ticket or a fabrication drawing. The configuration is far less certain. Boom length and angle, outrigger extension, counterweight fitted, tire pressure on a rubber-mounted pick, and the quadrant the load swung into all change within a shift, several without anyone recalculating anything.
Reconstruction therefore establishes configuration as rigged rather than as planned, from as-found geometry, photographs, telematics and any indicator record.
Deductions and the capacity actually available
Gross chart capacity is not what is available to hoist. The hook block, headache ball, slings and spreader beams, an erected jib or auxiliary boom head, and any attachment carried stowed all deduct from it, as does the hoist rope suspended below the boom tip on long-boom configurations.
This is where a lift that appeared to be inside capacity turns out not to have been. A rigging assembly heavy enough to matter is easy to omit from a mental calculation and difficult to omit from a reconstruction.
Structural limit or stability limit
Chart entries are governed by two different physical limits, and manufacturers normally mark which applies. At short radius the limit is structural: boom sections, pins, turntable and carrier carry only so much regardless of stability. At longer radius the limit is stability, set as a defined fraction of the load that would begin to tip the machine, the fraction differing between crawler and wheel-mounted equipment.
The two produce different wreckage. A structural exceedance buckles or fractures the boom with the machine still on its outriggers; a stability exceedance rotates it about a tipping axis.
Outrigger reactions are not distributed weight
An outrigger-supported crane concentrates the weight of machine, counterweight and load into a few pads, and the distribution shifts as the boom swings. The pad on the loaded side can carry a large multiple of what dividing gross vehicle weight by four would suggest, over a contact area of a few square feet.
That reaction is calculable, and manufacturers publish either the values or the method. It is the quantity to compare against what the ground could support, and machine weight is no substitute for it.
What the ground was assumed to bear
OSHA's construction crane standard makes ground conditions an express obligation. 29 CFR 1926.1402 requires equipment to be assembled and used on ground that is firm, drained and graded sufficiently, with supporting materials where needed, and places a duty on the controlling entity to disclose known hazards beneath the setup area.
Behind it sits a geotechnical question, allowable bearing pressure, rarely asked on site. Uncompacted backfill, a recent utility trench, a buried vault, saturated fine-grained soil after rain, and proximity to an open excavation all reduce what the ground will hold. None are visible from the cab.
Mats, cribbing and the load path below the pad
A mat spreads a pad reaction over a larger area only if it is stiff enough and large enough to do so. An undersized or over-flexed mat concentrates load near the pad instead of distributing it, and cribbing assembled from mismatched timber can crush, roll or rack under an eccentric reaction.
The as-found condition of mats and the impressions left in soil are among the first evidence destroyed once recovery equipment tracks across the setup area.
Loads the chart never contemplated
Charts assume the load is picked vertically and moved smoothly. Dragging a load sideways to break it free, arresting a swing, a sudden stop in hoist or swing, and a load that snags and then releases all impose side loading or dynamic amplification the chart does not account for.
Wind belongs to the same category. A large flat load presents surface area, and wind acting on it at the operating radius adds to the overturning moment while pushing the load out of plumb. Manufacturers state maximum permissible wind speeds, and recorded wind is a routine input.
Which assumption failed, and whose it was
Separating these questions matters because they resolve to different records. A chart violation reaches the operator and whoever approved the lift. A configuration error can reach the assembly crew. A ground failure reaches whoever assessed the site and specified the mats, together with the controlling entity's disclosure duty under Subpart CC.
ASME P30.1 provides the planning framework these obligations sit within, and a lift plan recording boom length, radius, net capacity, calculated ground bearing pressure and mat sizing is both better practice and better evidence.
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.