Two Documents That Have to Agree
A mobile crane arrives carrying a book of numbers the manufacturer established on a level, prepared test pad, then parks on a site that is none of those things. Planning a lift means forcing those two realities into agreement before the hook takes weight. Half the problem lives in the load chart — what the machine can hold, in one stated configuration, at one stated radius. The other half lives under the outrigger floats — what the ground will carry without settling, punching through or sliding. Real capacity is the smaller of the two, and confidence in one half buys nothing from the other.
Failures rarely involve both halves at once. Far more common is a faultless chart calculation set down on ground nobody proved: a float over a backfilled service trench, a mat bridging soft fill behind a new retaining wall, a pad that was firm on Tuesday and saturated by Thursday. The mirror image also happens — an immaculate engineered platform under a crane rigged in a configuration the chart page never covered, or slewing into a quadrant with less capacity than the one the lift was checked in.
Sequence follows from the split. Chart work comes first because it fixes the configuration, and configuration fixes the outrigger reactions the ground must then carry. Ground work either confirms those reactions are supportable or sends you back to the chart with a new constraint. Two or three passes around that loop is normal on a constrained site; one pass usually means somebody assumed something.
The Chart Is a Set of Conditions, Not a Number
Every capacity figure belongs to a page, and the page has an identity: machine model, boom length, boom mode or jib configuration, counterweight fitted, outrigger extension, and the quadrant of operation. Pull a number from the fully extended outrigger table while the crew sets up on intermediate extension because a kerb was in the way, and the lift is already wrong before anyone touches a lever. Counterweight is the same trap — the chart assumes a stack that has to physically be on the machine, in the arrangement drawn.
Notes pages carry the conditions the tabulated numbers depend on, and they are part of the chart rather than commentary on it. Typical conditions: the load hangs freely, the machine stands level within a stated tolerance, the supporting surface is firm and uniform, wind is below a stated limit, and the tabulated figures are gross with deductions still to come. Manufacturers word these differently, and the wording that governs is the one printed in the manual for that serial number, not the one remembered from the last machine.
Two different failure modes sit inside one table. Short radius with heavy loads is usually structurally limited — boom sections, slew ring, cylinders — and the warning before failure can be very short. Long radius is usually stability-limited, capacity set as a defined percentage of the tipping load, with the percentage depending on machine type and on the standard in force. Where the shaded or bracketed regions change from one governing mode to the other tells you what kind of margin remains, and how the machine will misbehave when it runs out.
Everything the Hook Carries Before It Carries the Load
Chart figures are gross. What the crane actually delivers to the load is what remains after everything else hanging below the boom head is subtracted: hook block or ball, the reeved rope below the head, an erected or even stowed jib, auxiliary sheaves, and then the entire rigging assembly — slings, shackles, spreader or lifting beam, chain blocks, tag lines. Anything mounted for this job and not present when the chart was written is a deduction, and the omitted spreader beam is the classic one.
Load weight deserves the same scepticism. A nameplate, a fabrication drawing and a supplier's estimate are three different qualities of evidence, and the last of those has been wrong by a wide margin often enough to justify weighing. Ask what the figure includes: retained water, ice, mud, entrained concrete, contents left in a vessel, attached platforms, insulation, packaging. Where uncertainty cannot be removed it has to be carried as declared margin rather than optimism, and that margin belongs somewhere the operator will see it.
Utilisation — net load over net capacity — becomes the number the whole site argues about, and reasonably so. Organisations set planning thresholds above which a lift moves into a higher category: more review, an engineered plan, a named supervisor, sometimes a physical trial. Track it early, because a figure that creeps upward as rigging is finalised is the normal way a routine pick turns into a complex one.
The chart page, the deductions and the load weight only mean something once they resolve into a single utilisation figure, which decides here and now whether this pick stays routine or changes category.
Read from YOUR crane's chart, at this radius and configuration.
Mass of the item being lifted.
Slings, shackles, spreader beam, lifting frame.
The block reeved on the machine.
Allowance for wind and dynamic effects.
Chart utilisation
70.8 %
Within capacity for the chart figure entered. This is arithmetic, not a lift plan.
- Gross lifted load
- 12,485 lb
- Rated capacity
- 17,635 lb
- Margin remaining
- 5,150 lb
- Load alone as a share of capacity
- 56.25 %
- Rigging, block and allowance
- 2,565 lb
- Gross load in tonnes
- 5.66 t
They open the calculator with your figures already in it
Crane Lift Utilisation Calculator: 70.8 % — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
What this calculation does not cover
- This is arithmetic on figures you supply, not a lift plan and not an engineering approval. A lift plan covers ground bearing, proximity hazards, exclusion zones, wind limits, the rigging arrangement and the sequence — none of which is here.
- Rated capacity must come from the chart for the actual machine in the actual configuration. Radius, boom length, counterweight and outrigger extension each change it, sometimes dramatically.
- Wind is not modelled. It acts on the load's surface area rather than its mass, and a large light panel can be a harder lift than a small heavy one. The manufacturer's wind limit is a stop, not a factor.
- Ground bearing pressure under outriggers or tracks is a separate and frequently governing check.
Radius Refuses to Stay Still
Radius is measured from the centre of rotation to the centre of gravity of the load, and it changes twice in every pick — once at the lift point, once at the set point. Plan against the worse of the two, plus the path between them. Tape measures and site plans disagree with reality often enough that the radius should be confirmed on the machine, with the boom in the working configuration, rather than scaled off a drawing produced before the piling rig moved.
Under load the boom deflects, and deflection pushes the load outwards, so the operating radius on a loaded boom is longer than the unloaded one. Manufacturers state whether their tabulated radii already account for this, and that statement decides whether extra allowance is owed. Slewing adds the second variable — capacity commonly differs over the side and over the rear, and a lift checked in one quadrant is not automatically valid in the one it swings through.
Wind acts on the load's sail area rather than its weight, which is why light, large panels are the ones that hurt people. Manufacturers publish a maximum permissible wind speed for the configuration; gusts rather than the average are what break a lift, and the reading that counts comes from boom-tip height, not from ground level beside the site cabin. Add dynamics: snatching a load, freeing one that is stuck or frozen down, hoisting too quickly, or swinging hard all put forces through the machine that the static table never contemplated.
Underneath the Percentage: What the Outriggers Actually Push
Outrigger reactions are not the gross load divided by four. During slew the majority of weight transfers onto two floats, and the peak reaction at a single corner can approach — or exceed — the combined weight of the crane and its load. Reactions should come from the manufacturer's software or a documented calculation method for that machine, and they should be produced for the worst slew position, not the parked one.
On the other side of the interface, what the ground can accept is governed by the geotechnical standard in force, the site investigation and the temporary works regime the project runs under — not by a generic table of soil types remembered from somewhere. Made ground, uncontrolled fill, reinstated trenches and the zone behind retaining structures all behave differently from natural soil a few metres away. Buried services, culverts, basements, tanks, drainage runs and old foundations are each a reason a competent-looking surface fails under a float.
Evidence-gathering happens before the crane is ordered, not on the morning of the lift. Ask for the ground investigation report, the service drawings and the as-built records; where those do not resolve the question, trial pits, hand probes, dynamic cone penetrometer testing or plate loading fill the gap. Record what was found, because the crane pad counts as temporary works in most regimes and someone has to check and accept it against a design.
Where the report hands over soil parameters instead of an allowable figure, somebody still has to convert them into a capacity the reactions can be judged against — do it while there is time to change the machine.
The soil's cohesion.
The soil's angle of internal friction.
The overburden pressure at the footing's base level.
The soil's unit weight below the footing.
The footing's width.
Ultimate bearing capacity
22,300 psf
This gives ULTIMATE bearing capacity — a real design divides this by a factor of safety (commonly 2.5-3.0) to get the allowable bearing pressure, and applies shape/depth/inclination correction factors for non-strip footings, which this simplified strip-footing calculator omits.
- Nc
- 30.14
- Nq
- 18.4
- Nγ
- 22.4
They open the calculator with your figures already in it
Shallow Foundation Bearing Capacity Calculator (Vesic Factors): 22,324 psf — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 22,300 psf — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- Ultimate capacity at shear failure, with no settlement check. On sand and on stiff clay the working pressure is usually set by total and differential settlement rather than by bearing failure, so a footing can clear this figure and still move more than the structure above it will tolerate.
- Groundwater is not in the model. There is no water table input, and when water stands at or above founding level both the surcharge term and the 0.5×γ×B×Nγ term have to be built from effective stresses — the unit weight field will not even accept a submerged unit weight, its floor being 14 kN/m³ (89 pcf). Reduce q and γ yourself before entering them, or the capacity reads high.
- Strip footings under a vertical, centrally applied load only. No shape, depth, load-inclination or ground/base-slope factors are applied, so a square or rectangular pad, a circular base, a raft or an eccentrically loaded footing falls outside it; eccentricity also calls for the effective-width reduction B − 2e, which this does not do.
- General shear failure is assumed at every friction angle. Loose sands and soft compressible clays fail by local or punching shear, where these factors overstate capacity, and no reduction to c or tanφ is made for it.
- One uniform soil, taken to extend through the whole failure wedge — roughly one to two footing widths below the base. Layering is invisible to it: a soft stratum under a firm crust, or fill over natural ground, returns the crust's capacity. This is a screening figure against parameters someone else measured, not a foundation design, and it does not replace a site investigation or an engineer's allowable bearing pressure.
Building Ground That Matches the Chart
Mats and pads exist to convert a concentrated reaction into a pressure the ground will accept, and they do that only as well as their stiffness allows. A flexible timber mat under a heavily loaded float does not distribute across its whole plan area; pressure concentrates beneath the float and tails off, so the effective bearing area is smaller than the area you can see. Steel and engineered composite mats are stiffer and spread further, at the cost of weight, handling plant and lay-down space.
Below the mat, a proper working platform is its own design: granular material of specified type and thickness, compacted, sometimes with geosynthetic reinforcement, over a prepared and drained subgrade. Guidance for platform design is published by industry bodies and referenced in most temporary works regimes, and the resulting thickness is a design output rather than a habit carried from the last job. Rain changes the answer — a platform signed off in dry weather can lose capacity after a night of it, so re-checking after weather is a scheduled activity, not a favour.
Detail at the float decides whether any of it works. Full and even contact between float, packing and mat; no rocking; no bearing on a kerb edge or a protruding lump of rock; cribbing built square and interlocked where height is needed; and a stated minimum distance from the crest of any excavation or slope, measured rather than eyeballed. Set-up level is checked with the machine on outriggers before rigging and again afterwards, because floats settle into ground that looked firm.
What carries an outrigger reaction
- Outrigger float — the machine's own steel foot, handing down a reaction that can approach the combined weight of crane and load through a plan area barely larger than a dustbin lid
- Packing and cribbing — takes up the gap and holds the float level; built square and interlocked, or the whole assembly rocks and loads one corner of the mat alone
- Mat — turns a concentrated reaction into a pressure, but only over the area its own stiffness can reach — flexible timber spreads across less than the plan area you can see Crane Outrigger/Mat Ground-Bearing Pressure Calculator
- Working platform — specified granular material laid and compacted in lifts, to a thickness that comes out of a design rather than out of memory, and that one wet night can take straight back Standard/Modified Proctor Compaction Percentage Calculator
- Prepared subgrade — the only part of the stack nobody manufactured; reinstated trenches, uncontrolled fill and forgotten basements all present the same competent-looking surface from above Shallow Foundation Bearing Capacity Calculator (Vesic Factors)
With reactions known and mats chosen, one question remains at the interface between the two halves of the plan — whether the pressure arriving at the subgrade is smaller than what that subgrade will take — and it gets settled here before anything is craned.
The maximum reaction load this outrigger or crawler track transmits to the ground, from your crane's certified load chart.
The width of the cribbing mat or outrigger pad footprint in contact with the ground.
The length of the cribbing mat or outrigger pad footprint in contact with the ground.
The site soil's allowable bearing capacity, from geotechnical data or a competent person's assessment.
Applied ground bearing pressure
2,070 psf
The pressure under this mat is at or below the allowable bearing capacity shown with it, which you entered. This checks GROUND bearing pressure only, from a reaction load you supply off your crane's own manufacturer-certified load chart and rigging plan — it calculates no crane capacity, stability, ballast or outrigger reaction, all of which come from the manufacturer's certified data and a qualified rigger or operator per OSHA 29 CFR 1926 Subpart CC, and the allowable figure comes from your own geotechnical data or a competent person's site assessment. Sitting under the figure you entered is not clearance to lift. The chart, the ground, the rigging and the plan are all outside this arithmetic.
- Outrigger reaction force
- 33,081.3 lbf
- Mat contact area
- 16 sq ft
- Allowable bearing capacity (user-supplied)
- 3,132.82 psf
They open the calculator with your figures already in it
Crane Outrigger/Mat Ground-Bearing Pressure Calculator: 2,068 psf — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 2,070 psf — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- The load is spread evenly over the whole mat footprint you enter. Nothing checks whether the mat is stiff enough to do that — a small outrigger float on a flexible timber crib concentrates pressure under the float over a fraction of the mat — and the mat's own bending and shear capacity is never checked, so the mat can fail before the ground does.
- The comparison is against one surface bearing number. It does not look at what is under the mat: a stiff crust over soft ground, punching through into a weaker layer, settlement, or capacity lost to rain, saturation, thaw or a nearby trench, excavation, slope or basement wall.
- Buried services, culverts, vaults, backfilled trenches, and setting up on a suspended slab, podium deck or basement roof are not modelled. Where any of those sit under the mat the governing limit is the structure's capacity, not the soil's, and this calculation will not see it.
- Only a static, centred, vertical reaction is modelled. Slewing, boom swing, out-of-level setup, wind and side loading shift the reaction onto one corner and load the mat eccentrically, driving peak edge pressure well above the average this reports.
- This is a screening comparison with no factor of safety applied, not a crane setup design. It does not calculate crane capacity, stability, ballast or the outrigger reaction itself, and it does not replace a competent person's assessment of the actual pad.
When the Halves Will Not Meet
Disagreement between chart and ground is a normal outcome, not a crisis. The levers available: shorten the radius by repositioning the crane or the delivery, lengthen the boom to clear an obstruction without adding radius, add counterweight or a superlift where the chart supports it, upgrade to a larger machine, enlarge the mats, improve or rebuild the platform, split the load, or plan a tandem lift with two machines and a documented load-sharing arrangement.
Every lever pulled on one side moves the other. A larger crane resolves a utilisation problem and immediately worsens the ground problem, because it arrives heavier and pushes harder through bigger floats. Bigger mats reduce pressure and add crane time and handling. Repositioning to shorten the radius may put a float directly over the drainage run you were avoiding. Iterating is the work, and the plan is finished only when both halves close at the same time.
Authority to stop belongs to more than one person, and it has to be usable in practice. Late load-weight discoveries, ground that fails a check after rain, a wind reading over the limit, a mat that will not sit flat — each is a reason to hold, and holding is cheap set against a machine on its side, a closed road, an investigation and a fleet-wide stand-down. Write the hold points into the plan so that stopping is a listed step rather than an act of individual courage.
Choosing between these levers means comparing a mat pressure against a geotechnical allowable, and the two halves of the plan almost never arrive written in the same units.
The pressure in kilopascals, as written in the geotechnical report or metric load schedule.
Bearing pressure
3,133 psf
Converted using an exact defined factor of 20.88543423315013 psf per kPa. The digits convert; the report's allowable-versus-ultimate wording and its settlement limit do not.
- Conversion factor applied
- 20.89 psf per kPa
They open the calculator with your figures already in it
kPa to psf Bearing Pressure Calculator: 3,133 psf — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 3,133 psf — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
A metric geotechnical report lands on a desk where the footing schedule is worked in pounds per square foot, and 150 kPa has to become something a US spreadsheet can size a pad from — about 3,133 psf. Two things travel badly. First, the report will write kN/m² as readily as kPa; they are the same unit exactly, so that part needs no arithmetic at all. Second, the figure is qualified — allowable or ultimate, net or gross, and usually tied to a settlement limit at an assumed footing width. The conversion carries the digits and none of the qualification, so read the sentence around the number before it enters a design. And keep psf clear of psi: they differ by a factor of 144.
The Plan on Paper and the Brief on the Ground
Roles are defined by whichever standard governs the site, and titles differ — appointed person, lift director, lifting supervisor, competent person — but the structure is consistent: someone plans, someone supervises execution, someone operates, someone slings and signals. Most regimes also grade lifts by complexity, and the grade sets how much of the plan must be written, checked and independently accepted before anyone tracks a machine onto the pad.
A lift plan that stays in the site office has done nothing. What reaches the ground is the brief: the sequence, the exclusion zone and who keeps it clear, the single nominated signaller and the comms channel, the weather limits and who reads the anemometer, the tag line arrangement, and the parking or recovery position if the lift is aborted mid-pick. People who will never open the plan should leave that briefing able to say what they personally do and when they stop.
On the day the order is set-up, level check, mat and packing inspection, function tests, then a trial lift — take the load just clear, hold, and watch the machine, the floats and the ground before committing. Re-check level once the load is on; re-check the platform after rain, after another plant item has tracked across it, and at the start of every shift the crane stands there. Change control applies to all of it: a different load, radius, mat layout or quadrant means the plan is revisited, not adjusted verbally at the hook.
Before the crane is booked
Six things that have to exist on paper before a machine is ordered — three belonging to the chart, three to the ground.
- The exact chart page — Model, boom length and mode, counterweight fitted, outrigger extension, quadrant. Print it and attach it to the plan.
- A rigging deduction list — Block, ball, rope, jib, slings, shackles, spreader, tag lines. Weighed where possible, not carried forward from the last job.
- Load weight with a stated source — Nameplate, drawing, weigh ticket or estimate — record which, and carry declared margin where it is an estimate.
- Outrigger reactions for the worst slew position — From the manufacturer's software or a documented method for that machine, never from gross load divided by four.
- Ground evidence — Investigation report, service and as-built drawings, plus trial pits or probes where those leave a gap. Note anything buried under a proposed float.
- Mat and platform schedule — Mat type and size against effective bearing area, platform specification and thickness, and a named person to check and accept it.
Opens the calculators above on one screen with the dimensions from this article already filled in. Quantities only — this site publishes no price list, because local prices vary too much to publish honestly.