Ground bearing pressure, done properly - the guide

Ground bearing pressure is the contact pressure one support puts on the ground:

$R$ is the reaction on that outrigger or track - not the crane's weight, and not an average - and $A$ is the area that actually bears - not automatically the mat you bought. Both halves are where real calculations go wrong: the reaction depends on the slew angle, the counterweight and the footprint, and the bearing area depends on what the mat can genuinely mobilise through its thickness. Get those two right and the check itself is one division against the allowable from the geotechnical report.

A slewing crane concentrates load toward the boom side. For the classic symmetric four-outrigger case with equal stiffness, rigid-body statics gives each support

and the moment terms routinely move a corner by half the average or more. In the worked example below the chart itself says so: 210 kN on the loaded pair against a quarter-share of 150 kN - 40% higher, before any partial extension or asymmetry makes it worse. Size the pads for the average and the loaded corner is undersized by exactly that margin.

An outrigger load calculator that asks only for the crane's weight is computing the dashed line. The real question is the bar - per support, at the worst slew angle, with the counterweight where it actually is.

As the boom slews, each outrigger's reaction rises and falls; the worst case for the front-left pad is a different angle from the worst case for the rear-right, and neither is necessarily over-side or over-corner. On the default machine model, the engine's full-circle sweep finds the peak reaction with the boom at 230° - 669 kN, which is 29% above the worst support at the declared over-front orientation. A calculation checked at one guessed angle simply never sees that number.

The tool sweeps the circle at a 1° grid with refinement around every candidate extremum and reports the governing angle per check - drawn on a polar rose so the shape of the problem is visible. If you have chart reactions at several slew angles, enter them all: the worst set governs, found the same way. The methodology page records how the sweep works and why the reported angle is always an evaluated sample.

• Path A - declared. Enter the reactions from the manufacturer's chart or planning software, one set per slew angle. The tool validates each set against the declared gross load - a set that does not balance is reported with its discrepancy and excluded, never quietly rescaled - and everything downstream of a valid set is exact.
• Path B - estimated. No chart yet? The tool solves a rigid body on compression-only supports from the machine's masses and geometry. Every derived number is labelled an ESTIMATE - in the panel, on the figures, in the report - and one click switches to declared reactions when the chart arrives. A support that would go into tension is released and re-solved; a machine that cannot stand on three supports gets a refusal, not a number.

The outrigger float is small - a fraction of a square metre - so the pressure on top of the mat is high: $q_{float}=R/A_f$, which is 840 kPa in the worked example. That pressure governs the mat's own structure: contact crushing, cantilever bending of the overhang, and shear at the float edge. The ground never sees it directly - what the ground sees is the load spread over the effective bearing area the mat can mobilise:

This is why crane mat size calculator and outrigger pad size calculator questions have two answers. Plan area helps the ground check - but only as far as the spread cone reaches; area outside the cone is cargo. Thickness is the real lever: it deepens the cone, spreads the load wider and stiffens the mat's own bending. And nothing about the mat changes the float pressure on its top surface - if the mat material cannot take $R/A_f$, the fix is a different mat material or a bigger float, not a bigger mat. The tool checks both levels and names which one governs; its inverse solver proposes the thickness or plan size that passes, and refuses with the reason when no pad dimension can fix the failing level.

Every mat vendor's marketing assumes a generous spread angle; the honest position is that the angle is a property of the mat's stiffness and construction that you declare and justify. This tool defaults to ψ = 0° - no credit - and its raw default example fails the ground check loudly at utilisation 5.6 because of it. Declaring a spread angle is a real engineering trade: the ground pressure drops, and the claimed cantilever loads the mat's own bending and shear at the same time. The declaration is stamped into the report's assumptions with the rest of your inputs.

This is the question every failing ground check ends in, and the instinct - buy bigger mats - is usually the wrong answer. The table below is the engine re-running the worked example with one input changed at a time, at ψ = 0 and at the declared ψ = 35°. Read down the ψ = 0 column first.

Three things fall out of that table, and none of them is obvious:

• With no spread declared, mat geometry does nothing. At ψ = 0 the thicker mat and the bigger mat both leave the ground utilisation at exactly 3.15 - unchanged, to the last digit. The ground is seeing the float's footprint and nothing else, so buying more mat buys nothing. The declaration is the gate; until it is made, no purchase order fixes this check.
• Once spread is declared, thickness is the productive lever. At ψ = 35° the extra 150 mm takes utilisation from 0.80 to 0.55. Thickness widens the dispersion cone and stiffens the mat against the float pressure - the only lever that helps both levels at once.
• A bigger mat still does nothing. Even at ψ = 35°, going to 2.4 m square leaves utilisation at 0.80 - identical to the 1.8 m mat. The cone through 350 mm never reaches the existing mat edge, so the extra plan area is outside the load path. Plan area only starts paying once the cone would spill past the edge without it.

How to calculate ground bearing pressure, end to end: take the governing reaction $R$ at the worst slew angle; divide by the effective bearing area $A_{eff}$ the mat genuinely mobilises; compare against the allowable bearing pressure from the geotechnical report divided by your factor of safety. Three numbers, each with a named provenance - the chart, your declared spread, the ground investigation.

The factor of safety deserves the same honesty as the spread angle. Industry guidance describes a factor of 1.5 to 3.0 on outrigger loading, scaled to how well the hardstand's bearing capacity has been evidenced - proof-rolled and tested at one end, assumed from a desk study at the other. The tool surfaces that band as guidance text beside the input and warns when your declared factor sits below it, but never applies it for you.

With no geotechnical report and only soil parameters to hand, the opt-in classical route computes a named textbook ultimate resistance - Vesić or Meyerhof drained, Skempton undrained - and divides it by your visible factor. On the worked example's dry sand at φ′ = 30° it composes 419 kPa ultimate, ÷ 1.50 = 279 kPa capacity against the 214 kPa demand. The result is labelled ULTIMATE ÷ FoS on every surface, and a non-dismissible note says a site-specific geotechnical investigation supersedes it - because it does. The methodology page names every factor in the composition.

The tool offers three ways to put a capacity against your demand, and they are not interchangeable. Pick by what is actually in your hands:

Run the same ground through all three and they disagree - which is the point. On the worked soil (φ′ = 30°, γ = 18 kN/m³, founded 0.5 m down, with a 400 kPa allowable on the report and FoS 1.50):

One trap worth naming, because the standard is explicit and the interface cannot enforce it: Design Approach 1 is not satisfied by either combination on its own. EN 1997-1 requires the limit state to be verified for Combination 1 and Combination 2, and the tool evaluates the one you select. Run both and take the worse. The check card says so on both DA1 selections.

For timber and composite mats the tool offers two routes. The declared route takes allowable contact, bending and shear stresses straight from the mat supplier - fast, and only as good as the datasheet. The coded route runs EN 1995-1-1 properly, and the price of admission is that you declare the things the code needs:

• Characteristic strengths $X_k$ - from EN 338, EN 14080 or the supplier. Those grade tables are not held in this product's library, so the tool ships no grade list: you declare the numbers, and the report records that you did.
• Service class - a mat on a construction site is service class 3. That is not a formality: it cuts $k_{mod}$ substantially against the indoor classes.
• Load duration - a lift lasting hours is short-term; a mat left under a standing crane for a month is not. $k_{mod}$ moves with it, and the honest answer is the shortest-duration action in the combination.

Run the worked mat through it - a glulam in service class 3, short-term, with declared characteristic strengths of 24.0 / 3.5 / 2.5 MPa in bending, shear and compression ⊥ grain:

The ranking is the useful part. Compression perpendicular to the grain governs at 0.80, while bending - the mode most people check first - sits at 0.03, an order of magnitude away. A crane float is a small, hard patch pressing into a soft face of timber; it crushes long before it breaks the mat in bending. If you take one thing from this section, check the bearing face.

A crawler spreads its load along two tracks, and the pressure is anything but uniform. The resultant's lateral position splits the load between the tracks; each track then carries its share with a longitudinal eccentricity $e$. Inside the middle third the distribution is trapezoidal; beyond it, part of the track lifts and the pressure goes triangular over a reduced contact length:

On the crawler example the sweep finds the peak track pressure of 164 kPa with the boom at 159° - in full contact there, but the same machine goes into partial contact at other radii, and the tool flags the reduced contact length whenever it happens, draws the pressure diagram on the plan, and carries the peak into the ground check. Crawler reactions always come from the machine model - per-track chart reactions are not an export format - so every crawler number is labelled an estimate.

The classic site rule - stay back from an excavation by at least its depth - is the 45° load-dispersion construction, and the tool applies it as a geometric flag: declare the face, its depth and the clear distance, and every bearing area is tested against the zone. The same construction covers a slope crest and a buried duct, culvert or basement. A fired flag means one thing, stated plainly: a temporary works design is required. The flag never analyses the excavation's support, the slope's stability or the buried structure's capacity - those are engineering designs, not geometry.

One honest edge worth knowing: a slope face at or under 45° can never fire the geometric flag for a bearing area clear of the crest, because a 45° dispersion line cannot daylight on a gentler face - though a pad at or past the crest line itself still fails, whatever the angle. That is a statement about the construction - not a statement that the slope is safe. Global slope stability is a different discipline and deliberately out of scope.

The setup: a 60 t class all-terrain crane, fully rigged, gross load 600 kN. The chart declares reactions of 210 / 210 / 90 / 90 kN with the boom over the front. Timber mats 1.8 × 1.8 m, 350 mm thick, on ground with a 400 kPa allowable from the geotechnical report.

1. The reactions. The declared set balances the gross load exactly, so it is a valid design point. The loaded pair carries 210 kN - 40% above the W/4 average of 150 kN.
2. Level 1 - float on mat. 210 kN over the float's 0.25 m² is 840 kPa on the timber - utilisation 0.34 against the declared contact allowable, with mat bending at 0.02 and shear at 0.14 for the declared spread.
3. Level 2 - mat on ground. The declared ψ = 35° through 350 mm grows the bearing area to 0.98 m², so the ground sees 214 kPa. Capacity is the 400 kPa allowable ÷ FoS 1.50 = 267 kPa - utilisation 0.80, and the ground check governs the run at 0.80.
4. The verdict, in words. The two-level card reports that the mat is big enough for the ground and strong enough for the float, names the governing level, and the factor ledger shows every term - the declared spread, the declared FoS, nothing hidden.

1. Gross load includes the crane, counterweight, rigging and the hoisted load.
2. Reactions come from the chart or planning software - per support, per slew angle.
3. The worst slew angle is found by sweep or by scanning every chart set, not assumed.
4. Partial outrigger extension declared per corner - the case the average cannot see.
5. Float dimensions from the machine data sheet, not guessed.
6. Mat spread angle declared and justified, or left at zero.
7. Allowable bearing pressure from the geotechnical report, with its provenance recorded.
8. Factor of safety declared against how well the ground is evidenced.
9. Excavations, slopes and buried services declared, with clear distances taped, not paced.
10. Every estimate labelled as one - and replaced with chart numbers before the lift plan is issued.

How do you calculate ground bearing pressure for a crane?

Take the governing outrigger reaction at the worst slew angle - from the manufacturer's chart, or from a labelled rigid-body estimate. Divide it by the bearing area the mat genuinely mobilises: the float area grown through the mat thickness at your declared spread angle, capped at the mat edge. Compare against the allowable bearing pressure from the geotechnical report divided by your factor of safety. The pressure itself is one division; the engineering is in the reaction, the area and the factor.

Can I just divide the crane's weight by four?

No. That gives the average reaction, and no outrigger carries the average: slew, counterweight, load radius and partial extension routinely put 40% or more above it on the loaded corner. The chart or a per-support solve gives the real distribution - and the worst angle for each support is found by sweeping, not guessing.

What size crane mat or outrigger pad do I need?

Two checks, not one. The mat's plan area helps the ground only as far as the spread cone through its thickness reaches - area outside the cone carries nothing. And the mat's own structure must take the float pressure on its top surface, which no amount of plan area changes. Thickness is usually the productive lever: it widens the effective bearing area and stiffens the mat at the same time. The calculator's inverse solver proposes the passing size and refuses honestly when no mat dimension can fix the failing level.

What allowable bearing pressure should I use?

The one in your geotechnical report - there is no defensible generic number, and this tool ships none. If you only have soil parameters, the classical route computes a named textbook ultimate resistance and divides it by your declared factor, labelled as exactly that; a site-specific ground investigation supersedes it.

What factor of safety applies to outrigger loading?

Industry guidance describes 1.5 to 3.0, scaled to how well the hardstand's bearing capacity has been evidenced. The source document for that band is not held in this product's library, so the band appears as labelled guidance beside a factor you declare - the tool warns below it, and never applies it for you.

How is crawler crane track pressure different?

The load spreads along two tracks instead of concentrating on four pads, but it is anything but uniform: the lateral split follows the resultant, and each track's longitudinal distribution is trapezoidal only while the resultant stays inside the middle third - beyond it part of the track lifts off and the pressure goes triangular over a reduced length. The tool computes both regimes, flags the transition, and treats every crawler number as a labelled estimate from the machine model.

How close to an excavation can a crane outrigger be?

The classic site rule is to stay back at least the excavation depth - that is the 45° dispersion construction this tool draws from your declaration. Inside that zone the honest answer is not a bigger mat: it is a temporary works design for the face, by an engineer who can see it. The flag tells you which side of that line you are on; it never substitutes for the design.

The methodology page documents every check, route, gate and source - including the named classical formulations, the superseded-edition labelling on the EN steel route, and the refusal semantics. The calculator opens on templates that pass with visible margin - including the worked example above - and its explain mode annotates every result surface in place.

Ground bearing is one part of the lift plan. The attachment side of the same plan is covered by the lifting lug guide, the lifting trunnion guide and the spreader beam guide - the same traceability posture, pointed at the rigging instead of the ground.