How is crawler track ground pressure calculated?

As a distribution rather than a division. The machine's resultant sits somewhere in the rectangle the two tracks define, and the pressure under each track varies along its length depending on where. Inside the middle third of the track the distribution is trapezoidal and non-zero everywhere; at the edge of the middle third it is triangular with a peak twice the average; outside it the track lifts at one end and the peak climbs quickly on a shorter contact length.

Are crawler tracks better for the ground than outriggers?

Substantially, and it is the main reason crawlers are the default on soft sites. On the same machine, the same 300 kN load at the same radius and the same ground, the worked crawler reaches 253 kPa and passes at 84 percent, while the same machine on outriggers with 3.0 m mats reaches 403 kPa and fails at 134. The reason is area: two tracks give around 15 square metres of contact and four mats give under two that a calculation may credit.

How much does crawler shoe width matter?

Pressure is inversely proportional to it, so a great deal, and it is the one lever a hire decision actually has. Going from 1.1 m shoes to 0.8 m on the worked machine took the peak track pressure from 253 kPa to 347, a 37 percent increase, and turned a passing setup into a failing one. The same crane model runs on more than one shoe width, so it is a question to ask rather than a property to assume from the designation.

What is different about a crawler travelling with a load?

Four things a static check does not cover. The ground changes, because a travelling machine crosses ground nobody assessed. The pressure distribution moves, and a machine pitching over a slight rise can be in partial contact when it was in full contact standing still. Dynamic effects from starting, stopping and slewing add to the static pressure. And the haul route becomes temporary works with a designer, a specification and an inspection regime, exactly as a standing area does.

Why report a contact mode alongside the track pressure?

Because full contact and partial contact are different regimes and a percentage alone hides which one you are in. In full contact a small change in radius or slew produces a small change in pressure. In partial contact the track is lifting at one end and the same small change produces a large change, so the margin implied by a utilisation is not the margin you actually have. An average pressure - load over track area - is not a bearing pressure at all; it is the mean of a distribution whose peak can be twice it or worse.

Crawler crane ground pressure is not an average

The short answer

A crawler and an outrigger are different bearing problems. One is a long strip whose pressure distribution changes shape; the other is four patches whose magnitudes change.
On the same machine, the same load and the same ground, the crawler passes at 84 percent and the outriggers fail at 134. Tracks are far gentler on the ground and that is their main advantage.
A crawler's number is a distribution, not a load. Under an eccentric resultant the pressure goes trapezoidal, then triangular, then the track lifts at one end and the pressure concentrates in what is left.
Shoe width is the one lever a hire decision has. Going from 1.1 m shoes to 0.8 m took the pressure from 253 kPa to 347 and turned a pass into a fail.
A crawler travelling with a load is a different case again, and it is the one most likely to be outside what was checked.

Two genuinely different bearing problems

An outrigger delivers a force at a point and the question is what area it reaches. A track delivers a distributed pressure over a strip and the question is what shape that distribution takes.

Outriggers. Four discrete supports. Each is a force, the float turns it into a pressure over a small area, and the mat's job is to spread it further. Every question is about magnitude and area, and the calculation is a division.

Crawler tracks. Two long strips. The machine's resultant sits somewhere within the rectangle they define, and the pressure under each track is distributed along its length in a way that depends on where. The calculation is not a division; it is a distribution.

What a division would give you

q_{mean} = \frac{R}{L b}

$R$: is the reaction on one track
$L$: is track contact length
$b$: is shoe width

This is the mean of a distribution, not a bearing pressure. The peak can be twice it under full contact and considerably more once contact is partial.

Three regimes, and knowing which one you are in is most of the work:

Full contact, trapezoidal. The resultant sits inside the middle third of the track length. The pressure varies linearly along the track and is non-zero everywhere. This is the comfortable case.

Full contact, triangular. The resultant sits at the edge of the middle third. Pressure at one end is zero, and the peak at the other is twice the average.

Partial contact. The resultant sits outside the middle third. The track lifts at one end, the contact length is shorter than the track, and the peak pressure climbs quickly, because the same load is now carried on less of the machine.

That progression is why a crawler's ground pressure is reported as a peak with a contact mode beside it. A single average number tells you almost nothing about which regime the machine is in.

The same machine, two ways of standing

Same weight, same load, same radius, same ground. On tracks it passes and on outriggers it does not, and the difference is area.

A 250 t class machine with 300 kN on the hook at 14 m, on ground declared good for 300 kPa.

Crane Ground Bearing Pressure & Outrigger Pad Calculator · computed at page render

On crawler tracks: 7.0 m long, 1.1 m shoes

The engine sweeps the slew and reports the worst track pressure with its contact mode.

Peak track pressureworst at 25.0 degrees of slew, left track	253kPa
Contact modethe resultant sits inside the middle third	full contact
Contact lengththe full track	7.00m
Crawler track contact check	51.8%
Ground bearing utilisation	84.2%

Governing check: Ground bearing - declared allowable (route R1)84.2% utilisationPass

Passing, and the contact mode is the useful half of that answer. Full contact means the machine is comfortably within its own base, which is a different statement from a percentage.

Open this example in the calculator

Crane Ground Bearing Pressure & Outrigger Pad Calculator · computed at page render

The same machine on outriggers

Identical masses, identical load, identical radius, identical ground, on a 3.5 m outrigger base with 3.0 m mats 0.5 m thick and 30 degrees of spread declared.

Worst outrigger loadFL at 41.2 degrees	764kN
Area the calculation creditsfrom a 9.0 m2 mat	1.90m2
Pressure on the groundagainst 253 kPa on tracks	403kPa
Ground bearing utilisationagainst 84.2% on tracks	134.3%

Governing check: Pad bearing pressure on ground134.3% utilisationFail

Failing, on the same ground the crawler passed on. The reason is area: two tracks give 15.4 square metres of contact, and the four mats credit 1.9 each, so 7.6 in total. Half the area, and the load is concentrated on the worst of four supports rather than spread along a strip.

Open this example in the calculator

The same machine, load, radius and ground: 84 percent on tracks against 134 on outriggers. That gap is why crawlers are the default on soft sites, and why narrowing the shoes closes it fast.

Shoe width, the one lever a hire decision has

Pressure is inversely proportional to it, and unlike an outrigger float it is genuinely a choice.

Crane Ground Bearing Pressure & Outrigger Pad Calculator · computed at page render

The same crawler on 0.8 m shoes

Identical machine, identical load, identical track length and gauge. Only the shoe width changed.

Shoe widthagainst 1.1 m	0.8m
Peak track pressureagainst 253 kPa	347kPa
Increase	37.5%
Ground bearing utilisationfailed	115.7%

Governing check: Ground bearing - declared allowable (route R1)115.7% utilisationFail

A 27 percent reduction in shoe width bought a 37 percent increase in pressure and turned a passing setup into a failing one. Wide shoes are usually available on a hire and they are usually the cheapest ground improvement there is.

Open this example in the calculator

Two things follow.

Ask what shoes the machine is coming with. It is a question with a number attached, and the answer changes the ground calculation before anybody buys a mat.

Do not assume shoes and track length from the model. The same crane model runs on more than one shoe width, and as the example shows, three hundred millimetres of it is the difference between passing and failing.

Travelling with a load

A different case, and the one most likely to be outside what was checked. The machine is moving, the load is swinging, and the ground under the tracks is fresh every metre.

Crawlers travel with loads suspended, which mobile cranes on outriggers cannot do. That capability is why crawlers are chosen, and it introduces four things a static check does not cover.

The ground changes. A static setup is checked on one patch of ground. A travelling crawler crosses ground that has not been checked, and the whole route needs the same treatment as a setup position.

The pressure distribution moves. A crawler pitching over a slight rise concentrates its load at one end of the track, which is the partial-contact regime. A machine that is comfortable standing still can be in a different regime moving.

Dynamic effects are real. Starting, stopping and slewing while travelling all add to the static pressure, and the manufacturer's travel-with-load restrictions exist because of that.

The route becomes temporary works. A crawler haul route is a working platform with a length. It has a designer, a specification, a construction and an inspection regime, exactly as a standing area does.

Checking a crawler properly

Seven things, and three of them have no equivalent on an outrigger calculation.

01Is the machine standing or travelling?
Standing, for the lift
Check the setup position, swept through the permitted slew arc. The worst track pressure is at a specific slew angle, and it is not usually over the front.
Travelling with the load
Check the whole route, plus the manufacturer's travel restrictions. Grade limits, speed limits and the ground along every metre of the route.
02What contact mode does the worst case produce?
Full contact, trapezoidal
The comfortable case. Report the peak and the mode together.
Partial contact
The track is lifting at one end and the peak climbs fast. This is where a small change in radius or slew produces a large change in pressure, so the margin is not what the percentage suggests.

A crawler ground check

01Get the shoe width and track length from the machineNot from the model designation. The same model runs on several shoe widths.
02Sweep the slew arcThe governing angle was found rather than assumed, and it is named in the output.
03Report the contact mode with the pressureFull or partial contact, and the contact length. A percentage alone hides the regime.
04Check both tracksThey are not symmetric once the boom is off the centreline.
05Treat the route as groundIf the machine travels with the load, every metre of the route is a setup position.
06Apply the manufacturer's travel restrictionsGrade, speed and load percentage limits, taken from the chart rather than from practice.
07Watch for local softeningA crawler works the same ground repeatedly. Ground that was adequate on the first pass may not be on the fifth.

Six ways a crawler check goes wrong

Three are about the distribution and three are about travel.

1. An average pressure reported instead of a peak. Load over track area is not a bearing pressure; it is the mean of a distribution whose peak can be twice that or worse.

2. The contact mode not stated. Full contact and partial contact are different regimes. Under partial contact the peak pressure rises faster than the load does, because the contact length is shrinking at the same time.

3. One slew angle checked. As with outriggers, the governing case is found by sweeping.

4. The route not checked. A machine that travels with the load crosses ground nobody assessed.

5. Manufacturer travel restrictions ignored. Grade and speed limits are part of the chart.

6. Shoe width assumed. It is the single largest lever in a crawler ground calculation and it is decided by whoever supplies the machine.

Common questions

How is crawler track ground pressure calculated?: As a distribution rather than a division. The machine's resultant sits somewhere in the rectangle the two tracks define, and the pressure under each track varies along its length depending on where. Inside the middle third of the track the distribution is trapezoidal and non-zero everywhere; at the edge of the middle third it is triangular with a peak twice the average; outside it the track lifts at one end and the peak climbs quickly on a shorter contact length.
Are crawler tracks better for the ground than outriggers?: Substantially, and it is the main reason crawlers are the default on soft sites. On the same machine, the same 300 kN load at the same radius and the same ground, the worked crawler reaches 253 kPa and passes at 84 percent, while the same machine on outriggers with 3.0 m mats reaches 403 kPa and fails at 134. The reason is area: two tracks give around 15 square metres of contact and four mats give under two that a calculation may credit.
How much does crawler shoe width matter?: Pressure is inversely proportional to it, so a great deal, and it is the one lever a hire decision actually has. Going from 1.1 m shoes to 0.8 m on the worked machine took the peak track pressure from 253 kPa to 347, a 37 percent increase, and turned a passing setup into a failing one. The same crane model runs on more than one shoe width, so it is a question to ask rather than a property to assume from the designation.
What is different about a crawler travelling with a load?: Four things a static check does not cover. The ground changes, because a travelling machine crosses ground nobody assessed. The pressure distribution moves, and a machine pitching over a slight rise can be in partial contact when it was in full contact standing still. Dynamic effects from starting, stopping and slewing add to the static pressure. And the haul route becomes temporary works with a designer, a specification and an inspection regime, exactly as a standing area does.
Why report a contact mode alongside the track pressure?: Because full contact and partial contact are different regimes and a percentage alone hides which one you are in. In full contact a small change in radius or slew produces a small change in pressure. In partial contact the track is lifting at one end and the same small change produces a large change, so the margin implied by a utilisation is not the margin you actually have. An average pressure - load over track area - is not a bearing pressure at all; it is the mean of a distribution whose peak can be twice it or worse.

Sources

Every document below is linked at its publisher or regulator. Xarpis reproduces no standard text; where a clause is named, the identifier is given so you can find it in your own copy.

ASME B30.5Mobile and Locomotive Cranes
ASME · paid document
Construction, installation, operation, inspection and maintenance of mobile cranes in the US, including load rating and the requirement to operate within the manufacturer's chart. It governs the machine; the ground it stands on is 29 CFR 1926.1402 and the calculation is yours.
29 CFR 1926.1402Ground conditions (Cranes and Derricks in Construction)
US Occupational Safety and Health Administration · free to read
Requires ground supporting a crane to be firm, drained and graded sufficiently for the manufacturer's specifications, and places the duty to prepare it, and to disclose voids and buried services, on the controlling entity. It states a duty; it prescribes no calculation.
29 CFR 1926 Subpart CCCranes and Derricks in Construction
US Occupational Safety and Health Administration · free to read
The whole US construction crane subpart, free in full: ground conditions, assembly and disassembly, power line clearance, operator qualification, signals, inspection and multiple-crane lifts. The index page, because the duty a reader needs is usually two sections away from the one they searched for.
EM 1110-1-1905Bearing Capacity of Soils
US Army Corps of Engineers · free to read
A complete, free engineering manual on bearing capacity: the general bearing capacity equation, its shape, depth, inclination and groundwater corrections, and the settlement checks that usually govern before capacity does. The one document that lets a reader do a real ground capacity calculation without buying anything.

Run the check properly

Reading about a calculation is not the same as being able to hand one over. These tools produce the traceable record.

Spreader Beam Design Calculator Vessel Upending & Tailing Lift Calculator

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