Crane Outrigger Pad Calculations — UAE Soil-Bearing Numbers
Outriggers concentrate the entire weight of the crane plus the load onto four pads. Get the pad size wrong and the unit sinks, tips, or both. UAE soils are not what European or US load charts assume — sabkha behaves nothing like compacted gravel. This article gives the real-world bearing capacities and a practical sizing rule for a UAE site.
Why outrigger pads matter
A 50T mobile crane has gross vehicle weight around 36 tonnes. Lifting an 8 tonne load at 30 m radius shifts the centre of gravity dramatically — the loaded outrigger can take 70%+ of the total weight. That's roughly 30 tonnes on one corner of the crane, transmitted through one outrigger float into the ground.
If the float is 600 mm × 600 mm (typical factory float), the contact pressure is ~83 t/m² — far above the bearing capacity of any natural UAE soil. Without a pad, the float sinks. With a properly sized pad, the load is spread across enough area that the soil holds.
Calculating outrigger force
The crane manufacturer's load chart shows the outrigger reaction force at each boom angle and load combination. For a worst-case fast estimate without the chart:
Assume the loaded outrigger carries 70% of (crane GVW + load + counterweight) on the heaviest lift. For a 50T truck crane lifting at maximum radius, that's 70% × (36 t + 8 t) ≈ 31 t on one outrigger.
Always verify against the manufacturer's outrigger reaction chart for the actual configuration. The 70% rule is a sanity check, not a substitute.
UAE soil bearing capacities (real numbers)
Indicative ultimate bearing capacities measured on UAE sites by EIAC-recognised geotechnical labs:
| Surface | Bearing capacity (t/m²) | Notes |
|---|---|---|
| Engineered concrete pad | 40–80+ | Dependent on slab thickness and reinforcement |
| Asphalt road (compacted base) | 30–50 | Hot weather lowers — afternoon lifts can sink into asphalt |
| Compacted gravel | 25–35 | Common at construction sites; reliable |
| Compacted desert sand | 15–22 | If properly compacted — uncompacted is half this |
| Loose desert dune sand | 5–8 | Outriggers will sink without large mats |
| Sabkha (coastal salt flat) | 3–10 | Highly variable — wet sabkha can be near-zero |
| Reclaimed land < 5 yrs old | 10–20 | Confirm with site geotech report |
| Hajar mountain rock | 50+ | Solid but irregular surface — pad still recommended |
Use the lower end of each range for design. Wet sabkha after rain is the single most dangerous UAE surface for crane work — refuse the lift until the surface dries or proper mats are deployed.
Pad size formula
Required pad area (m²) = Outrigger force (tonnes) ÷ Allowable bearing capacity (t/m²)
Add a safety factor of 2.0 for stable surfaces (compacted gravel, asphalt) and 3.0 for variable surfaces (sabkha, reclaimed land, loose sand).
Worked examples
25T QY25K-II on compacted gravel
Outrigger force: 70% × (29 t + 4 t) ≈ 23 t. Bearing capacity: 30 t/m². Required area: 23/30 = 0.77 m² × 2.0 SF = 1.54 m². Use a 1.25 m × 1.25 m pad.
50T QY50KA on loose desert sand
Outrigger force: 70% × (36 t + 8 t) ≈ 31 t. Bearing capacity: 6 t/m². Required area: 31/6 = 5.2 m² × 3.0 SF = 15.5 m². Use a 4 m × 4 m mat (or a multi-mat layout). On loose sand the only safe pads are large timber mats.
80T XCT80-Y1 on asphalt road
Outrigger force: 70% × (45 t + 12 t) ≈ 40 t. Bearing capacity: 35 t/m² (asphalt). Required area: 40/35 = 1.14 m² × 2.0 SF = 2.3 m². Use a 1.5 m × 1.5 m pad.
130T QY130K-I on engineered concrete pad
Outrigger force: 70% × (60 t + 25 t) ≈ 60 t. Bearing capacity: 50 t/m² (concrete). Required area: 60/50 = 1.2 m² × 2.0 SF = 2.4 m². Use a 1.6 m × 1.6 m pad — concrete sites are forgiving.
Crane mats vs steel pads vs timber
- Steel pads (factory float extensions): 600–1,200 mm square, fine for stable surfaces. Useless on sabkha.
- Timber crane mats (typically 4 m × 1.2 m × 100 mm hardwood): the workhorse for variable UAE soil. Use multi-layer crosshatch on sabkha or loose sand for additional spread.
- Composite mats (HDPE / fibre): lighter, more expensive, increasingly common on tier-1 UAE sites where mobilisation logistics matter.
- Custom steel mats: for repeated heavy lifts at the same location, fabricated steel mats with grouser bars give the best load spread per kg of mat.
Pre-lift outrigger checklist
- Read the manufacturer's outrigger reaction chart for the configuration.
- Identify the surface type and confirm bearing capacity (request site geotech if unsure).
- Calculate required pad area, apply safety factor.
- Lay pads. For sabkha, lay a sand bed first then the mat.
- Extend outriggers fully and level the crane on its outriggers, not on the chassis.
- Test-lift at low height first; if any pad sinks, stop and re-pad.
- Keep a load-cell or pressure gauge eye on the boom-hoist and the LMI display throughout.
Need outrigger calculations for a specific lift?
Send us the crane model, the load, the radius, and the surface type — we'll work the calculation and send back a written reaction-force diagram you can hand to the lift supervisor.
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