Outriggers, pads and ground bearing pressure

R 03 mWhat Alberta and Ontario require

Alberta. If outriggers are installed on a mobile crane or boom truck, they must be extended and supported by solid footings before use¹. A second rule requires a mobile crane with outriggers to be set up on load-bearing floats or pads of adequate size, strength and rigidity¹. The Code defines a mobile crane as a crane other than a boom truck, the same term s.88 uses to adopt CAN/CSA Z150-98 (R2004)¹. So the pad rule names mobile cranes, while the footing rule names both. For a boom truck the duty is a solid footing, and on most ground a pad is how you get one.

Ontario. Outriggers must be extended as the load chart requires, rest on blocking that supports the crane and its maximum load without failure, deformation or settlement, and be set up only after the operator has considered the ground bearing pressure². The ministry's guideline says who supplies what: the manufacturer's specifications should give the pressures the crane exerts, and the operator or employer should confirm the ground and the set-up location with the constructor while the lift is planned³.

British Columbia goes further on the hardware. Outrigger beams and stabilizers must be marked to show when the needed extension is reached, floats must be secured to the outrigger jacks, and a boom truck may stand only on a surface able to support it and its load, with blocking or cribbing sized so the ground's bearing capacity is not exceeded⁴.

The load chart assumes all of this. IHSA notes that full on-outrigger ratings apply only when every beam is fully extended and every tire is clear of the ground; partial extension needs its own chart, or the on-tires rating applies⁵. Every number on the chart also assumes the ground under the pads does not move.

R 06 mHow outrigger pressure is estimated

Pressure under a pad is force divided by area. Queensland's mobile crane code, a non-Canadian reference, writes it as P = F / A and gives the minimum pad area as the force divided by the allowable ground pressure⁶.

The force. The best number is the manufacturer's maximum outrigger load for the configuration, which Ontario's guideline expects the specifications to give³. Ask the supplier for it. Without it, Queensland's code, a non-Canadian reference, approximates the worst outrigger as 0.65 × (crane mass + lifted load)⁶. Its worked example is also a non-Canadian reference: a 40 t crane lifting 20 t on 0.64 m² of timbers puts about 60.9 t/m², roughly 597 kPa, under one outrigger⁶.

Example: one pad, four sizes

Example inputs, not a typical truck: a boom truck weighing 20,000 kg lifting 2,000 kg including rigging. With the 0.65 approximation, the worst outrigger carries about 14,300 kg, or 140 kN.

Doubling the pad's width cuts the pressure to a quarter. Compare the result with an allowable pressure for your ground from an engineer, not with a table on a website. One t/m² is about 9.81 kPa.

R 09 mWhy an empty hook can be the worst case

The hardest-working outrigger is not always under the heaviest load. Queensland's code, a non-Canadian reference, says the greatest outrigger force occurs at the point of tipping, at a high boom angle with maximum counterweight and an empty hook, or with the boom directly over a pad⁶. The CICA and CANZ guidance note, another non-Canadian reference, warns that in some load cases most of the crane's total load can sit on one outrigger when the boom or counterweight is slewed over it⁷.

What that means on site:

• Size every pad for the worst case, not the average. Total weight divided by four understates the pad the boom swings over.
• Check the whole swing. A pad that is fine with the boom over the rear may carry most of the truck as the boom passes over its corner.
• Put the same footing under all four. The weakest pad decides the set-up, because the swing will find it.
• Mind the quadrant. IHSA notes capacity is normally greatest over the rear, less over the side and may be nil over the front⁵. Position the truck so the heavy part of the lift happens where the chart allows it.

R 12 mGround: what the tables say, and what they do not

No Canadian regulator we found publishes allowable ground pressures for crane outriggers. Two tables get quoted anyway, and both need a label.

Queensland's typical values are a non-Canadian reference, and the code itself says site investigation governs. Where ground types are mixed, the poorer one sets the value (Queensland, non-Canadian reference)⁶.

The BC Building Code values are Canadian, but they are for Part 9 house and small-building footings, not crane outriggers. They show one thing the Queensland table does not: on gravel, sand or silt, the allowable pressure drops to 50% when the water table is within one footing width below the bearing surface⁸.

Neither table knows what is under your site. Fill, frost, thaw, a buried tank or a parking garage roof can sit under a surface that looks like compacted gravel. Ontario's guideline names previously excavated soil and voids such as underground parking and utility vaults³.

R 15 mPads, timbers and mats

A pad spreads the outrigger force over more ground and gives the float a level, solid seat. Alberta's test for mobile cranes is adequate size, strength and rigidity¹; Ontario's is no failure, deformation or settlement that affects stability².

• Pads. In BC, floats must be secured to the outrigger jacks whenever outriggers are used⁴. A thin, flexible pad bends under the float and spreads less than its size suggests, so stiffness counts as well as area.
• Timber cribbing. Queensland's code, a non-Canadian reference, calls for timbers at least 200 mm wide and 75 mm thick, with gaps of no more than 25 mm in the top and bottom layers⁶.
• Mats. IHSA says chart ratings apply only on ground firm enough to keep the crane level, and that timber or steel mats must be used on soft ground or backfill⁵.

Level matters as much as area. In IHSA's example table for one crane, 1° out of level cost 10% of chart capacity with a short boom at minimum radius, 30% with a long boom, and 50% at 3°⁵. BC requires the frame to be level within the manufacturer's limits, with level indicators the operator can read⁴. A pad that settles mid-lift takes the truck out of level with the load in the air.

Ask the supplier

• What pads travel with the truck, and their size and material.
• Whether mats or cribbing are available for soft ground, and at what cost.
• The maximum outrigger load for the configuration you need.
• The outrigger spread at full extension, so you can check the space.

R 18 mNear trenches, slopes and soft edges

An outrigger near an edge loads ground that has somewhere to go. BC is explicit: when a boom truck will be used next to an excavation, slope or backfilled area, a qualified person must decide where it sets up⁴. Ontario's guideline says to consult an engineer when previously excavated soil or voids are a concern³.

The rule of thumb most often quoted comes from the CICA and CANZ guidance note, a non-Canadian reference: set the nearest outrigger at least as far from a trench as the trench is deep, assuming a 45° zone of influence⁷. Example: a 2 m deep trench keeps the outrigger at least 2 m back from the edge. Treat that as a first look, not an approved clearance.

Backfill over a recent trench is the other trap: it looks like the ground around it and is not. IHSA's answer for soft ground or backfill is mats⁵. Retaining walls and basement walls are edges too. The trench has its own depth and protection rules; CivilConstruction.ca covers trench safety and shoring by province.

R 21 mWhen to call a geotechnical engineer

Call a geotechnical engineer, or the constructor's engineer, before the truck arrives when:

• the set-up is over fill, backfill or ground dug before, or over a void such as an underground parking structure or a utility vault³;
• the truck sets up next to an excavation, slope or backfilled area, where BC already requires a qualified person to choose the spot⁴;
• the ground is wet, soft, frozen or thawing, or the water table is high;
• the pad you need is bigger than anything the truck carries;
• the lift is heavy. Queensland's code, a non-Canadian reference, calls for geotechnical certification of the ground before lifts such as bridge beams of 10 t or more, or loads of 50 t or more⁶.

A slab or parking deck is also a structural question. Give the engineer the manufacturer's maximum outrigger load, the pad size, the set-up position and the lift radius and direction. What you want back is an allowable pressure, or a pad or mat design, for that spot.

R 24 mUsing the pad calculator

The outrigger pad calculator runs the arithmetic above on your numbers:

1. Truck and crane mass, from the registration or a weigh slip, and the load plus rigging.
2. The maximum outrigger force from the manufacturer, if you have it. It replaces the 0.65 estimate.
3. The pad shape and size: square, round or rectangular.
4. The allowable ground pressure, from your engineer or a geotechnical report. Queensland's values can be picked, each labelled as a non-Canadian reference; the BC Part 9 values are shown for comparison only.

It returns the pressure under one outrigger in kPa and t/m², the pad area your allowable pressure needs, and a flag when the pad is too small. Example: the 140 kN outrigger above on a 0.8 m square pad, against 200 kPa from an engineer, shows about 219 kPa and a minimum pad of about 0.70 m², roughly 0.84 m square, so the pad is flagged.

It does not know your ground, the swing or the edge distance. Use it to describe the job to a supplier and an engineer, never to decide that a set-up is safe. To size the truck itself, start with the boom truck size estimator and the load chart guide.