NCCCO Ground Bearing Pressure and Outrigger Setup 2 — Questions and Answers

Question 1: A crawler crane weighs 200,000 pounds and each track pad has a total ground contact area of 2,000 square inches. What is the ground bearing pressure under each track in pounds per square inch (psi)?

50 psi (Correct answer)
100 psi
200 psi
400 psi

Correct answer: 50 psi

Ground bearing pressure = Total weight divided by Total contact area. Each track supports 100,000 lbs over 2,000 sq in, so GBP = 100,000 divided by 2,000 = 50 psi per track.

Ground bearing pressure (GBP) is the force per unit area exerted on the ground by the crane's support system. For a crawler crane: if the machine weighs 200,000 lbs and each track has 2,000 sq in of contact area, each track carries approximately 100,000 lbs (assuming even distribution). GBP = 100,000 divided by 2,000 = 50 psi. This value must be compared to the allowable bearing capacity of the ground at the site. If the soil can only support 40 psi, mats or other load-spreading devices must be used to increase the contact area and reduce the pressure below the allowable limit. Geotechnical data for the site should always be reviewed before crane setup.

Question 2: Why must outrigger pads be used on outriggers even when working on concrete surfaces?

To protect the crane's outrigger floats from damage
To spread the point load over a larger area and reduce pressure on the concrete and underlying soil (Correct answer)
Concrete does not require pads; pads are only for soil
To provide a level surface for the outrigger

Correct answer: To spread the point load over a larger area and reduce pressure on the concrete and underlying soil

Outrigger pads spread the concentrated point load from the outrigger float over a larger area, reducing pressure per square inch on the concrete slab and the soil below. Concrete slabs can fail if point loads exceed the slab's capacity.

Even reinforced concrete can fail under concentrated point loads from crane outriggers if the load exceeds the slab's design capacity. An outrigger float's small area concentrates load to a very high pressure. Outrigger pads (timber cribbing, crane mats, or manufactured pads) spread this concentrated force over a larger area, reducing the pressure to within the slab's capacity. Additionally, concrete may be underlain by utilities, voids, or weaker sub-base material. A structural engineer should evaluate working on concrete slabs in crane pick zones, especially near foundations, basements, or underground structures.

Question 3: During a pre-lift check, the crane operator observes that one outrigger pad is sinking into soft ground more than the others. What is the appropriate action?

Continue the lift but watch carefully for further sinking
Stop the lift immediately and add more cribbing or a larger pad to distribute the load (Correct answer)
Reduce boom angle to lower the load on that outrigger
Lift the load quickly before the ground fails completely

Correct answer: Stop the lift immediately and add more cribbing or a larger pad to distribute the load

Differential outrigger settlement indicates the ground at that point cannot support the load. The lift must stop immediately and the setup corrected by enlarging the bearing area or improving the ground support before continuing.

Outrigger sinking is a warning sign of imminent ground failure, which can lead to crane upset. Any differential settlement during setup or lifting must be treated as an immediate hazard. The operator must stop all crane movement, lower any suspended load safely if possible, and re-evaluate the ground support at the affected outrigger. Solutions include adding timber cribbing, using larger steel pads or crane mats, or moving the crane to firmer ground. OSHA 29 CFR 1926.1402 requires that the ground be firm, drained, and graded and that adequate blocking or cribbing be used under outrigger floats to prevent settling or shifting.

Question 4: What is the primary purpose of extending outriggers to their full width before making a lift?

To level the crane automatically
To maximize the crane's tipping fulcrum distance and increase rated lifting capacity (Correct answer)
To provide electrical grounding for the crane
To allow the operator to see the outrigger pads more clearly

Correct answer: To maximize the crane's tipping fulcrum distance and increase rated lifting capacity

Fully extended outriggers widen the crane's support base, pushing the tipping axis farther from the center of gravity, which significantly increases the crane's rated lifting capacity and stability.

Outrigger extension increases the distance between the crane's center of gravity and the tipping fulcrum (the line between adjacent outrigger pads). A wider base means the crane would have to tilt much farther before reaching the tipping point, providing greater stability. Load charts are computed for specific outrigger configurations — fully extended, mid-span, or on rubber (wheels) — and rated capacities are highest when outriggers are fully extended. Using a partially-extended outrigger configuration results in significantly reduced rated capacities, and the operator must use the appropriate load chart section. ASME B30.5 requires that the load chart specify capacities for different outrigger positions.

Question 5: What does the term 'soil bearing capacity' refer to in the context of crane setup?

The total weight of soil at the job site
The maximum load per unit area that a soil can safely support without excessive settlement or shear failure (Correct answer)
The depth to which outrigger pads must be buried
The moisture content of the soil at the site

Correct answer: The maximum load per unit area that a soil can safely support without excessive settlement or shear failure

Soil bearing capacity is the maximum pressure (in psi or psf) that a particular soil type can support without failing. The outrigger ground pressure must not exceed the soil's bearing capacity.

Soil bearing capacity is a geotechnical measure of how much load a given soil type can support. Typical values range from very low for soft clay (around 250 psf or 1.7 psi) to high for dense gravel or rock (8,000 psf or higher). When setting up a crane, the ground pressure from outriggers must not exceed the soil's allowable bearing capacity. If it does, the soil will shear or settle, potentially causing crane overturn. Outrigger pads increase the load-bearing area, reducing pressure per unit area. Site-specific soil reports or cone penetration test data should be used to determine soil bearing capacity on critical lifts.

Question 6: OSHA 29 CFR 1926.1402 requires the ground to be firm, drained, and graded before crane use. What additional requirement is stated for supporting surfaces under outriggers?

Surfaces must be paved with asphalt or concrete
Adequate blocking, cribbing, or pads must be used to prevent settling or shifting of the crane (Correct answer)
Ground must be excavated and filled with gravel to 3 feet depth
Soil tests must be performed by a licensed engineer before every lift

Correct answer: Adequate blocking, cribbing, or pads must be used to prevent settling or shifting of the crane

OSHA 1926.1402 specifically requires adequate blocking, cribbing, or outrigger pads under outrigger floats to prevent the crane from settling or shifting during operations.

29 CFR 1926.1402(b) states that equipment must not be assembled or used unless ground conditions are firm, drained, and graded to a sufficient extent so that, in conjunction with the use of outriggers, blocking, cribbing, or other supporting materials, the equipment manufacturer's specifications for adequate support and degree of level are met. This means the crane operator and lift supervisor are responsible for assessing ground conditions and using appropriate support materials. The requirement applies to all supporting surfaces — soil, pavement, floor slabs, bridges, and elevated structures — and is critical for preventing the most common cause of crane accidents: ground failure under outriggers.

NCCCO Ground Bearing Pressure and Outrigger Setup 2

A crawler crane weighs 200,000 pounds and each track pad has a total ground contact area of 2,000 square inches.
What is the ground bearing pressure under each track in pounds per square inch (psi)?