Ground Bearing Pressure and Outrigger Setup Flashcards

An outrigger float applies 90,000 lbs to the ground. The soil's allowable bearing capacity is 1,500 psf. A contractor installs 6 ft × 6 ft cribbing (36 sq ft) beneath the float. Is this setup within limits, and what is the resulting ground bearing pressure?

Answer: No — the resulting GBP is 2,500 psf, exceeding the allowable by 1,000 psf

GBP = Load ÷ Cribbing Area = 90,000 lbs ÷ 36 sq ft = 2,500 psf. This exceeds the 1,500 psf allowable by 1,000 psf. To meet the allowable limit, the minimum cribbing area must be 90,000 ÷ 1,500 = 60 sq ft (e.g., a roughly 7.75 ft × 7.75 ft mat). A common mistake is assuming a large-looking mat is automatically sufficient without doing the math.

A mobile crane's boom is slewed directly over the front-left outrigger at near-maximum radius. Which statement BEST describes the resulting outrigger load distribution across all four pads?

Answer: The front-left outrigger may bear close to 100% of the load reaction, and the diagonally opposite rear outrigger may lift off entirely

When the boom is positioned directly over one outrigger, the tipping line runs between that outrigger and the nearest adjacent one — concentrating nearly the full suspended-load reaction at the near corner. The diagonally opposite outrigger can lift completely off the ground. This is why the maximum single-outrigger load must be verified for every slew position, not just an average 25% assumption.

A crane's load chart provides capacity ratings for 'full extension' and '50% extension' outrigger configurations. The operator has extended the outriggers to 75% of full extension width. Without explicit manufacturer guidance permitting interpolation, what is the CORRECT method for determining allowable lift capacity?

Answer: Use the 50% extension rating, as no intermediate chart value exists and the more conservative value must apply

Load chart values are discrete, tested configurations. Interpolating between them is only permitted if the crane manufacturer explicitly provides a method or authorizes it. When the actual setup falls between two configurations without that authorization, the operator must use the more conservative (lower) applicable chart value — in this case, the 50% extension rating. Using a higher rating without manufacturer backing violates ASME B30.5 requirements.

A crane is staged on asphalt over an underground parking deck rated at 2,200 psf. The outrigger load is 110,000 lbs. The crew installs cribbing measuring 7 ft × 7 ft (49 sq ft). Is this setup compliant, and why?

Answer: No — the resulting GBP is approximately 2,245 psf, which exceeds the deck's 2,200 psf rating

GBP = 110,000 lbs ÷ 49 sq ft ≈ 2,245 psf. This exceeds the deck's 2,200 psf rating by 45 psf — a seemingly small margin that is still non-compliant. Asphalt does not add rated structural capacity. The cribbing would need to be at least 110,000 ÷ 2,200 ≈ 50 sq ft (e.g., 7.1 ft × 7.1 ft). Underground structure limits are absolute; there is no rounding tolerance.

During site preparation, a geotechnical report classifies the soil as 'stiff clay.' Using standard allowable bearing capacity references, which range most accurately represents the expected capacity of this soil type?

Answer: 4,000–6,000 psf

Standard geotechnical classification assigns approximate allowable bearing capacities as follows: soft clay ≈ 1,000 psf, medium clay ≈ 2,000 psf, stiff clay ≈ 4,000–6,000 psf, hard clay ≈ 8,000 psf. Confusing 'stiff' with 'medium' or 'soft' is a common error that leads to undersized cribbing. A geotechnical engineer should confirm site-specific values before any critical lift.

When building a timber cribbing stack to gain height under an outrigger float on uneven ground, what is the generally accepted maximum height-to-base-width ratio (H:W) to prevent the stack from becoming unstable under dynamic crane loads?

Answer: 3:1

The industry-accepted maximum height-to-base-width ratio for timber cribbing stacks is 3:1. A stack that is taller than three times its base width becomes prone to toppling, especially under the dynamic shock loads that occur during crane picks and load swings. If additional height is required beyond this ratio, the base footprint must be proportionally expanded. Many rigger training programs and site safety plans treat 3:1 as an absolute limit.