Ground Bearing Pressure and Outrigger Setup 1 Flashcards

A mobile crane has a maximum single-outrigger load of 85,000 pounds. The outrigger float measures 14 inches by 14 inches. What is the ground bearing pressure under that float?

Answer: Approximately 434 psi

Ground bearing pressure = load ÷ contact area. The float area is 14 × 14 = 196 square inches. 85,000 ÷ 196 ≈ 434 psi. This value must then be compared against the allowable soil bearing capacity to determine whether additional cribbing is needed.

A geotechnical report lists the allowable soil bearing capacity as 2,880 pounds per square foot (psf). What is this value expressed in pounds per square inch (psi)?

Answer: 20 psi

There are 144 square inches in one square foot, so to convert psf to psi you divide by 144. 2,880 ÷ 144 = 20 psi. This conversion is essential when comparing outrigger float pressure (typically calculated in psi) to soil capacity values reported by engineers in psf.

An outrigger exerts a downward load of 72,000 pounds. The allowable soil bearing pressure at the site is 24 psi. What is the minimum outrigger mat area required to keep ground pressure within safe limits?

Answer: 3,000 square inches

Required area = load ÷ allowable pressure = 72,000 ÷ 24 = 3,000 square inches. The outrigger mat must spread the load over at least this area so that the pressure transmitted to the soil does not exceed the site's rated capacity.

Per ASME B30.5, what is the maximum out-of-level condition permitted for a mobile crane before a lifting operation begins?

Answer: 1% of grade (approximately 0.57°)

ASME B30.5 requires the crane to be level within 1% of grade before any lift. Exceeding this tolerance shifts the crane's center of gravity, alters boom geometry, and invalidates the load chart ratings, which are all developed assuming a level machine.

Due to a congested job site, a mobile crane cannot extend its outriggers to full width and must operate with outriggers at the mid-span extension position. What is the correct procedure?

Answer: Use the manufacturer's load chart for the mid-span (intermediate) outrigger extension setting

ASME B30.5 and crane manufacturers provide separate load chart columns for full, mid, and minimum outrigger extensions. The operator must use the column corresponding to the actual extension used; applying an arbitrary percentage reduction is not an acceptable substitute and can be either unsafe or overly restrictive.

When placing timber cribbing beneath an outrigger pad, which characteristic of the cribbing is MOST critical for safe load distribution?

Answer: Sufficient total bearing area and structural capacity to spread the load without crushing or buckling

Cribbing works by distributing the concentrated outrigger load over a larger soil contact area. If the cribbing lacks adequate bearing area or compressive strength, it will crush or deflect, concentrating the load rather than spreading it. Orientation, finish, and exact height are secondary to the cribbing's ability to safely carry and distribute the applied force.