NCCCO Ground Bearing Pressure and Outrigger Setup 3 — Questions and Answers
Question 1: When calculating ground bearing pressure under an outrigger float, you divide the outrigger load by which value?
- The crane's gross weight
- The float contact area (Correct answer)
- The number of outriggers deployed
- The boom length
Correct answer: The float contact area
Ground bearing pressure (GBP) equals the outrigger reaction load divided by the contact area of the outrigger float or pad.
Question 2: A crane is set up on asphalt in summer heat. What is the primary concern regarding outrigger pads?
- Asphalt reflects heat and expands the pads
- Asphalt can soften and allow pads to sink or shift under load (Correct answer)
- Asphalt is too rigid and may crack the pads
- Asphalt always has higher bearing capacity than soil
Correct answer: Asphalt can soften and allow pads to sink or shift under load
Hot asphalt loses bearing capacity and can deform under concentrated outrigger loads, causing the crane to shift or tip.
Question 3: Which outrigger configuration provides the greatest crane lifting capacity?
- All outriggers fully extended and set (Correct answer)
- Two front outriggers extended, rear on tires
- All outriggers at mid-span extension
- Rear outriggers only, fully extended
Correct answer: All outriggers fully extended and set
Full outrigger extension maximizes the tipping fulcrum distance, providing the greatest load chart capacity.
Question 4: What term describes the tendency of a crane to rotate about its tipping axis when a load is lifted?
- Side loading
- Overturning moment (Correct answer)
- Dynamic amplification
- Structural deflection
Correct answer: Overturning moment
The overturning moment is the rotational force created by the load and boom weight acting against the crane's stability.
Question 5: Before placing timber cribbing under outrigger pads, what must be confirmed about the cribbing material?
- It must be painted or sealed to resist moisture
- It must be sound, free of rot, and of adequate dimensions to distribute load (Correct answer)
- It must be exactly 4 inches thick per layer
- It must be metal-banded to prevent spreading
Correct answer: It must be sound, free of rot, and of adequate dimensions to distribute load
Deteriorated or undersized cribbing can fail under load; sound lumber of correct dimensions is required to safely spread the outrigger load.
Question 6: A crane manufacturer's load chart specifies 'on outriggers, fully extended.' Using mid-range outrigger extension means:
- Capacity remains the same if the boom is shortened
- The operator must use a reduced capacity or a separate mid-extension load chart if provided (Correct answer)
- Capacity increases because the crane is lighter at mid-extension
- The load chart does not apply and any load can be lifted
Correct answer: The operator must use a reduced capacity or a separate mid-extension load chart if provided
Load charts are configuration-specific; lifting at mid-extension without an applicable chart or derating violates safe operating procedures.
Question 7: When outriggers are deployed over a utility vault or underground structure, the primary hazard is:
- Electrical interference from underground cables
- Collapse of the underground structure under the concentrated outrigger load (Correct answer)
- Outrigger cylinders corroding from ground moisture
- Hydraulic pressure loss due to cold underground temperatures
Correct answer: Collapse of the underground structure under the concentrated outrigger load
Underground vaults, culverts, and utilities may not support the concentrated point loads from crane outriggers and can collapse suddenly.
NCCCO Ground Bearing Pressure and Outrigger Setup 3
When calculating ground bearing pressure under an outrigger float, you divide the outrigger load by which value?