Mobile Crane 339A: Ground Bearing & Outrigger Sizing
Challenging the Interprovincial Red Seal Mobile Crane Operator (339A) Examination demands far more than basic rote memorization. Candidates must prove operational competence under strict Canadian safety codes and technical mandates. In this study guide, our technical team analyzes ground bearing pressure and outrigger pad sizing governed by CSA Z150 Safety Code for Mobile Cranes.
📐 Examination Blueprint & Competency Profile
- Regulatory Standard: CSA Z150 Safety Code for Mobile Cranes
- Core Technical Domain: Ground Bearing Pressure and Outrigger Pad Sizing
- Examination Focus: Diagnostic troubleshooting, mathematical parameter calculation, and strict safety compliance.
Core Technical Competencies & Exam Strategy
Red Seal exam questions are deliberately crafted around realistic jobsite dilemmas. Rather than asking for definitions, the examiners present an operational scenario where you must:
- Accurately reference governing formulas, tables, and minimum clearances.
- Isolate faulty components systematically before executing destructive tests.
- Ensure strict lockout/tagout, PPE compliance, and provincial jurisdictional safety.
Step-by-Step Technical Problem Breakdown (3 Practice Scenarios)
Work through these 3 representative Red Seal exam questions. Analyze the question prompt, cross-examine the options, and verify your rationale against the official technical explanations below:
An apprentice rigger in Winnipeg is helping size timber mats for a crane that will set up over soft clay, and asks what decides the pressure an outrigger puts on the soil. The lift plan lists the crane weight, the load, and the maximum reaction at each outrigger. How is the ground bearing pressure under one outrigger found?
- A) The crane's shipping weight divided equally among all four of the outriggers
- B) That outrigger's maximum reaction force divided by the area of the mat under it
- C) The boom length and boom angle at the time of the lift, whatever the load weight
- D) The width of the outrigger beam where it enters the box on the carrier frame
Correct Answer: B
Technical Rationale: Ground bearing pressure = the force on that outrigger / the bearing area of the mat or pad. The maximum reaction comes from the manufacturer's outrigger load data for the lift and swing range.
During a heavy lift plan in Calgary, the manufacturer's outrigger reaction table shows one jack will put 60,000 lb on a 3 ft by 3 ft timber crib pad. The geotechnical engineer wants the pressure under the pad to compare with the soil report. What is the ground bearing pressure?
- A) About 20,000 psf
- B) About 10,000 psf
- C) About 6,670 psf
- D) About 15,000 psf
Correct Answer: C
Technical Rationale: Pad area = 3 x 3 = 9 sq ft. Pressure = 60,000 / 9 = about 6,670 lb per square foot.
An outrigger jack on an all-terrain crane in Halifax will put a maximum force of 250 kN onto a square engineered timber mat 2.0 m on each side. The lift engineer must compare the pressure with the soil's allowable bearing pressure in the site report, which is given in kPa. What is the ground bearing pressure?
- A) 31.25 kPa
- B) 125 kPa
- C) 500 kPa
- D) 62.5 kPa
Correct Answer: D
Technical Rationale: Mat area = 2.0 x 2.0 = 4.0 m². Pressure = 250 kN / 4.0 m² = 62.5 kPa.
Key Examination Takeaways & Code Review Checklist
When reviewing ground bearing pressure and outrigger pad sizing ahead of your official sitting:
- Time Allocation: Maintain a strict pace of approximately 90 to 100 seconds per question. Never spend more than 3 minutes stuck on a single calculation.
- Cross-Referencing: Verify table footnotes and exception clauses in CSA Z150 Safety Code for Mobile Cranes—exam questions frequently test specific conditions rather than general rules.
- Elimination Protocol: Immediately discard distractors that violate fundamental Canadian Occupational Health and Safety (OH&S) standards or invert standard diagnostic sequences.
🎧 Watch & Listen: 30 Mobile Crane Operator (339A) Practice Questions & Worked Answers
Weighted to the official Red Seal Mobile Crane Operator (339A) exam breakdown, read aloud with a 5-second pause before each worked answer and code explanation.
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