A mobile crane is lifting a 15,000-pound load at a 60-foot radius. The crane's load chart shows a capacity of 18,000 pounds at this radius. What is the actual safety margin for this lift?
Correct Answer
B) 16.7%
The safety margin is calculated as (18,000 - 15,000) ÷ 18,000 = 0.167 or 16.7%. While this is within the crane's rated capacity, many companies require higher safety margins for critical lifts.
Why This Is the Correct Answer
The safety margin is calculated by finding the difference between the crane's rated capacity and the actual load, then dividing by the rated capacity. With an 18,000-pound capacity and a 15,000-pound load, the margin is 3,000 pounds. Dividing 3,000 by 18,000 gives 0.167 or 16.7%. This represents the percentage of unused capacity available as a safety buffer.
Why the Other Options Are Wrong
Option A: 20%
20% would result from incorrectly calculating (18,000 - 15,000) ÷ 15,000 = 0.20, which uses the actual load as the denominator instead of the rated capacity. The safety margin formula requires the rated capacity as the denominator.
Option C: 25%
This is incorrect because 15,000 pounds is less than the crane's rated capacity of 18,000 pounds at 60-foot radius. The lift is within safe capacity limits, though the safety margin may be considered low by some company standards.
Memory Technique
Remember 'CLC' - Capacity minus Load, divided by Capacity = safety margin percentage
Reference Hint
OSHA 1926 Subpart CC - Cranes and Derricks in Construction, specifically load chart requirements and capacity calculations
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