Predicting Weight Distribution from Occupant Load Using a Monte Carlo Method

The Federal Aviation Administration (FAA) and Coast Guard recently adapted increases in the average passenger weight used to calculate load and conduct safety analysis and tests in multiple modes of transportation. The Federal Transit Authority (FTA) has proposed similar measures. The increased passenger weight requirements were created in response to the Center for Disease Control's (CDC) documented rise in weight among the country's citizens and followed crash or failure incidents in which a cause was overweight equipment. The current certification requirements under CFR 49, Part 567 state that Gross Vehicle Weight Rating (GVWR) of a motor vehicle shall not be less than the sum of the unloaded vehicle weight, rated cargo weight and 150 pounds (68 kg) times the number of designated seating positions. Actual occupant weight distributions versus certified weight per occupant seat causes a potential conflict between a vehicle's in-use weight versus its certified GVWR. A Monte Carlo method using a midsized bus example was conducted to determine the statistical probability that adult passengers and rated cargo would result in weight distributions that exceeded tire load capability, Gross Axle Weight Rating (GAWR), or GVWR. A discrete distribution of occupant weight modeled on the 2006 CDC anthropometric reference data was utilized. The analysis examined buses loaded with fewer occupants than seating positions by assuming a uniform probability that a seat would be occupied. Results demonstrated that load conditions and usage restrictions can be identified that decrease the probability of operating in a condition that exceeds a weight rating.