To produce extreme force effects, the rare axles spacing shall be varied between which range?

• A. 2 to 6 ft
• B. 30 to 60 ft
• C. 6 to 12 ft
• D. 14 to 30 ft

Answer: (D)

Understanding how a moving load interacts with a bridge hinges on where the axles sit relative to the supports. The farther apart the axles are within a practical range, the larger the moment arm you can create between the load and the supports, which increases the support reactions and the bending moments in critical sections. This is why spacing the axles within a mid-to-long range tends to produce the most severe force effects on the structure.

The range of fourteen to thirty feet captures realistic long wheelbases found in heavy vehicles, allowing the load to straddle multiple supports and align with critical locations on the bridge. Spacings much smaller concentrate the load too closely and reduce the lever arm, yielding less extreme effects. Spacings much larger, while possible, are less representative of typical vehicles and often don’t produce the worst-case responses for common bridge spans. Therefore, the fourteen to thirty feet range best tends to maximize the potential force effects.