What do bridge formulas determine regarding axle weight?

Bridge formulas are critical in regulating how much weight can be safely carried by axles on vehicles, particularly as it pertains to the impact on road infrastructure, including bridges. The correct answer focuses on the relationship between axle spacing and weight distribution.

The formula is designed to ensure that vehicles do not exceed certain axle weight limits when the axles are close together. When axles are spaced closely, the impact on the road surface and bridge structures can be significant. Thus, the bridge formula effectively allows for less maximum axle weight when axles are positioned closer together. This is to prevent the excessive concentrated load that could potentially damage roadways and bridges, which have weight limits designed to protect their integrity.

The other options address misleading points. Some suggest that closer axle spacing would permit greater weight, which contradicts the purpose of the bridge formula. Others incorrectly imply that tire size or vehicle size significantly alters the fundamental principle of the bridge formulas concerning axle weight limits. The bridge formula strictly applies to the spacing and load distribution across axles to maintain safety and structural integrity on the road.