Enacted by Congress in 23 CFR 658.17, Bridge Formula B mathematically limits the maximum weight allowable on any group of two or more consecutive commercial truck axles (W = 500 [LN/(N-1) + 12N + 36]). Its purpose is to prevent heavy concentrated wheel loads from producing excessive bending moments, fatigue cracking, or structural collapse in highway bridge spans.

Even if total Gross Vehicle Weight is legal (e.g. 78,000 lbs) and individual tandems are legal (33,000 lbs each), the vehicle can commit a major bridge violation if the internal axle spacing between the drive tandem and trailer tandem (the "inner bridge") is too short. For example, a 4-axle group spanning only 26 feet is limited by Bridge Formula B to 57,000 lbs; carrying 66,000 lbs on that inner span creates a 9,000 lb federal violation.

A standard tandem has axle spacing of 40 to 96 inches (typically 49 to 54 inches) and is legally capped at 34,000 lbs on interstate highways. A spread tandem with spacing greater than 96 inches (typically 120 inches / 10 feet) is legally classified as two separate single axles, allowing 20,000 lbs on each axle (40,000 lbs total for the group), provided outer bridge limits are satisfied.

Standard interstate legal limits without permits are 20,000 lbs single, 34,000 lbs tandem, and 80,000 lbs gross. Loads exceeding 80,000 lbs up to approximately 120,000 lbs require routine state DOT overweight permits. Loads between 120,000 and 160,000 lbs require route surveys, certified escorts, and bridge engineering analysis. Loads above 160,000 to 200,000 lbs are classified as Superloads, requiring highway patrol escorts and specific bridge load stress modeling.

The 5th wheel slider transfers tractor weight between the front steering axle and the rear drive tandem. Sliding the 5th wheel forward moves weight onto the steer axle and relieves the drive tandem. Sliding it backward moves weight onto the drive tandem and lightens the steer axle. Improper positioning can overload steer tires beyond their maximum sidewall ply rating (e.g. 12,350 lbs).