Bridge Formula Calculator

Because Formula B is protecting bridges from concentrated load, not protecting the road surface or the truck's own components. A given total weight applied over a longer wheelbase spreads the bending stress a bridge span experiences over more of its length, so federal law rewards a longer axle spread (larger L) or an added axle (larger N) with a higher legal weight for that axle group. This is why a 5-axle tractor-trailer can legally gross 80,000 lbs while a 2-axle straight truck cannot -- more axles spread farther apart carry more weight without exceeding any single bridge span's design load.

Because federal law (23 U.S.C. 127) sets 34,000 lbs as an independent ceiling on any "tandem axle" -- two axles spaced more than 40 and not more than 96 inches (3.33 to 8 ft) apart -- regardless of what Formula B alone would allow at that spacing. Once a 2-axle group's spacing passes roughly 7 ft, Formula B's own number climbs above 34,000 lbs, but the tandem cap still governs. This calculator reports the lowest of the Bridge Formula result, the 34,000 lb tandem cap (when it applies), and the 80,000 lb federal gross weight limit as the effective legal weight for the group.

No -- it evaluates a single axle group, the same unit the Bridge Formula itself governs and the same way a weigh-station officer checks a scale ticket. A tractor- trailer is checked group by group: steer axle, drive tandem, and trailer tandem or tridem each have their own spacing and weight, and each must independently satisfy Formula B before the overall 80,000 lb gross vehicle weight is even relevant. This calculator itself checks Formula B and, when the group is a 2-axle tandem, the 34,000 lb tandem cap -- it does not check the 20,000 lb single-axle cap, since that limit applies to a single axle rather than an axle group and this tool's minimum group size is two axles; verify a steer axle's weight against 20,000 lbs separately.

It rearranges Formula B to solve for the axle spacing L that would make your entered Actual Weight exactly legal under the formula alone, holding the number of axles fixed -- in effect, how far apart the axles would need to be spread to carry that weight. It does not account for the independent 34,000 lb tandem-axle cap or the 80,000 lb GVW cap, both of which apply regardless of spacing. If Actual Weight already exceeds 80,000 lbs, no amount of axle spread makes the load legal -- Formula B only governs how weight is distributed across a group's axles, not the truck's total weight, so the fix in that case is removing weight, not spreading axles further. Likewise, for a 2-axle group already at or under the 34,000 lb tandem cap, no extra spacing is needed; the Min Spacing figure is only a real route to legality when Formula B itself, not one of the two independent caps, is the binding constraint.

No -- those are the federal limits for the Interstate Highway System under 23 U.S.C. 127. States can and do set different limits on non-Interstate roads, and many issue overweight/oversize permits that allow higher weights on specific routes under specific conditions. Bridges can also carry their own posted weight limits below the federal formula's result. Always confirm against the specific state and route before relying on a Formula B calculation alone for a real trip.