Cracking the Code: Understanding FHWA Bridge Law and its Impact on Intermodal Drayage

Greetings Drayage Community!

Today, we're diving deep into the world of the Federal Highway Administration (FHWA) Bridge Law and how it affects not only ContainerPort Group, but the entire intermodal drayage community.

In 1975, Congress enacted the FHWA Bridge Formula, which requires all trucks to comply with federal weight limits in order to prevent bridge damage and ensure safety on the road. This formula is based on the concept of axle spacing, which means that the closer the axles are together, the less weight they can carry without causing damage to the bridge.

Now, you may be wondering, "How does this affect me?"

As a general rule of thumb If you are loaded, you will almost always travel with a 20’ slider chassis in the “Open Position” to provide additional axle spacing as to stay in compliance with the axle spacing bridge law. In most cases using the most common tractors such as Freightliners and Internationals you can haul roughly 39,000 LBS using a tandem axle 20’ slider chassis assuming the load is evenly loaded and not all to the front or the back of the container. Conversely, using a 20’ TRIAXLE slider chassis, you can generally get roughly 44,000 lbs of cargo into a container and remain compliant when in the “open position”. The extra axle the triaxle provides adds to the overall length allowing that cargo to get up to 80,000 gross and more if you need to permit higher.  Obviously, this will be affected by the type of tractor you have. Day cabs are usually a little shorter wheelbase and are lighter overall. It's best to measure using the tractor you drive daily to ensure you know that your truck and chassis combo are compliant when hauling a loaded container.

Please note that if the container is EMPTY the chassis can travel in the closed position.

It's important to note that if a driver is not compliant with axle spacing requirements in the bridge law due to the equipment not being in the open position, it is considered the fault of the driver. It's essential to slide the equipment to the open position to allow the weight to be hauled on the highway in a compliant manner.

If a driver fails to extend or open a 20’ chassis, while it’s loaded, the overweight fine that may potentially be assessed will not be something that we would NOT be able to bill back to a customer. It would be billed back to the driver. It would also result in CSA violation points that stick to a driver for 3 years.

To help you better understand how to comply with these regulations, let's take a closer look at the FHWA Bridge Formula. This formula is based on the following equation: W = 500(LN/N-1) + 12N + 36), where W is the maximum weight in pounds that can be carried on any group of two or more axles, L is the distance in feet between the extreme axle, and N is the number of axles in the group being considered. This may seem complex, but it's crucial to ensure that your equipment follows FHWA regulations.

Bridge Law

To further illustrate this point, let's consider an example.

If you're hauling a 20' container with a 20' triaxle chassis, the maximum weight limit is 44,000 lbs. If the chassis has an axle spacing of 4 feet, the maximum weight limit for that group of axles is 22,400 lbs. If the spacing is increased to 8 feet, the maximum weight limit for that group of axles is 44,800 lbs.

As you can see, the spacing between the axles plays a crucial role in determining the maximum weight that can be carried. To view the Bridge Formula table click here

Compliance with FHWA regulations is crucial to ensuring the safety of our roads and bridges. As a company that values safety above all else, we at ContainerPort Group take these regulations seriously and encourage all members of the intermodal drayage community to do the same.


Mark Castaneda Bio


About the author: Mark Castaneda is a Regional Safety Manager headquartered in Kansas City, MO and has been with CPG since 2016.