Bridges aren’t supposed to have weight restrictions on them. That is, they don’t come with weight restrictions on them when they’re new. So a bridge with a weight restriction on it is a sign that something has gone wrong and the bridge does not meet current standards.
The maximum weight that a vehicle is allowed to carry on the Interstate System per federal law is 80,000 pounds gross vehicle weight (with a max of 20,000 pounds per axle). That’s 40 tons. That limit applies to every inch of pavement, not just the bridges. Since this is a known cap, a new Interstate bridge will be designed to accommodate an 80,000 lb GVW load on it. You could say the bridge’s weight limit is 80,000 lb/40 tons but that doesn’t really have much meaning, because a load higher than that would be illegal to transport on public roads anyway, and the road leading up to the bridge has the same weight restriction. (In practice, the bridge doubtlessly will be designed to have a little bit of let to it just in case some idiot tries to squeak by a few hundred extra pounds.)
Now, note that that law applies to the Interstate System only, because the federal government only has a governing interest in the Interstate System (and other roads that together make up something called the National Highway System) because they partially fund it. Most long-distance roads are owned and funded by the states. The states could theoretically set lower standard weight limits and/or design bridges with lower weight limits...but in practice they don’t.
One, because all of that 80,000 lb GVW traffic on the Interstate system has to go somewhere when it exits the system.
Two, because a group called the American Association of State Highway and Transportation Officials (AASHTO, who are best known for picking the road numbers) maintains a catalog of standard components for making bridges that meet Interstate System requirements. Engineers are expensive on a per-hour basis, so if you can direct your engineer to use standard components and make a standard bridge, that’s a lot cheaper than having them design a bridge from scratch to go over the creek in Nowheresville. As a result, most new bridges meet Interstate standards and have an 80,000 lb GVW rating even if they aren’t on the Interstate system. (This is also why all new bridges kind of look the same, but we’re not worried about how boring the bridges are for the sake of this post.)
So a bridge only has an explicit weight limit if it has been damaged in some way (through failure to properly maintain it usually) or because it predates the application of Interstate System standards and the standard AASHTO bridges.
Older bridges often have other problems in addition to the weight limits: many older designs are what we call “fracture critical”, which means that if one component of the bridge fails the whole thing collapses. Modern bridge designs have redundancy designed into them so that if one beam fails the other beams will carry the load until the damaged beam can be replaced. Older bridges also often don’t meet other standards, like height (16 ft clearance) and width (12 ft per lane plus 14 ft for shoulders) requirements.
Biden isn’t advocating eliminating weight limits and letting it be a laissez-faire free-for-all where trucks can just go wherever they want. He’s advocating for replacing bridges that carry weight limits with new ones that don’t have them.
