due to vehicle weight limits. I don't know what the limits currently are but they were something like 65,000 pounds for the entire vehicle back in the late 1970s. The limit could be higher today but I doubt it's high enough to allow a 40 ton payload. I may be wrong but I suspect the maximum payload you could carry on a single truck would be somewhere in the range of 20 tons, give or take a few.

Furthermore, if I were going to try to blow out a section of a bridge, I'd use a demolitions technique known as tamping to direct most of the blast downwards. Using that technique, you'd have about half or less of your weight as explosives covered by a layer of something heavy, such as sandbags. Using tamping, a good demolitions man can cut a massive steel beam with just a few pounds of C4.

Perhaps the easiest "solution" during this warning period is to prohibit heavy trucks from the bridges. Failing that, then set up a portable inspection station on the approaches to the bridges and inspect each truck with bomb sniffing dogs. That would definitely have an economic impact because it'd slow the movement of the trucks but it'd really reduce the chances of someone damaging one of the bridges.