FYI the posted load limit on a bridge is not the weight that, if exceeded once, would cause the bridge to immediately collapse. Rather, it’s the weight that, if exceeded regularly, would cause the bridge to require repairs and maintenance much sooner than otherwise. This is why school buses (and emergency vehicles like fire trucks) are legally allowed to ignore the posted weight limits on bridges. I’m a school bus driver and this is actually one of the questions that comes up on the school bus certification exam (there’s also a bridge in my district with a 5 ton weight limit that a number of our buses regularly cross despite weighing about 10 tons empty).
Even if the authorities wanted to post an absolute, it’s-going-to-collapse weight limit for a bridge, it wouldn’t really be possible, as this would have more to do with per-axle weight, ground pressure and vehicle speed, not to mention length of the vehicle compared to the length of the bridge. Nor would authorities want to post any such number without a large safety factor.
FYI the posted load limit on a bridge is not the weight that, if exceeded once, would cause the bridge to immediately collapse. Rather, it’s the weight that, if exceeded regularly, would cause the bridge to require repairs and maintenance much sooner than otherwise. This is why school buses (and emergency vehicles like fire trucks) are legally allowed to ignore the posted weight limits on bridges. I’m a school bus driver and this is actually one of the questions that comes up on the school bus certification exam (there’s also a bridge in my district with a 5 ton weight limit that a number of our buses regularly cross despite weighing about 10 tons empty).
Even if the authorities wanted to post an absolute, it’s-going-to-collapse weight limit for a bridge, it wouldn’t really be possible, as this would have more to do with per-axle weight, ground pressure and vehicle speed, not to mention length of the vehicle compared to the length of the bridge. Nor would authorities want to post any such number without a large safety factor.