Ok I'm clearly talking to an expert here but isn't there a factor of vibration from vehicle traffic to take into account? And sure humans if packed into a bridge might be heavier than cars per square foot, but they're usually not packed like sardines on a bridge.
Sure but again, just like packing people onto a bridge like sardines, it's an unusual circumstance. While vehicles driving all day over a bridge is not unusual. And even vehicles getting stuck on a bridge due to some traffic issue is not unusual either.
One has to consider what is daily use, not just the exceptional cases.
We have a car bridge designed to withstand exceptional load, and a pedestrian bridge designed to withstand exceptional load. The commenter assumes that on average, the car bridge's load is much closer to it's maximum load than the pedestrian bridge's load, and in consequence, the average wear on the car bridge should be higher than on the pedestrian bridge. As such, the pedestrian bridge should have a much longer lifetime, and the commenter assumes that this is the reason old Roman bridges are still standing.
It probably is not correct, because the pedestrian bridge max load is probably something like "what if an idiot drove a car down it" whereas the car bridge max load is "what if it was full of very heavy trucks and one burst into flame".
You also have to take failure modes into account, and how degraded the bridge can get before it's "unsafe".
Somewhat. There are a bunch of 'failure' modes that you want to avoid (either actual failure or service level issues), and some bridges are going to be closer to one mode than another.
It's a little like memory pressure vs cpu vs disk vs network. There are some services that aren't going to ever hit one of them because the others are limiting first. Memcached is never going saturate disk.
If you've got a bridge that's 'stiff' or 'heavy' (like truss/beam/whatever) resonances are unlikely to be an issue. If you've got anything light and flexible or with cables, you need to be thinking _hard_ about resonance, in all the modes, torsional being one that's bitten designers bad in the past.
