The reason why we still have roman bridges standing is because the stress contribution of a human walking on it is basically zero. But start bringing in cars and trucks and see it crumble in a week
Pedestrians are heavier than cars in terms of pounds per square foot.
Bridges are designed for extreme loads, including pessimistic pattern loading, and the requirements for pedestrian bridges aren’t any less safety critical.
(edit: I designed this bridge: https://www.google.com/maps/@47.597616,-122.3297804,3a,75y,7...)
Where are you from? I haven't seen an "annoyingly narrow" ped/cycle bridge in Sweden or Belgium.
You're not American or British, ehe? ;)
I’d say most of London’s busy pedestrian/cycle bridges and tunnels are annoyingly narrow.
Even taking cyclists out of the equation, they can get congested at times, particularly with tourists stopping to take photos from the middle etc.
So a cyclist can either be polite and move at pedestrian speed, which is annoying for the cyclist. Or weave and dodge pedestrians at speed (typically Deliveroo/UberEats riders on e-bikes) which is annoying and dangerous for pedestrians…
Love this quote: Friday marked the 32nd anniversary of the walk across the Golden Gate Bridge, an event The Chronicle's Peter Hartlaub once referred to as "the largest clusterf— in Bay Area history where no one actually died."
I admit that I chuckled at this.
Let's try to find some data ... ... ... A fully loaded Hanson cement lorry seems to be 32 tonnes / 2.55 m / 9.15 m, which is about 1.4 tonnes per square metre. That corresponds to 20 x 70 kg people per square metre, which would be a disaster whether or not the floor gives way, unless it's a very special kind of crowd (acrobats or something).
One has to consider what is daily use, not just the exceptional cases.
generally it's the exceptional case that it's designed around though
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.
Their question is: is this assumption correct?
You also have to take failure modes into account, and how degraded the bridge can get before it's "unsafe".
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.
In terms of maximum load you are right, but wear and tear is completely different.
Trucks are terrible, pedestrians and bikes have zero impact.
Truck loading is very large, but pedestrians do have a non-zero impact - look at the solid marble steps of ancient buildings, for example.
Pedestrian and bike infra for majority of cases in dense areas is objectively better in all metrics except human laziness and entitlement
There are some Roman bridges that still get automobile traffic (Römerbrücke, Puente Alcántara) or were only recently pedestrianized (Puente Romano). There are more probably, that's just what I found after skimming some Wikipedia pages.
If that would be the case, armys would still march on bridges. But they don't.