Yeah, the potential loading is annoying. I remember wondering why predestrian/cycle bridges were always so annoyingly narrow despite having low loads. The issue is of course that they have to be strong enough to support being crammed with hundreds of people when everyone is there to watch the local fireworks/rowing race/anything else that's happening, even if that happens almost never.

If you built a rope-bridge, I'm sure people would beware of cramming it. Maybe it's possible to build designs that discourage.

Where are you from? I haven't seen an "annoyingly narrow" ped/cycle bridge in Sweden or Belgium.

> “Where are you from? I haven't seen an "annoyingly narrow" ped/cycle bridge in Sweden or Belgium.”

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…

>If you built a rope-bridge, I'm sure people would beware of cramming it.

You're not American or British, ehe? ;)

Or many other countries, I've seen enough video of bridge collapses to know that if people CAN cram on it, at some point they WILL cram on it.

This is interesting! But how does a dense crowd of people compare to a queue of lorries carrying cement or building materials in terms of mass per square metre?

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).

Lots of roads and bridges have a limit on axle load permitted on them. You can't do that with pedestrian bridges.

See also this story about the Golden Gate: https://www.sfgate.com/local-donotuse/article/golden-gate-br...

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."

> "There were cheers as some people started to hurl bicycles over the railing," he wrote. "A stroller tumbled down and sank beneath the waves 220 feet below. 'Throw the baby, too,' people yelled, laughing.

I admit that I chuckled at this.

there's more to it than just pounds per square foot. speed over the bridge matters too, which is why you get speed restrictions on some bridges

If these early '00 bridge builder games have taught me anything, you want to go as fast as possible, as you might reach the other side as the bridge is still in the process of collapsing ;)

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.

Famous example of pedestrian bridge resonances: https://en.m.wikipedia.org/wiki/Millennium_Bridge,_London#Re...

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.

> 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

I'm not sure I understand that comment. "Well, yeah, the bridge collapsed but it was an exceptional case. It worked fine for typical daily use."

Here is how I understand the comment:

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?

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".

Sounds good to me, ship it.

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.

> Bridges are designed for extreme loads, including pessimistic pattern loading, and the requirements for pedestrian bridges aren’t any less safety critical.

In terms of maximum load you are right, but wear and tear is completely different.

Trucks are terrible, pedestrians and bikes have zero impact.

> 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.

I think though that the pedestrian wear is only on the roadway surface, rather than structural wear from, say, trucks vibrating enough to crack the bridge or throw off bits of concrete.

That's some pointless nitpicking. Ok, humans have near-zero impact to the point of needing double-digit decades to erode a surface. Happy now?