The best thing about pedestrian/cycling bridges and tunnels is that they cost soooo much less money to build since they have so much laxer requirements.
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.
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.
> “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…
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).
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.
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.
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.
> 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.
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.
Tell that to the engineers of the Millennium Bridge in London. Resonance from foot traffic caused it to start swaying, requiring shoring up with massive dampers.
No matter the traffic, a bridge should always be built to avoid resonance. Unstable dynamical systems do not care if it's a truck or a feather, they'll resonate and collapse.
>But start bringing in cars and trucks and see it crumble in a week
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.
I'm a Notjustbikes superfan and love my bike, but container ships are a marvel of efficiency and emissions per kg per km are probably better for the ship.
1 tonne = 1000kg so 16 / 1000 = 16mg CO2 per km per kg
Of course, that will massively depend on diet, conditions, and whether the bike trip was taken purely for transporting goods or whether I needed a workout anyway and it's a two birds with one stone situation. Then again, the sheer efficiency of container shipping might be why I take it as given I can buy some random cheap item manufactured 10,000 km away for a few euro and thereby bring about Jevon's paradox.
I'd still take a EuroVelo route to Canada though if it's on offer!
I feel like the greatest trick they recently played on us is to focus everything on CO2...
CO2 is but a small part of the equation, container ships release other nasty things. Even for cars co2 isn't the main issue, particulates from tires and brakes will fuck you up faster than any amount of co2
> The two main pollutants from a ship’s emissions are Nitrogen Oxides (NOx) and Sulphur Oxides (SOx). These gases have adverse effects on the ozone layer in the troposphere area of the earth’s atmosphere which results in the green house effect and global warming.
Well yeah, if container ships wouldn't be as horribly efficient, manufacturing everything in China, or shipping various stages of garments halfway around the world, which leads to the finished product having traveled more than those that have worked on it will in their lifetime, wouldn't be possible. The problem with container ships is not just the CO2, but also that they burn the worst of the worst fuel (https://en.wikipedia.org/wiki/Fuel_oil#Environmental_issues).
Imagine a cyclist who exists on a diet of beans, beer and dairy. Perhaps slightly lower CO2 emissions but vastly more methane and various mercaptans :D
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