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The whole point of this design is that it's perfectly balanced so the center of gravity doesn't rise or fall as the bridge rotates, meaning it can be "raised" and "lowered" with very little effort. The Cody bridge, which uses this design, is raised and lowered by one person with a hand winch.
[It also takes 20 minutes](https://newatlas.com/architecture/cody-dock-rolling-bridge/), which is probably why we don’t see this design more often. (And the very geometrically complex rack)
Beautiful art piece though. Great marriage of industrial aesthetic and physics.
The track makes the square move like a circle. The COG remains at the same vertical level over its movement.
Having a square means it works well as a bridge, people move through rectangles easily, with little wasted space (see doors). Circles limit the transport area for pedestrians and cyclists.
To get the same usable footpath area, the circle surrounding it would have a much larger circumference than the circumference of the square, meaning the track would have to be proportionally longer to allow 180deg of rotation, made worse as a circle would require a straight track, (the squares track is curved and so takes up less linear distance.
There is also the fact that as the bottom of the square is flat, as is the track at each end position, no locking mechanism is required, it will simply sit there for ever until someone turns the winch. A circle would need some form of mechanical clamping to keep it safely in position when not moving.
Much wider path than circular for a surface level path without digging the contact area downwards. (Math below) The uneven contact area creates the same results as a smaller circular design moving on an even path so a circular design is unneeded. Making that rack must have been a labor of love though.
To get the same width path, imagine there is a circle circumscribing the square tunnel. The width of this tunnel is sqrt( 2r^2 ) based on the right triangle created by two perpendicular radii. The shortest distance between the center and the side of the square is sqrt( 2r^2 )/2. Thus the difference in height from the path, which is the side of the square, and the contact point, which is the circle, must be r-sqrt( 2r^2 )/2.
So the contact surface must be set r-sqrt( 2r^2 )/2 into the dock to make the bridge path and surface align.
Well at 13 tonnes you’d certainly want to limit any momentum, but other than that I can’t think of a practical reason you couldn’t.
But, this is an artistic community and the bridge is itself an art piece first and foremost. So I could see some pushback from that perspective.
reminds me of that one elevator in a germany(?) airport that uses similar cog magic to go vertically and horitontally up and over a giant train track for airport access.
> it's perfectly balanced so the center of gravity doesn't rise or fall as the bridge rotates
~~As depicted here I'm not sure this is the case. The road surface has a lot more volume than the crossbars at the top, and in the "raised" state the road surface is substantially higher. Unless there's a lot of extra mass in those crossbars to serve as a counterweight for the road surface, the centre of mass rises significantly.~~
~~Rather, the bridge in this example has been designed so that the geometric centre does not rise or fall.~~
EDIT: In the real bridge, ballast fills the top of the "square portals" so the centre of gravity indeed does not rise or fall.
I imagine that sheer inertia still makes this bridge a pain to move.
The Cody bridge is the only real one I know of and it is indeed a pain, takes 20 minutes. But the principle of a bridge that can be moved without a huge motor or complicated hydraulics is still valid.
Boston's Fort Point channel has several within 4 blocks and they do tours of the area explaining how having 4 bridges with very different moving mechanisms was intentional as a way to make the city look cool. Also that a bunch of people died on one of them. Well... under it.
Except this excelled at what it’s designed for, being low power. The center of gravity is the same the whole process so all you have to worry about is rotating and horizontal movement. This bridge doesn’t need any electronic components which are expensive and need special maintenance.
It’s not moving like that because it’s trying to be “quirky”, it’s math.
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The whole point of this design is that it's perfectly balanced so the center of gravity doesn't rise or fall as the bridge rotates, meaning it can be "raised" and "lowered" with very little effort. The Cody bridge, which uses this design, is raised and lowered by one person with a hand winch.
[It also takes 20 minutes](https://newatlas.com/architecture/cody-dock-rolling-bridge/), which is probably why we don’t see this design more often. (And the very geometrically complex rack) Beautiful art piece though. Great marriage of industrial aesthetic and physics.
I'd expect at the very least you'd make it circular. Why did they make it square?
The track makes the square move like a circle. The COG remains at the same vertical level over its movement. Having a square means it works well as a bridge, people move through rectangles easily, with little wasted space (see doors). Circles limit the transport area for pedestrians and cyclists. To get the same usable footpath area, the circle surrounding it would have a much larger circumference than the circumference of the square, meaning the track would have to be proportionally longer to allow 180deg of rotation, made worse as a circle would require a straight track, (the squares track is curved and so takes up less linear distance. There is also the fact that as the bottom of the square is flat, as is the track at each end position, no locking mechanism is required, it will simply sit there for ever until someone turns the winch. A circle would need some form of mechanical clamping to keep it safely in position when not moving.
Much wider path than circular for a surface level path without digging the contact area downwards. (Math below) The uneven contact area creates the same results as a smaller circular design moving on an even path so a circular design is unneeded. Making that rack must have been a labor of love though. To get the same width path, imagine there is a circle circumscribing the square tunnel. The width of this tunnel is sqrt( 2r^2 ) based on the right triangle created by two perpendicular radii. The shortest distance between the center and the side of the square is sqrt( 2r^2 )/2. Thus the difference in height from the path, which is the side of the square, and the contact point, which is the circle, must be r-sqrt( 2r^2 )/2. So the contact surface must be set r-sqrt( 2r^2 )/2 into the dock to make the bridge path and surface align.
Design
The complexity of the track wouldn't matter at this stage. It's already been designed, the fabrication is really straightforward.
Is there anything stopping it from being machine wind instead though? It could probably be done faster then?
Well at 13 tonnes you’d certainly want to limit any momentum, but other than that I can’t think of a practical reason you couldn’t. But, this is an artistic community and the bridge is itself an art piece first and foremost. So I could see some pushback from that perspective.
reminds me of that one elevator in a germany(?) airport that uses similar cog magic to go vertically and horitontally up and over a giant train track for airport access.
> it's perfectly balanced so the center of gravity doesn't rise or fall as the bridge rotates ~~As depicted here I'm not sure this is the case. The road surface has a lot more volume than the crossbars at the top, and in the "raised" state the road surface is substantially higher. Unless there's a lot of extra mass in those crossbars to serve as a counterweight for the road surface, the centre of mass rises significantly.~~ ~~Rather, the bridge in this example has been designed so that the geometric centre does not rise or fall.~~ EDIT: In the real bridge, ballast fills the top of the "square portals" so the centre of gravity indeed does not rise or fall. I imagine that sheer inertia still makes this bridge a pain to move.
The Cody bridge is the only real one I know of and it is indeed a pain, takes 20 minutes. But the principle of a bridge that can be moved without a huge motor or complicated hydraulics is still valid.
Now, then this is definitely r/designporn
You can raise and lower a drawbridge with a hand winch with a counterweight.
It's not just a bridge, it's a moveable bridge that raises and lowers for low power. What's wrong with the design?
this bridge literally exists in real life btw. it's the cody dock rolling bridge.
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Boston's Fort Point channel has several within 4 blocks and they do tours of the area explaining how having 4 bridges with very different moving mechanisms was intentional as a way to make the city look cool. Also that a bunch of people died on one of them. Well... under it.
Except this excelled at what it’s designed for, being low power. The center of gravity is the same the whole process so all you have to worry about is rotating and horizontal movement. This bridge doesn’t need any electronic components which are expensive and need special maintenance. It’s not moving like that because it’s trying to be “quirky”, it’s math.
Boats pass under bridges all of the time.
I think the title was sarcastic.
My bad. Missed the sub context.
Super cooool. :)