3048: Suspension Bridge

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Suspension Bridge

Title text: As a first step, they can put in a secondary deck, to help drivers try it out and find out how fun the jumps are. After a while no one will use the old flat deck and they can remove it.

Explanation

A suspension bridge works by exploiting the strength in tension of a cable, or series of links, in what is usually described as a catenary curve (but see later) suspended between towers or other elevated positions and firmly anchored to the ground at either end. Such a cable, or parallel cables, can span a large gap, across which an arch (with compressive forces) or cantilever (with compression below and tension above) bridge structure would be more difficult. The slung droop of the connection, and various other issues (the susceptibility to resonance from moving loads, as well as the sheer impracticality of travelling along this link) means that the surface of the usual road itself (or footway, in the case of a non-vehicular bridge) is suspended from this cable by vertical (and perhaps diagonal) suspending stringers of suitable lengths to maintain a more level track. This usually means that the greatest clearance beneath the traversable part of the bridge is not far below the middle of the main curve of the cable, which has the closest (or direct) attachment to the bridging 'deck'. Unlike the free-hanging cable, the addition of the weight of the road and the large number of vertical cables required now forces the cable to take the path of a similar parabolic curve, rather than a true catenary, and may also have to further flex significantly according to wind, temperature and the changing loads of passing traffic with significant mass.

Randall suggests 'improving' the suspension bridge by having just the catenary curve, needing much less structure, and giving an increased clearance for anything passing beneath (in this case, a tall-masted sailboat) if they pass closer to the supporting towers. This, of course, means that the traffic can only cross upon the cables themselves, in a way that is not explained at all by this side-section view. Due to the steep nature of the way the cable must pass over the supports, this produces a steep gradient up to and then down from the tower which sends traffic temporarily onto a free ballistic trajectory, assuming it has enough speed. This is described as "fun jumps", as if it is how bridges should work. This would likely inflict damage on the car, and might result in unfortunate accidents. Some bridges are designed for traffic to go on top with a cable underneath, but these are only used for shorter spans and can flex significantly.^{[citation needed]}

The title text suggests a gradual introduction of this new form of bridge, a practice which is common when introducing new large-scale societal changes. According to Randall, the bridges would at first be a sort of hybrid between the old and new design, where any driver could choose between taking the catenary curve or driving along the tried-and-true flat road. As more and more people sample the "fun jumps" path, he claims, word will spread about how much 'better' this path is, and once it's been widely adopted, the flat road will be discontinued. In reality, a more likely result would be that drivers who take the catenary curve quickly spread word about how dangerous this path is (and/or spread themselves, and their wrecked vehicles, all around the terrain that the bridge is supposed to cross), and any usage would quickly disappear.

Transcript

[In one panel two different designs of suspension bridges are shown in cross section one above the other.]

[At the top a normal suspension bridge is drawn. It spans a water way with two towers about a quarter of the way from the banks on either side. The distance from the road part of the bridge to the water is about a third of the total distance from the bridge to the bottom of the water. The suspension cables goes above the straight road up to the top of the towers in a curve and almost down to the road in between the towers. Vertical suspenders (cables) hangs down from the suspension cables holding the road in place (8 on either side and 17 between the towers). On the road there are ten vehicles going either way, evenly spread out across the part of the bridge that are over water. There is a large truck, a smaller truck, a van and then 7 regular cars. Above the drawing is a caption:]

Traditional suspension bridge:

[Beneath this a different version of a suspension bridge is drawn. It spans the same water way, and also has the same two towers. But here ends the similarities. Instead of suspension cables holding a straight road, the road is now build in the same curve that the cables had in the first drawing. So the cars need to drive up to the top of the towers along a curves road and then down again in the middle between the towers. There are thus no cables of any kind. And no straight road at a fixed height above the water. On this hilly road there are also ten vehicles going either way. To the left three regular cars are driving up the steep incline toward the left tower. A fourth car has just past over the top, and a dotted line behind it indicates that it is flying above the road after having speed over the top. In the middle of the road between the towers where it is closest to the water, there is a large truck also going towards the right. The road is clearly bending under the weight of the truck. Near the top of the right tower another regular car is driving up the incline. Just above it near the top of the right tower another car has made a large jump out from the top of the tower going left, a dotted line indicating a quite high jump taking it a couple of car heights above the top of the tower. On the right side of the tower, two cars (one very small) drives left up towards the tower and behind them a final car drives down towards the right bank. There are three labels. The first is beneath the first section of the bridge to the left, where three arrows points to the underside of the road with the label beneath these arrows. The second is just right of the left tower where a double arrow indicate the height of the road above the water. An arrow point up to this arrow from the label that are written down in the water. Near the right tower, in the same spot as the double arrow is near the left tower, there is a sail boat with a high sail that can pass under the road near the tower, where the road is higher up than in the middle (or all the way in the normal version drawn above). The third label is written above the right tower, where an arrow points from the label to the dotted line of the car that makes the high jump from the top of the tower. Above the drawing is a caption:]

Improved suspension bridge:

[Label 1:] Less cable

[Label 2:] More clearance

[Label 3:] Fun jumps

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Discussion

What if a car doing one of the fun jumps lands on a car that didn't do a fun jump? Should we mandate that every car does a fun jump for this reason (and yay, fun jump!)? 198.41.227.115 22:08, 7 February 2025 (UTC)

Radar speed management, vehicles are only let on the bridge if their expected land point is between the vehicles around them. 172.68.55.80 22:48, 7 February 2025 (UTC)

Carrot/stick question. If they do a jump, they get their toll refunded on the far side of the bridge. Maybe give them double refund if they do a flip. 172.69.246.135 04:07, 8 February 2025 (UTC)

The same would happen as on a normal road. If you drive faster than the car in front of you and do not break you will run into it. Only difference here is that you cannot break after jumping so if the one in front of you did make a small jump and then hit the brakes you could hit him. But if you drive very fast up towards a car on a normal road, and they hit the brakes, you will likely also hit them. Even though you do have a chance to brake as opposed to after a jump. So I do not see this as any different than a regular road. Only problem is someone will find it so fun to drive fast towards the top, they might forget to think about safety. But that is already a real problem on normal roads... You cannot stop someone from entering the bridge based on their speed before thet enter the bridge... So the radar comment makes no sense to me!? --Kynde (talk) 08:20, 9 February 2025 (UTC)

Do suspension bridges still work when built this way? Those pilings look much easier to sway to me. Maybe they are just buried a little more securely. 172.68.55.80 22:48, 7 February 2025 (UTC)

Yep! With "classic" suspension bridges (like the Golden Gate Bridge), there is no fixed connection between the deck and the towers. With differring loads and temperatures the deck can move up and down about 30cm at tower level (and almost 2 netres at midpoint). Some modern bridges do ise a fixed connection or hinge between deck and tower. IIVQ (talk) 12:58, 8 February 2025 (UTC)

This looks a lot like a stressed ribbon bridge. 172.69.64.132 22:59, 7 February 2025 (UTC)

This exact proposal, the 'dangling road', is actually The Meta in Poly Bridge 2. Poly Bridge 3 tried to tweak the numbers to make this strat less effective, but a variant of it continues to persist to this day. (This probably goes in the trivia section, because it seems like Randall came to this concept from first principles.) 172.71.151.65 01:07, 8 February 2025 (UTC)

I came to the comment section to see if somebody already said that. The other thing I would have said is I wonder if RCE reads XKCD… if not, I think someone should ask him to do a blind reaction to this comic. I'd love to see his response :) -- Angel (talk) 19:18, 8 February 2025 (UTC)

It's funny to me, there's a level in Jak II for the PS2 that's literally just this idea: for whatever reason the city's central palace is held up by five suspension bridge-esque wires, and one level has you crawling along wire #4 to get inside the palace (the catch? The wires are covered in security robots) 172.71.159.7 01:54, 8 February 2025 (UTC)

I like the fact that vehicles are clearly on flexible cables, not some rigid material formed into a catenary shape. There's a truck between the two towers that is visibly depressing the cable it's riding. Nitpicking (talk) 03:57, 8 February 2025 (UTC)

Happy happy joy joy. No dogs allowed on this bridge. 162.158.42.87 04:35, 8 February 2025 (UTC)

If you add a downward pressure to a rope that's significantly stronger than it's normal tension, you end up with a parabola 162.158.137.212 17:47, 8 February 2025 (UTC)

Related: The Wooden bridge Tatzlwurm" in Essing Germany by Richard Johann Dietrich: [1]. Maybe it could be included in the explanation. Rps (talk) 20:46, 8 February 2025 (UTC)

Not sure I think this is really related. It is also possible as opposed to Randall's proposal here ;-) --Kynde (talk) 08:20, 9 February 2025 (UTC)

Another example is the Karlssteg in Freiburg im Breisgau, Germany. [2] 108.162.241.160 (talk) 15:46, 10 February 2025 (please sign your comments with ~~~~)

It would seem that Randall has re-invented the https://en.wikipedia.org/wiki/Simple_suspension_bridge 172.68.54.207 (talk) 15:17, 9 February 2025 (please sign your comments with ~~~~)

More so the mostly 'simple'-derived Stressed ribbon bridge. Unless you consider that development more a version of the non-simple type, with the descenders degenerated completely into the deck structure through high tension, i.e. a step beyond the "structure hiding" levels of other low-profioe suspension bridges. 172.71.178.78 15:37, 9 February 2025 (UTC)

I seem to recall a bridge on the overseas highway (Florida Keys) that was originally an arch bridge above a grade-level rail line. When they wanted to "upgrade" the route for cars, cars wouldn't fit into the narrow rail bridge, so they planked up and over the arches, and it was apparently quite a scary drive. I can't seem to locate it via google, although I'm sure I recall seeing it on my drive to key west about 20 years ago. I suspect it's gone now. RandalSchwartz (talk) 20:34, 9 February 2025 (UTC)

Ahh, it was the Bahia Honda Railroad Bridge. A few pictures of the structure (mostly torn down now) can be seen here. RandalSchwartz (talk) 22:55, 9 February 2025 (UTC)

Find it on Google Earth at 24°39.3142'N 81°17.5182'W. Mostly still there as of image date (2023, if I read correctly). 172.71.146.78 15:43, 10 February 2025 (UTC)
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