Bridge-design

[Galladon he/him]

Hi there this is my first post and besides I am from Germany so bear with me here. :)/>

My question is this: "how are the bridges designed".

This might seem like an easy question but I didn't yet find an easy answere to it.

Sure there are plenty of describtions in the book but the concept is seriously lacking in detail, eg.:

- The construction underneith which is used to push the bridge over how is it retracted?

- How do you adjust for different hights?

- what do you do whenst you are on the other side?

the only solution I see is that the construct underneith must be symetrical to each side so you can do the same thing retracting as pushing over.

I am sorry if this post is confusingly written but I would greatly apprechiate any input as to the actual (or possible) design of the bridgecrews bridges.

[Branf she/her]

Hi! I'll try my best to answer your questions.

In my understanding the bridges are indeed symmetrical, with the bridge men turning around to return so that those who were at the back are now at the front, etc.

As for different heights I remember a passage where Kaladin describes shorter men padding their shoulders to match the taller bridge men. I'm not sure how much more detail I could give you with out the book in front of me but I hope this was helpful.

[Galladon he/him]

Oh sorry this was not what I had in mind with different heights, I ment between the plateaus.

As to the other parts, the pushing over is easy and clear but I imaging the retracting like trying to drag a log of firewood across an open barrel with your fingertips, The resistance is just too great.

[Kurkistan he/him]

More on heights: IIRC, most bridge teams were intentionally made up of men of similar heights, with only our beloved Bridge Four having a wide spread.

[Galladon he/him]

Again I am sorry I know about carrying those bridges but try pushing a matchstick over a hole with sides that are different heights.

Plateau-heights!!! sorry about the confusion

[king of nowhere]

2 options for that:

1) cross the chasm in a place where you are higher

2) use a sort of ramp so that the bridge will point upwards

I have a design in my mind but I am unable to describe it.

[Honor]

I think if examined too closely the way the bridges are described and function may not be entirely realistic. As to height differences, I always imagined the plateaus being at very close to the same elevation, due to the way the bridges are described as being pushed and pulled across. Again, in the real world, that would not be very probable. And if the far side side is higher than the side you are on, I don't really see a realistic way you would be able to quickly push the bridges across as described in the book.

I don't know that there is much specific description on how exactly the bridges are contructed to be pushed and pulled across. I agree that it would be a difficult task to push a bridge weighing hundreds of pounds (perhaps 1 ton) across rocky ground, there would be a lot of friction if there isn't wheels or at least some sort of narrower skids to support the weight.

I do seem to recall the bridges being long enough to easily cross the span and leave enough length on what ever side you are on to push or pull it across.

[Meg]

I like quoting :-)

It seemed that they'd intentionally chosen a point where the chasm was narrow and the first plateau was a little higher than the second. The bridge was twice as long as the chasm's {is this a typo?} width here.

(TWoK, Chapter 6, Bridge Four)

Aside from the abrasion of the bridges, wouldn't the (sure existing) lots of loose (smaller or bigger) stones work as, err, little wheels?

[Windrunner he/him]

Well Roshar, the Shattered Plains in particular, is mostly flat rock, with few loose stones and the like. They get blown away into chasms if they're small enough, or if they're too big, they gradually become one with the plain, due to crem. Not a lot of loose stones lying around.

[dj26792 he/him]

the bridges are described as exceptionally light weight and polished wood does slide quite easily especially on well weathered and therefore slippery stone, as to differing heights I can only speculate that they deal with it by lifting the bridge once its almost all the way across, it would be difficult but with ropes it would be quite doable and reasonably fast if you had enough man power on the ropes, which was obviously not a problem due to the abundance of slaves.

[Senor Feesh]

Hard to describe without diagrams, but using the quote Meg provided -

If the bridge length is more than twice the width of the chasm, then simple physics will see you get the bridge across with no worry about losing the bridge down the chasm, and no need for rope.

Consider the lip of the first chasm as the pivot point. Because of the length of the bridge, at least half the bridge will remain over the first plateau whilst the front is being pushed into empty space. By the time enough of the bridge is beyond the lip to cause it to pivot downwards, the lead end of the bridge is already over the second plateau, and will simply slide along the ground. Once everyone has crossed, they simply pull the bridge from the other end, with the same physical principle stopping the bridge from falling.

I believe there's also a quote in the book that the return route will differ due to the necessity of needing the landing point on each plateau to be lower for this to work (though that may be an invention of my brain trying to support this method of plateau traversal).

[Inkthinker]

Man, I wish we hadn't lost our TWG thread. We did something like 10 pages on this subject, working out the math.

The end result, as I recall, was more or less as Senor describes. Part of the problem is that Brandon never clearly states how wide the average chasm is. We ended up deciding that the widest (navigable) gaps are probably no more than 15-20 feet across, which allows the 40-foot bridge just enough leverage. Opposite plateaus must be always slightly lower than the current one, though they don't have to be lower by much, and it's important to remember that few if any plateaus have a level, even surface... it's not uncommon for two adjacent plateaus to both have points that are higher than their neighbor, thanks to uneven ground.

Think about the realistic ability of an average, athletically fit long jump... most people can probably barely make a 6-7 foot gap comfortably, and that's unencumbered. Add armor and weapons, and that's even more unlikely. Now, consider that you don't want your men arriving at the battle exhausted from marching and leaping across chasms, so you probably drop the bridge for any gap more than 3-4 feet across. The average gap is probably about ten feet wide, which is still WAY further than you want to try and jump across, but plenty short enough for a 40-foot bridge.

TL;DR the chasms are probably not as wide as you may imagine.

The record for long jumps is just over the 25-foot range, but bear in mind, again, this isn't about setting records... it's about repeated leaps on long marches over uneven terrain. The Parshendi can grasshopper their way across the Plains, but humans don't do that action. If you can avoid having your soldiers do any leaping at all (especially as a failed jump would mean losing a man and equipment) then that's the way you want to handle it.

Honestly, I'm less confused about Sadeas's bridges than I am about Dalinar's chull-towed towers. How the heck do they get THOSE across the gaps they're bridging? Presumably in stages (tower 1 bridges, tower 2 crosses, tower 2 bridges, tower 1 crosses, etc) but I've yet to suss out the design that lets the tower cross over the gap that it bridges itself.

Edited February 15, 2013 by Rubix
Please use the Edit button instead of double posting. Thanks!

[Chicken]

Chasms are narrow enough that scouts can jump over them with poles. It was somewhere in the book. Also they never go back the same route so they always push from high to low.

[Rubix he/him]

Man, I wish we hadn't lost our TWG thread. We did something like 10 pages on this subject, working out the math.

You mean this TWG thread? It's all still there, it's just hosted on our site in a read only mode.

[Windrunner he/him]

I could be misremembering, but I think that Brandon said that there's a chance that some diagrams of the bridges will show up in Stormlight 2. As for the original post about how the bottom looks, I have some ideas. I might do a diagram and post it here.

[Satsuoni he/him]

Ok, kvothes incoming:

It was a wooden bridge, around thirty feet long, eight feet wide.

He’d been a little low in his assessment; looked like about thirty-five to forty men per bridge. There was room for five men across—three under the bridge and one on each side—and eight deep

As mentioned above:

It seemed that they’d intentionally chosen a point where the chasm was narrow and the first plateau was a little higher than the second. The bridge was twice as long as the chasm’s width here.

Hence, "narrow" chasm is about 15 feet, aka 5 meters - not something you can jump easily.

They weren’t natural, these chasms. This one started narrow, but as it ran toward the east, it grew wider—and deeper—incredibly quickly. At only ten feet long, the crack was already wide enough that it would be difficult to jump.

Though you can jump with a stick:

The scouts crossed chasms with jumping poles, moving very lightly from plateau to plateau without the need of bridges.

Depth:

In some few places, the distance from the chasm floor to the edge of the plateau above was only about forty feet. In most places, however, it was closer to a hundred or more.

The chasms were wider at the bottom, perhaps a result of highstorms.

30 feet is about as much as Shardbearer can jump:

The chasms between these plateaus were unusually large, almost too wide for the bridges to span.

Those chasms might be too wide for a Shardbearer to jump

Bridge construction:

It sloped down at the front and back, and had no railings. The wood was thick, with the largest boards for support through the center. At least there were rods on the bottom to use as handholds. He couldn’t see a thing; there was an indentation for his head, but wood cut off his view to all sides.

And that is about it...

[dj26792 he/him]

I feel like the lack of rails makes it significantly more difficult to make the bridge rigid enough to extend across a 20 foot chasm... basic mechanics is that the thickness of the bridge dictates how rigid it will be, adding a rail would make it significantly stronger, even just a kerb would be incredibly beneficial.

Also I mentioned the ropes as a way of lifting the end of the bridge to make it easier to lift the bridge slightly to get it onto a slightly higher platform, they would obviously be unnecessary if the bridge was going to a arrive at the other side of the chasm higher than the other plateau.

[Galladon he/him]

Do the scouts really polejump? I allways thought they used those poles to lay it arcoss the chasm and then "climbe" to the other side. Besides how would that even work? Imean the chasms are way too deep to kind of "swing over" and you polejumping is mostly about height not distance.

My main thought was allways how to eliminate friction (cause if you don't you'll never be able to push it across- or at least I don't see it)

I had some interesting ideas concerning the bridges weight:

-so here are some calulations(sorry they are metric; I will translate the solution):

-jump.worldrecord: 9meters

-lower gravity (on rochar) + s.plate = jumpdistance of 15m = max.chasmwidth

-Bridge.length= 2*max.chasmwith (as seems the dominating opinion)

-low density wood: 500kg/m³

-Bridge.width=3m (3 Bridgeman underneith)

-max.carryweight(man) = 40Kg (low gravity)

-max.Bridgeweight=25(min number of men)*40kg= 1ton

-usage: Kavalary(500Kg horse, 100Kg Man, 100kg other)two abreast

-Calc.: m³ of wood= max.Bridgeweight/density= 2m³ of material

Bridge.thick= material/(length*wdith)= 2m³/75m²= 2.7cm = 1 inch = average possible thickness of bridge

I mean I am no strucktural engineer but that seems to me like it wouldn't work.

So lets assume that Shardplat is not that strong and dalinar could only jump with it 5m(6yards) this would leave us with 3inches there it becomes plausible to have a construction that could withstand a Cavalry charge, but it only leave enough space for max 13 people to push 1 ton over (frictionkoeficcient about 1.2). (bridge 2*5m)

If we said we met in the middle at 15m(bridgelength; 50') that would leave us with say 2inches maybe enough with some VERY special wood but still the friction and comparetively less men to push would leve them unable to push it across.

[dj26792 he/him]

it would be much more structurally efficient to make a compact pratt-truss bridge design which would be stronger and thicker for the same amount of material and most importantly more rigid, I would imagine that for most of its length the bridge is at least 300mm-500mm thick with sloped ends to bring it down to a smooth entry and exit. This would be required to make it rigid enough to extend over the chasm without bending down and being too low to end up on the other side of the chasm. this also means less material in contact with the ground and therefore significantly less friction.

A bridge is much better than a solid piece of wood. which is what your calculations seem based off...

(I haven't done any calculations, this is all estimated, but equally I am studying to be an engineer and last year did an assignment where I designed a bridge so its educated estimation)

[Galladon he/him]

I did those calculations to get the max.portable material which is 2m³ - give me some credit would I be able to do those calcs without knowing that a flat piece of wood is a stupid way to make a bridge?!!! The average thickness is just a way for everyone to visualize how mch material it actually is - I mean I cant exactly imagine what to do with 2m³ just by looking at the numbers so I broke it down to visualize.

Besides we know it's flat and has only wooden support beams ( I think 3 ?!).

I might not be a structural engineer but I am an engineer - so I know my physics(-:

By the way Friction is not dependent on the area formula is:

F(friction) = F(normal; in this case gravitationalforce)*Koefficient

[dj26792 he/him]

yeah sorry its late here and I'm half asleep I should probably come back and do this properly tomorrow, it would be good practice for my structural subjects this semester.

What I should have said was that if you want to do a more thorough weight analysis for this sort of application some variation of the pratt truss bridge design would be ideal, and further information on such a design can easily be googled, I stand by 3 inches being far too thin for the bridge to be structurally sound, it wouldn't have a big enough moment of inertia to resist even the lighter Rosharian gravity. Without doing the calculations I'll stick with my last estimate of 300mm and come back tomorrow or sometime in the next few days and design the lightest bridge I can that has a chance of supporting the necessary weight and see if it is light enough for 25 men to reasonably carry it.

[Galladon he/him]

thats a good idea maybe I'll draw up some specks myself should be interesting. yeah it's early afternoon over here^^

[Satsuoni he/him]

I did those calculations to get the max.portable material which is 2m³ - give me some credit would I be able to do those calcs without knowing that a flat piece of wood is a stupid way to make a bridge?!!! The average thickness is just a way for everyone to visualize how mch material it actually is - I mean I cant exactly imagine what to do with 2m³ just by looking at the numbers so I broke it down to visualize.

Besides we know it's flat and has only wooden support beams ( I think 3 ?!).

How did you arrive from

It sloped down at the front and back, and had no railings.

To "flat", Galladon? Seems more like an arc to me...

Also, I think you severely underestimate carrying power of a single human. I was one carrying an Omikoshi, which was about 500kg and was comfortably carried by about 10 people (two poles), comfortably enough to shake it up and down (on purpose), and nobody in there was particularly buff. It took considerably less people to lift the thing, and 25 is a minimum number for that, it seems:

takes at least twenty-five men to lift a bridge.

, so you can easily double your weight estimates, and one-inch thick wood is actually plenty, as long as it has proper supports. (looking at 3-inch wooden walls of his house and 1-inch wooden ladder) I think.

[Galladon he/him]

sure and now run for couple of hours with that... I think my estimate is quite sound since have you ever tried to run with someone of about 45-50 kilos on your back? if so I bet you didn't do it for long. ANd if that doesn't convince you how do you suppose the sidecarry was done with only 5 members more? try carrieing something at your side...

But even if you say 50 kilos this would only 25% more material (easy conversion : +10kg/pperson => 0.5m³ material)

As to the slight arc I thought for calculation-purposes it's easier to just leave it be since it can't be a big arc:

1. Cavalry needs flat terrain to charge so lets say elivation of max 1 meter for every 10)

2. The higher the arc ; the more complex and heavy the structure underneith becomes

but calculations only get you this far:

If you have time and ideas please draw a bridge-spec and proove me wrong- I'm trying to do the same right now

So here is a prelimenary "result", I have no CAD-Program at home so sorry for the quality of this...

[king of nowhere]

do we have any idea how much weaker is roshar's gravity? is it 70% of our? 50%? That would make a huge difference. I think on heart it would be impossible to make portable bridges like that that could support a chivalry charge. Remember that lower gravity means not only that the bridge will wheight less, but it will also need to support less wheight.

On the other hand, lower gravity means that the people will probably be less strong. big muscles are developed to carry your own body. We would be a bit like superman to the inhabitants of roshar.

Also, we have no idea what kind of wood they have on roshar. it is possible that they have some wood stronger than what we have on real world.

Also, thanks to galladon for the scheme.