Bridge-design

[Kurkistan he/him]

It's 70% of earths, for a 6.87 m/s^2 falling rate.

Also, I recall that the wood used is of the somewhat cliched "light, but strong" variety. I could have sworn that Brandon said somewhere that he had to cheat by using wood about as heavy as balsa, but I can't find it.

Edited February 14, 2013 by Kurkistan

[Galladon he/him]

wow that would make the density on 1/5th of the lowest I have yet found making roughly 10 m³ of material availabel and with that in mind one wold only need a contraption on the end of the bridge to facilitate a handhold for all bridgemen to push it across. Under those circumstances it's possible to make a 300kg bridge no more problems with friction and so forth and didn't expect a 25% gravitational ascelleration reduktion which would mean the bridge would theoretically only "weigh" 225kg (this is a minimum-calculation).

Changes a lot but still the question is this the best design, do you guys have better ideas of what sanderson had in mind?

[Windrunner he/him]

Here's a quote for you guys:

JOHN OTTINGER

Your battle system involving bridges and plateaus is both complex and innovative. In writing these scenes, was a significant amount of research necessary, and did you encounter any difficulties when writing the sequences?

BRANDON SANDERSON

Yes to both questions. This is not going to be immediately obvious, but the big difficulty was in designing bridges that were mobile but also strong enough to support a cavalry charge. It took a lot of research and talk with my editor, looking at the engineering of it and the physics of the world to actually be able to create these things. I'm sure fans are going to try to diagram them out. That was one aspect of it: how were the bridges going to be set?

I approached this first from a "how would you actually fight on these plains?" direction. But also I wanted to evoke the concept of a terrible siege, with a man running with a ladder toward a wall. And yet that's been done so much. The Shattered Plains came from me wanting to do something new. I liked the idea of battles taking place in a situation that could never exist on our planet, what it would require, what it would take out of the people, and how it would naturally grow. And so I did a lot of reading about siege equipment. I did a lot of reading about weights of various woods, did a lot playing with the length, the span between the chasms, etc. One thing that people should know if they are trying to figure all this out is that Roshar has less gravity than Earth does. This is a natural outgrowth of my requirements both for the bridges and for the size of the creatures that appear in the book—of course they couldn't get that large even with the point-seven gravity that Roshar has, but we also have magical reasons they can grow the size they do. That's one factor to take into account.

Source

I hadn't ever noticed the quote about the bridges being curves. It does quite nicely solve the problem I was seeing with horses being able to charge on top of them, considering if they were flat, either the end would stick up or they would have to keep the end so close to the edge that there would be a significant risk of falling.

I'm working on a model myself at the moment, we'll see if it turns out. I'm rather out of practice and the program I'm using isn't ideal for the situation. Never thought I'd miss my old school's engineering software, but I sure do now. XD

EDIT: Also, Galladon, please don't double post. I can't merge your two posts together without losing that image, but you should just edit your original post instead of posting again. Thanks!

RubixEdit: Windy, you can always merge the top post into the bottom post...

[Honor]

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

I think it says somewhere that four horseman could charge abreast across the bridges. 3 meters seems too narrow for that, it would probably be more like 5 or 6 meters wide.

[Senor Feesh]

If anyone ever needs evidence that we obsess about these things far too much, I will point first to this thread

Seriously though, the effort I see here is incredible. If I was an author, I could only hope to have such a dedicated fanbase.

[Inkthinker]

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

That's fairly close to the diagrams I've drawn up in my notes, though I had less curvature (making it a flat, hollow box with wedge-shaped ends). Then again, I also missed the quote on curves, and I'm also not a structural engineer. Though I do know arcs are stronger, so I'd say it's a smarter design.

It's 70% of earths, for a 6.87 m/s^2 falling rate.

Also, I recall that the wood used is of the somewhat cliched "light, but strong" variety. I could have sworn that Brandon said somewhere that he had to cheat by using wood about as heavy as balsa, but I can't find it.

At the risk of shooting myself in the foot, it's the stumpweight tree. Which is conveniently very straight and light, with large, easily stripped leaves. Sort of a cross between palm and pine, I think.

Man, I'm gonna feel stupid if that's wrong. I'm batting low on trivia, lately.

[Windrunner he/him]

Dude, you're awesome! I just might have to wiki that fact

[Inkthinker]

Eeeeehhhh... I suggest seeking validation on the subject of whether or not they're the wood that the bridges are made from, but my notes suggest that it's so.

But those notes are from the 2009 edit, and Brandon changed the book up a bunch from there. It's probably right... Unfortunately, a quick search of my e-book edition reveals that the stumpweight tree is mentioned only three times in the canon text, and none of the mentions are in conjunction with the Shattered Plains, much less the bridges.

On the other hand, you can mix the sap with rockbud seeds and paste that all over a leeward surface to grow food. So... that's good to know.

The illustration locks down the visual at least, and so it's still pretty much the perfect wood for building anything. The stumpweight is practically purpose-designed as a commodity; they grow relatively straight, not very tall, and with an even grain. It does everything but actually grow in planks.

The only downside to the stuff is that it's hard to get planks more than 6 or 7 feet long.

Edited February 15, 2013 by Inkthinker

[Quandmeme]

If anyone ever needs evidence that we obsess about these things far too much, I will point first to this thread

Seriously though, the effort I see here is incredible. If I was an author, I could only hope to have such a dedicated fanbase.

 

No, I'm confused.

 

I am reading listening to WoK for the first time and am struggling to imagine the bridges. I'm listening to the audio book so I can't quote passages.

 

On the first day there is something about not being able to see when carrying the bridge. But then when moved to the front it is easier to run because you can see out.

 

Then on the day that Khaladin first "wakes" up to being a bridge leader some of the wounded are sheltered behind the bridge. I'm wondering whether it's more of a structure, and thanks for the drawing, but I can't see how that it is dropped in place from one side of a chasm.

[Cromptj he/him]

Here are the dimensions of the bridge, as described by Kaladin, "around thirty feet long, eight feet wide". This limits the maximum size of the chasms crossed by the armies at around 15ft across. This also means that you definitely cannot have a four abreast cavalry charge. You might be able to get four horses on the bridge, but even two side by side would be a very tight fit. The bridge would probably have to be at least 8-12 inches thick as well because it would have to support at least two more likely four, probably medium to heavy cavalry, which could exert a force of at least 3700-4700N each (adjusted for Roshar gravity). I have no idea how much force a horse kicks off the ground with, but an average medium warhorse weighs around 400kg on earth plus a rider of between 80-120kg depending on size of person and weight of armour so it seems about reasonable.
Assuming dimensions of around 10m x 2.5m x 0.25m, this gives us a volume of 6.25m^3. Since they really need about 25 bridgemen to carry a bridge in battle, the maximum any of them can carry while running is probably around 40-60kg on Earth. 60x25x0.7 is 1050kg. This gives the wood a density of 168kg/m^3, about the same as balsa wood.

Edited February 27, 2014 by Cromptj

[Humph]

There are quite a lot of unknown variables that still need to be nailed down. 

 

However, I get the impression that there are bridgemen underneath the bridge while carrying it, with portions of the bridge resting on their shoulders.  That implies that the bridge is composed of a top planking system with joists underneath, with a depth of joist a minimum of 14" -16" to allow for the headroom of the carriers.  Let us assume a minimum of 4 joists underneath the walking surface, two at the edges, and 2 in the middle.  Let us also posit say 5' of overlap of bridge with the plateau at either end, for an effective span of 20'.

 

In modern construction, a single 16" deep steel joist can span 20' while carrying a uniform load between 368 lbs/linear foot for a 16K2 joist to 550 lbs/linear foot for 16K5 joists and above; with 4 joists, that works out to 1,472 - 2,200 lbs/ft; i.e. a total uniform load between 15 and 22 tons that can be supported.

 

What we don't know is how to compare the strength of Rosharian wood to earth wood or steel.  If we assume say about 10% of the strength of steel [based off of 50ksi steel vs. 5 - 8 ksi strength of wood on earth], we would have an allowable uniform loading of 1.5 to 2.2 tons.  [Disclaimer:  this is actually an incredibly simplistic assumption.  Wood strength depends upon a number of factors, including grain direction].  Allowable uniform loading is much different than point loading, which would occur say when a single horse walks across the bridge.  Given the reduction in weight on Roshar it seems conceivable that a wooden bridge could support the weight of single horse/rider WALKING across the bridge over a 20' span.  However, a horse RUNNING across the bridge might be an entirely different story, as then we start having impact loads as well.

 

The top planking system would not need to be super thick either, as each plank would be spanning about 30" between supports.

 

One point to keep in mind, the deeper the bridge is, the stronger it would be.  A change of only 2 or 3 inches in depth can result in a fairly significant increase in strength, as the section modulus varies with the square of the depth.

[Inkthinker]

I'd forgotten this thread. I did a bunch of diagrams about a month ago, which are in another thread here somewhere... Isaac and I were more or less in agreement with the latest I did, though it did have the unfortunate design flaw of providing too much cover to the bridgemen.

-EDIT-

There we go: http://www.17thshard.com/forum/topic/5213-bridge-specifications/

Or if you just want the short version:

It ain't flawless, but it'll do for the moment. Suggestions and corrections (based on engineering or the text of the novel) are always appreciated.

Edited March 1, 2014 by Inkthinker

[The Rooster he/him]

That's so horrible. Prefect!

[Chrono she/her]

Inkthinker, I swear it says in The Way of Kings that the bridges are constructed out of Makam wood, which would presumably come from a Makam tree? It's in a Kaladin viewpoint somewhere in there.

[Awesomeness Summoned he/him]

Chapter 30:

All mobile bridges were made of a kind of wood called makam. It had a deep brown color, the grain almost hidden, and was both strong and light.

They use makam because it's strong for it's weight to support a heavy cavalry charge.

[Inkthinker]

Whup, there you go, text rules.

 

[Cromptj he/him]

Kaladin does say that the bridge is around 30 feet long, if the ramps made it longer, I think he would have mentioned them...

[Humph]

 

It ain't flawless, but it'll do for the moment. Suggestions and corrections (based on engineering or the text of the novel) are always appreciated.

 

Great artwork!

 

From an engineering standpoint, the inked design has two main load-bearing elements, at the edges (the trusses).  You have some lines in the middle that are inked lighter and so I assume they are meant more as ties than as load bearing elements.  The bridge would be stronger if you also have some additional primary members running underneath down the middle, for 4 or 5 main load bearing elements. This would also have the added benefit of cutting down the span of the bridge deck, enabling it to be thinner and of less weight.  These can also be in "truss" formation rather than solid elements.

 

You do correctly indicate the need for bracing tying the trusses/joist members together -- these prevent a mode of failure called lateral-torsional buckling, wherein the bottom chord of the truss or joist moves out of the vertical plane of loading, which would destroy its load-carrying capacity.  The number required would depend on a host of factors, including loading, member sizes of the chords and webs of the joists/trusses, truss depths, strength of the material etc.

[Galladon he/him]

so I tried to do the bridgedesign again on blender. this Is how I imagined them.  was really fun to make.

The curve gives more stability, place for 40 bridge men, supportbeams, curve landingpoles to compensate for different plateauhights.

Edited March 2, 2014 by Galladon

[dyring]

Love the artwork. One issue with it. (goes for both Inkthinkers and the last model by galladon)

 

(and someone correct me if I´m wrong, book loaned out so going from memory, wich despite frequent rereads is not always the best;), hard at work trying to get more ppl hooked on Brandon here;)

 

When they run towards the shaft at one point with Kaladin at the front, there is a notice about arrows hitting around his head. If there was a ramp such as your artwork have, that would be imposible, his head would be in cover.

 

Is it not possible that there are poles extending in front wich the runners in front(and back) are gripping? Thats how I always imagined them. Poles running along the length of the bridge, and a planked surface ontop, again with poles extending to the sides, front and rear to serve as hand/shoulderholds.

 

the drop in front and rear does not need to be that great a problem. A step close to a foot high, both horses and people can just step up and down that as they get on and off the bridge.

 

The pushing over procedure I´m glad to see is the exactely the same as I thought myself:)

[Natans he/him]

Wow very slick like I liked =)

 

Bonus points because the bridge floats!!! The gravity spren addition are nice touch

 

 

Take this upvote it my regards Crafty Sir =)

Edited March 3, 2014 by Natans

[Awesomeness Summoned he/him]

Bonus points because the bridge floats!!! The gravity spren addition are nice touch

 

Didn't you know, that's how Kaladin will eventually fly. He's gonna ride a bridge around like a flying carpet.

[Natans he/him]

Didn't you know, that's how Kaladin will eventually fly. He's gonna ride a bridge around like a flying carpet.

 

 I must politely  disagree this is a wrong assumption, Kaladin will never fly. He only be able to fall upward

[Inkthinker]

Kaladin does say that the bridge is around 30 feet long, if the ramps made it longer, I think he would have mentioned them...

 

My excuse is that Kaladin is a surgeon's son, not a carpenter's. He says "around" and "about" not "exactly", so I'm taking advantage of the weasel words.

[Inkthinker]

 

When they run towards the shaft at one point with Kaladin at the front, there is a notice about arrows hitting around his head. If there was a ramp such as your artwork have, that would be imposible, his head would be in cover.

 

Is it not possible that there are poles extending in front wich the runners in front(and back) are gripping? Thats how I always imagined them. Poles running along the length of the bridge, and a planked surface ontop, again with poles extending to the sides, front and rear to serve as hand/shoulderholds.

 

the drop in front and rear does not need to be that great a problem. A step close to a foot high, both horses and people can just step up and down that as they get on and off the bridge.

 

The pushing over procedure I´m glad to see is the exactely the same as I thought myself:)

 

Hence the flaw I mentioned, my bridge offers too much cover.

The thing is that the text also mentions arrows slamming into the wood beside his head... meaning that somewhere in the design, he needs to have wood on his shoulders or near/behind him. One idea that was given to me for mine is that the front rank places the leading edge on their shoulders, rather than the first crossbeam behind the ramp, so the first rank is more exposed and the wood is slamming into the ramp's leading edge.

I don't like the idea of too much break between the ground and the ramp edge, if only because any sort of leap or step up when men and horses are charging is a recipe for someone to trip and create a domino disaster. Maybe a couple inches (I've got the runners lower than the lip myself), but certainly not a foot.