way of kings Bridges

[Neithan]

How can Sadeas’s bridge crews push bridges over chasms without the bridges falling into the chasms? Pushing something that heavy over a chasm should result in it falling in unless every chasm is very narrow. The crews aren’t described as doing anything to the bridges to hold them up while pushing them. 

[Calderis]

@Kurkistan found this WoB a few days ago, and I felt it needed to be shared here. It answers all of the issues. It appears that Brandon had considered all of these things and either took them for granted in the writing, or they were edited out.

Quote

Questioner

How does a bridge crew set a bridge down across a chasm in front of them?

Brandon Sanderson

So... I may have to act this out.

*Proceeds to act it out, no video available*

I am Kaladin, running at the front of the bridge, right? Now there's a chasm there. I'm going to step out to the side and grab the poles to the side, and lower the front bridge down. Then these guys are going to lower theirs down while I run around to the back and grab the poles on the side, which extend out. And we're going to get several ranks and then we're going to shovetogether.

We're waited on this side, remember, we've got the weights that roll to one side or the other. So it's weighted on this side, so we get it over and then plot it down. We're looking for a place where we're a little bit above-- with the paths, alright? And then we go to the other side, your reposition the weights or the poles or whatever it is on the thing so that it's heavier on this side and then get everyone and then you pull it back across and then you can lift it.

Questioner

Okay, great, thank you very much.

Brandon Sanderson

Now also keep in mind a couple of things: 0.7 Earth gravity, a very important aspect for actually making this bridge weight work for carrying it; and it is a soulcast wood. The only way to get the numbers to work if you've got engineers, the weight-- you actually have to realize that we've got a wood that is slightly stronger, yet lighter, than what-- it's like a-- Peter came up, it's like a balsa, there's a soulcast wood that is really a great wood for this sort of thing.

That doesn't mean it's light, it's still a really heavy book

Questioner

*Laughter*

Brandon Sanderson

A really heavy bridge.

So there are counterweights on bridges that just aren't mentioned properly in the text. 

[Shadowmancer]

As I understand it, the bridges are significantly longer than the chasms themselves. The bridgemen pushing on the back of the bridge also adds a bit of weight. So the bridge doesn't extend past its center of gravity/tipping point, and therefore doesn't tip over.

Edited October 19, 2017 by Shadowmancer

[Fatling]

Yeah I was thinking the same thing as shadowmancer. But now that I think about it some more, there’s what, like forty people in a normal bridge crew? So twenty on each side, maybe three feet apart from each other? It seemed like they were pretty close to each other, but any closer than that and they would be tripping over each other. If the bridge is sixty feet or so in length, and it needs to be at least twice as long as widest chasm they will regularly encounter, that means the chasm is like thirty feet across. Did it say if the Parahendi can jump across all chasms or just the shorter ones? I just looked up the world record for the long jump and it’s about 29 feet. Seems legit. Someone will have to check those assumptions for accuracy, though, since I don’t have the book with me

[FiveLate]

The bridges were also made out of a super light weight wood.  Maybe something like bamboo?  Also, the use of some ropes attached near the front with Bridgemen near the back  pulling while the rest pushed would cause the front end to pivot upwards? Just some idle thoughts on how to make it work.

[Neithan]

1 hour ago, Fatling said:

Yeah I was thinking the same thing as shadowmancer. But now that I think about it some more, there’s what, like forty people in a normal bridge crew? So twenty on each side, maybe three feet apart from each other? It seemed like they were pretty close to each other, but any closer than that and they would be tripping over each other. If the bridge is sixty feet or so in length, and it needs to be at least twice as long as widest chasm they will regularly encounter, that means the chasm is like thirty feet across. Did it say if the Parahendi can jump across all chasms or just the shorter ones? I just looked up the world record for the long jump and it’s about 29 feet. Seems legit. Someone will have to check those assumptions for accuracy, though, since I don’t have the book with me

Early on in TWoK, it is stated that he bridges are “over 30 feet long,” so 60 feet seems unlikely. 

[Wreith]

WoK ch 6

Quote

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

I'm not exactly sure how that would work in both directions... guess you take a different path back.

[Alfa]

Wasn't there also something about counterweights on the bridges? Maybe there is a WoB about it, i don't know. I think he talked somewhere about it, but I don't know anymore and I don't have access to theoryland.

[amflare]

19 minutes ago, Alfa said:

Wasn't there also something about counterweights on the bridges? Maybe there is a WoB about it, i don't know. I think he talked somewhere about it, but I don't know anymore and I don't have access to theoryland.

I think on Dalinar's bridges there were.

[Wreith]

5 minutes ago, amflare said:

I think on Dalinar's bridges there were.

Quick search for "counter" only shows counterweight in reference to Navani's lifting fabrial in WoR.

I think you just have to make the logical assumption that 40 guys pushing at the back of the bridge is gonna have some downward force to it.

[Neithan]

7 minutes ago, FiveLate said:

The bridges were also made out of a super light weight wood.  Maybe something like bamboo?  Also, the use of some ropes attached near the front with Bridgemen near the back  pulling while the rest pushed would cause the front end to pivot upwards? Just some idle thoughts on how to make it work.

That rope thing is done in the Dragonsteel Bridge Four chapters, but isn’t mentioned in The Way of Kings.

[FiveLate]

Yup.

[kmosiman]

The biggest issue with the bridges is mapping out the route. There are permanent bridges close to the war camps, but the Parshendi occasionally destroy them in night raids so they don't extend out too far from the camps and the watchtowers. The upside of the permanent bridges is that they are probably longer than the mobile units, so they make it easier (faster route) to reach the outer plateaus.

The path to a given plateau is probably never straight since the bridges need to be placed in an area that is level with or higher than the next plateau and close enough to reach with a mobile bridge. In some cases the route back may not be the same as the route out because the plateaus don't match up correctly. This was part of the risk with the final expedition. If they didn't map their path correctly they could have been stuck without a way back.