Re: Tarkalor's Bridge

From: Joerg Baumgartner <joe_at_...>
Date: Tue, 5 Feb 2008 15:07:48 +0100 (CET)


Me:
>>> Tarkalor brought trade to Whitewall. Is it just me,
>>> or would that mean
>>> two-way travel across the bridge, i.e. two wagons
>>> wide?

Jane
>>Your logic is impeccable, but sadly removes the
>>dramatic possibilities of a bridge narrow enough to be
>>held by a single hero (or group of PCs).

Not really. Think of the Glienitzer Br�cke between western Berlin and Potsdam during the Cold War - lots of boulder-like blocks leaving only a snaking way through.

>>> The bridge spans a rather long gap.

>>True. And shortening it would make it too easy for the
>>gap to be bridged by other means.

>>> If it is a single arch rather than a
>>> viaduct with lots of columns downward, ...

>>If. Is it? I believe we drew it as a viaduct.

The artist's impression has a single long span across.

>> My
>>engineering days were a long time ago, but it seems to
>>me that that's a long way for a single span. I can see
>>four possibilites rather than two:
>>1) Arch - stone built, keystone in the middle, steep
>>up and down.

Every massive element also keystone-shaped.

>>2) Viaduct (what are the central pillars resting on,
>>and how tall are they?)

If I recall correctly, we are talking about a 100 m deep canyon here. The bridge was placed between two rocky outcrops probably overhanging a bit, on the far side from Whitewall there was a hilltop named (builder's) leap. (Sestarto isn't thought to be the builder any more, so we might as well change that hilltop name to fit.)

>>3) Suspension bridge. There's a nice White Cliff to
>>attack to at the city side, what do we have at the
>>other? Towers?

> Seems to rely on too modern a technology.

Agreed. The classical suspension bridge is a rope bridge, which probably was there during construction.

>>4) A really long, strong, single-piece slab - stone,
>>wood, whatever. It couldn't be done in RL, but Mostali
>>magic - maybe.

> Have to be stone, anything else would eventually perish.

Actually, could be two slabs of stone meeting in the middle.

I just came across an article about mathematicians calculating how much overhang you can get by simply stacking slabs of whatever. The classical answer (the harmonic stack) yields more than a full slab length for each side. More creative solutions (with counterbalance) yield up to 4 slab lenghts. While I don't expect Tarkalor to have had slabs stacked, this does offer some mechanical possibility if the bedrock on either side is part of the counterweights. All it takes is good rock for the slabs, and a slight inclination so that you do get a slight arch. And rock should be no great problem - plenty of useful varieties around.

> A fifth alternative
>
> Given that viaduct is a series of arches with a level
> platform between the tops. There is nothing to stop a
> single arch being built with the top on a level with
> the sides of the chasm and stones being used to fill
> the gap. Tacky ASCII drawing follows:
>
> __________ __ __________
> �********_/ \_********�
> �******_/ \_******�
> �****_/ \_****�
> �**_/ \_**�
> �_/ \_�
>

This (or a less massive solution with secondary arches resting on the main arch) is what I expect, too. The road top usually is allowed to rise somewhat, saving weight.

Another, picturesque way to do such a bridge can be seen here (right where I live):

http://upload.wikimedia.org/wikipedia/commons/8/83/Levensauer_Hochbr%C3%BCcke_%28um_1900%29.jpg

The road (and railway track) goes between the two arches. (The topmost layer is roofing...) The arches on that bridge are steel, of course, but the span may be wider than the Whitewall gap. (The bridge is 42 m above the water surface, for a scale)

>>> Backing up a heavily loaded wagon is hard. A trade
>>> route ought not make that a normal event.

>>I agree. If we were to go with the arch option, would
>>a passing place (and control point) at the top of the
>>arch make sense?

> Alternatively there could be a traffic control system
> with a guard at each end and a signalman in the middle.
> There could also be traditional times when priority is
> given to traffic in particular directions.

True, but that doesn't quite fit the image of Broyan, does it? What is the royal office called, signaller of the bridge? Chief traffic controller?

> Thinking about it a mid passing point isn't going to work
> unless only a few wagons are using it. If you have six
> wagons going into the city and eight coming out you need a
> passing point long enough to take the six wagons while the
> eight go past.

Besides, the middle is about the worst place for this, construction-wise.

> I'm not too bothered about making the bridge two and a bit
> wagons width. It only gives a frontage of eight to ten men
> so a party with a few followers could hold it. Not that
> holding the bridge that way makes sense - Lunar missile
> troops will shoot them down pretty quickly in such an
> exposed place.

As soon as no Lunar forces keep trying to push them back.

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