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Thursday, January 18, 2024

A Portable Bridge for DF Felltower

My players, like the original group back in the days of Raggi's Roughnecks, want to build a bridge.

Sadly, the notes about the cost and weight of that bridge are gone - they're not on the blog that I can find, and I can't find any notes in my email that details the cost. I know it was based on a GURPS Low-Tech ladder, but more costly. One of my players started there.

A wooden ladder costs $10 and 2.5 lbs x the square of the length in yards. So a 24' ladder is $640 and 160 lbs. By my figuring, a ladder isn't quite the tool for the job . . . and isn't designed to lay flat with a potentially heavy weight (say, a knight or barbarian carrying a fallen friend, or a pair of delvers with a heavy chest or stretcher between them) right in the middle.

So I just double the numbers to $20 and 5 lbs, ending up with $1280 and 320 lbs for a 24' portable bridge, with minimal spacing between planks, side rails, carrying handles, and loops on the ends to fit over iron spikes to keep it from shifting. It's a two-person job at least to carry it, four is better, and it's potentially looped for six to carry it.

Those are the stats I'm going with unless someone can provide me with specs that make sense and an explanation of why that is so. "That seems expensive and heavy" isn't convincing. It's a custom job, since there isn't much call for man-portable 24' briges in general situations, and the weigh doesn't seem crazy for a reinforced bridge designed for heavy loads. If someone else knows better I'm all ears, but I'll need something to back up the numbers.

9 comments:

  1. A 24' bridge, but how wide is the opening to be crossed. The deflection of the bridge and how much it can carry will depend a lot on how wide the gap spanned is. I'll get out my building with dimensional lumber engineering book tomorrow and take a look at weight vs span for different lumber pieces.

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    1. Here are some rough, back of the napkin estimates, without engineered drawings, etc.

      Dry 2 x 12 x 24 weighs ~132 pounds (this is modern dimensional lumber, ancient old growth timber would be bigger, substantially heavier and stronger), need between 3 and 4 joists plus internal ladder structure for a 2 foot wide "bridge" assuming no major knots or flaws which weaken the members
      That is 528# for just the "joists", plus fasteners/rope bindings, plus any framing/decking to make them into a "bridge" (I would GUESS adding 30 - 50% to total would be close)


      Green 4 x 12 x 24 weighs ~ 585 pounds (this is modern rough sawn lumber, ancient old growth timber would be substantially heavier and stronger), need 2 plus internal ladder structure for a 20 inch wide "bridge" assuming no major knots or flaws which weaken the members
      That is 1170 pounds just for the joists, ", plus fasteners/rope bindings, plus any framing/decking to make them into a "bridge" (I would GUESS adding 30 - 50% to total would be close)

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    2. Very helpful, thanks. So given dry wood, my guess wasn't too far off . . . we might just be able to go with $1280 and 640 and call it a day for a sturdy portable bridge.

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  2. I figured we'd do a rope bridge. My suggestion in the discord chat was anchor iron rings/hooks on either side; depending on the ceiling height you could put a few rings in the ceiling too. Accessible via Walk on Air, and the core support of the bridge could easily just be a cargo net, then lay some planks across to prevent a misstep from leading to the sort of crap we're trying to avoid.

    I've got zero interest in turning Felltower into a civil engineering exercise. A lay-flat ladder or permanent wood-frame bridge isn't the way to go here, though I could think of ways to modify a ladder that WOULD be suitable.

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    1. Whatever you guys feel works for you and are willing to front the cash for, that's fine with me. I just want to make sure the numbers aren't wholly Madeuppium and Guessium blends.

      And that we figure all that between sessions.

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    2. I'm totally fine with Madeuppium, but the MakerUpper better have a really strong explanation of how the established in world magic system makes it work.

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    3. I generally reserve that for GMs. And while DF does allow Faerie-made gear, I generally reserve that for a small subset of gear . . . not including portable bridges.

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    4. Comon, I bet the fae would make you a portable bridge ... two acorns wide, a half-walligator long, with a weight capacity of one and a half cats.

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