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Bridge Capacity

   
Bridge Capacity

I have a causeway supported by pillars of a known thickness at a known spacing. The thickness of the road surface is known.

Questions:
What is the the maximum local load of the bridge (e.g. if a giant walks across it)?
What is the maximum total load of the bridge (e.g. if an army of giants walk across it)?
How do the load tolerances change as I vary pillar spacing, surface thickness, pillar thickness, and materials?

In short, they're planks on sticks. No trusses. No arched support. No suspended support. Assume the pillars are sunk in and immobile

A lot would depend on the condition of the bridge. If it where badly maintained wooden bridge it might have difficulties standing up to the strain of a single medium sized individual.

Maximum total load would be difficult to calculate. The bridge would likely fail multiple times at localized points before the entire bridge would fail.

My best answer would be that the bridge would be as strong as it needs to be to carry out it's function, and a little stronger to stand up to unforeseen demands. No one would take the time and effort build a bridge capable of supporting a load of thirty tons if it is meant for light foot traffic.

I would say that as a general rule the weakest points would probably be capable of supporting a little under twice their expected maximum load.

Figuring out the load tolerances and variations of different parts of the bridge would be difficult, because they could vary wildly depending on construction techniques and materials used.

As I said in the shoutbox, it depends on style, technique, material and location.

Generally, a well built arch bridge should be able to support just about anything, as each support takes a share of the weight.

And, be sure to build with different materials, to avoid fun resonance collapses.

Assuming a general fantasy world and wooden bridge of a kind of simple footbridge construction (flat). I would say that the maximum it can carry is a heavy wagon per section, plus a little extra for safety. Bridges are horribly over designed usually. Rome built simple wooden plank bridges that could support a legion.

I would agree with odinps. Bridges probably can support about twice what they're going to be used for. What that means in terms of your situation depends on the game....

Really, it depends on which failure mechanism you're looking at. Flexure in an individual plank, flexure in an entire span, shear for a plank or a span, pillars failing in compression or flexure.

I suggest only looking at flexure for each individual span and compression for the pillar, just to keep things simple.

For a flexural failure, that's just when the giant is heavy enough that the span beneath him bends until it breaks due to the moment applied by the giant exceeding the moment capacity for the span. The moment the giant applies is a function of its own weight and the length of the span squared, as well as the location of the giant along the span, but just assume the giant is standing smack dab in the middle, since that's the worst case scenario.

For a compressive failure in a support pillar for a single giant, that's pretty obvious. Calculate the maximum axial force the pillar can take and if the giant exceeds that, then when the giant's standing on top of the pillar, it breaks.

Now, if you have a lot of giants, the point load of a single giant is instead a distributed load across the bridge. That's all. Intensity and span length squared are still the factors in flexure, and now span length becomes a factor in pillar compression, since each pillar is supporting the distributed load across a single span's length.

Moment capacity is a bit more complicated, as shapes and structures become far more relevant. And if you're using wood, it becomes even more complicated due to the odd nature of wood failure, in that it can fail and break, but still carry the weight until the giant crosses. It just damages the bridge badly.

Quote:
Originally Posted by P0L View Post
Usually, the bridge is as strong as needed for your plot.
That makes very little since, since the plot depends on the setting, rather than the other way around. You propose to build the story from the wrong end, by my perspective. After all, I have more than 20 wholly independent plots that need to be handled. There isn't "one plot."

What I meant was, that instead of building a highly detailed world that can live up to any contingency, you should instead tweak the details according to what would make a more interesting story.




 

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