How do wooden bridges hold the weight of trains?

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As the title states, how do wooden bridges hold the weight of trains?
I’m specifically speaking of steam locomotives in the late 1800s.

I know there is some sort of engineering involved, and I know there’s a lot of wooden beams and some sort of science behind the angles and supports but I just don’t get it and I would like to understand.

Thank you!

In: Engineering

Arches aid in supporting and distribution of the load. This is also why tunnels are circular.

The engineers know how much force a given beam can sustain. And as long as you design the structure in a way that the forces exerted on it never exceed what it can handle, everything is fine.

This applies to any building material. Weather it be wood, steel, concrete, or anything else

Any material, wood, stone, steel, bamboo, plastic, can be tested for how much weight it can hold, and then it’s just a matter of building with enough thickness of material to support the weight that’s going to be placed on the building or the bridge.

There’s also “structural” tricks; certain shapes help a structure become more rigid. [Triangles](https://datagenetics.com/blog/november12014/cover.jpg) make for very rigid structures without the need to have solid walls holding up all that weight. [A table](https://cdn-images.article.com/products/SKU365A/2890×1500/image57015.jpg?fit=max&w=1300&q=80&fm=webp) redirects the weight on it to the floor, through its legs; you put one ton of weight on that table, each leg will be compressed by a quarter of that weight (4 legs, weight gets distributed evenly). [Arches](https://www.designingbuildings.co.uk/w/images/a/a2/Arches.jpg) are a structural shape that redirect the weight of all the stuff above them sideways and down to the columns on their sides.

The engineering is possible because forces (like the weight of an object and gravity) can be decomposed by direction.

It’s like if you move in a NNE direction on your GPS, you’re moving northward, and you’re also moving a little bit eastward, and you can “decompose” your movement in terms of “north” and “east”, and then “add it” with any other person’s movements in terms of north and east – you add north with north, and east with east, and look at the result. If you’re walking NNE inside a train that’s moving SW, you “decompose” in terms of north-south and east-west and add up for the “overall” trajectory as seen from a satellite.

Anyway, forces can be decomposed the same way, and the [angle of a ladder](https://i.ytimg.com/vi/vtpE5synod8/maxresdefault.jpg) for example is a basic engineering exercise that just decomposes the effect of a person on the ladder in terms of “down” against the ground and “sideways” pushing against the wall. The ladder is designed to not slip, and the contact points of the ladder (the foot and the top) must not slip so that friction can stop the forces that the person’s weight exert on the ladder.

It’s a misconception that wood isn’t strong. Wood has a very high specific strength (strength to weight ratio). Higher than steel. The main reason you see structures transitioning to other materials has more to do with cost/availability/longevity of other materials and that metals have a better ability to be formed into very efficient structures.

When it comes to designing structures it really just comes down to distributing the weight. It’s like walking on sand. You tend to walk with your feet flatter. Large quantities of large “thick” beams distribute the train weight. It’s pretty simple really. Doing it efficiently is harder though.