How do we make houses and buildings out of wide wooden slabs that are strong enough to hold the people, furniture, and rest of the structure?
In: Engineering
Wood is a very strong material, trees, where wood comes from, can stand dozens to hundreds of feet tall.
And really, it is more than just second and third floors, one of the most common building types these days is a “Mixed” design where the foundation and first and possibly 2nd floor are steel and concrete, and then all of the floors above, up to 7 floors, are made of wood.
And it really is just engineering, you have a room, you know how big that room is, you can estimate how much load is going to be on that floor per square foot, pick how big your column and beams will be based on that estimated loading, multiply that by 1.5 for a little extra safety wiggle room, and you are off to the races.
Well, some wood is pretty strong.
There’s more to it: putting structural members “vertically” so that the weight falls on a stronger direction, making sure that support members are close enough together, ensuring that force is directed outward and downward well, etc etc.
But I think that your question is probably best answered by “wood is pretty strong”.
After all, trees are made out of it and some of them are super tall! And the thing restricting tree height isn’t the strength of the wood, but how high the trees can make water go up.
Two or three stories aren’t that tall. Wood is stronger than you think.
As for carrying weight and taking abuse, wooden ships are probably more impressive
Wood’s compressive strength is very strong. That means you can put a LOT of weight on it without crushing it. If you think about it, this makes sense, since trees are basically wood stacked dozens of feet in the air.
Commonly, [wood can withstand 5000 pounds per square inch](https://woodbin.com/ref/wood-strength-table/) without being crushed. That’s as much as concrete.
In practice, by following building codes that tell you what size lumber to use where. Multiple stories isn’t particularly difficult, because the boards in the walls are loaded mostly in compression, whereas the floor is loaded in bending. The sheathing keeps those boards upright and also helps keep them from buckling. One of the boards in your wall can take thousands of pounds of force in compression if you can keep it from buckling, and there are many such boards in each wall.
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