How do floors bend/shake but not break?

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I’ve had a bizarre phobia since I was young. I do not trust the structural integrity of most buildings and am always worrying things will collapse/break. One thing that really triggers my anxiety is when I can feel a floor shaking. If my one year old, 25 pound son can run by and make the floor shake, how can it hold substantially heavier items (like a fridge, washer, etc.)? Please only reassuring comments – I can’t handle this phobia getting any worse 🙂

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Strength has almost nothing to do with flexibility/stiffness.

A steel cable is incredibly strong but flexible as a noodle. A glass rod is as stiff as you could wish but easily broken by hand.

You’re worried about things breaking…that happens if the load gets bigger than the thing can hold. The safety factor of buildings is extremely high… they can hold far more load than they ever see in normal use.

Flexibility actually *helps* the building be stronger… it lets the load balance out and spread around to be carried by more of the structure. We don’t *like* flexibility, because we evolved walking on solid ground and it feels weird to us, so building designers intentionally make floors stiffer than needed to be to make us comfortable but this has nothing to do with structural weakness.

Building things to be both really strong and really stiff can be done… but it’s almost always extremely *heavy*. This is why concrete or stone buildings are so massive and stiff. Wood or steel construction is much more flexible, and light (“more airy”) but no less strong.

It’s good when things bend, because they’re avoiding breaking! Wood is a great building material because it is flexible and can take the stresses we put on it as walls and floors for years! Wood also vibrates in ways that make it better for musical instruments than, say, concrete. I don’t really know why that is and I am not smart. But so your floor boards will bend and also carry shockwaves. The floor shaking from someone running on it is just vibrations. If it’s shaking in an earthquake, that’d be something to worry about, but even then, buildings are designed to move a little so they aren’t brittle. Skyscrapers sway like mad in the wind or when the ground moves, and they don’t fall. I hope this is helpful!

All forces cause some bending in structural members. Everything is, to some extent, like a spring.

A 25 pound object hitting the floor can exert hundreds of pounds of force. This force causes a small but not unnoticeable deflection of structural members – a tiny fraction of an inch of movement. Since it is an impact, the force is brief, and vibration is caused, making this small movement even more noticeable to you as it happens over and over during the vibration.

A much heavier object will cause more deflection of the supports, but still well under the amount it would take to cause a yield.

Wood, the main structural element of most buildings, can bend a huge amount before it breaks. This is extremely useful, since it means that wood can withstand those huge force spikes from impacts, while something even stronger like glass would break from an impact.

If it helps to ease your phobia, you might consider the vibrations you’re hearing and feeling as ripples on the surface of a pond. That’s pretty much what they are. When your son runs across the floor, what you hear and feel is those ripples traveling through the floor (and the air) and bumping into your body.

Have you ever seen a picture of [the inside of an acoustic guitar?](https://www.reddit.com/r/Damnthatsinteresting/comments/ivakd0/inside_of_an_acoustic_guitar_looks_like_a_luxury/) Check it out. It looks cool. That’s basically what your house is. If you rapped your knuckles on the “floor” of that picture, you’d hear quite a loud sound as the vibrations traveled around, but there would be absolutely no danger of your knuckles breaking through. You’d have to hit it incredibly hard with your fist to break through. And a guitar is very thin wood held together with only glue! Your house is made of, and held together by, much, *much* stronger materials.

Sports stadiums all wiggle. It’s because a stiff building would crack under all that weight of people tramping around. Regular building floors also wiggle and flex, but not nearly as much. There’s space between floors so that all the cabling and insulation and pipes and stuff can fit in, and again so that the actual flooring can flex without cracking every time something hits it.