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I have never thought about the fact that manhole covers need to hold up to weight. Of course they do and it's perfectly normal and sensible. It's just not a thought I've ever had.
Technically it's the manhole covers that need to support the weight.
Manholes themselves are expected to do the opposite of holding weight
Manholes themselves are expected to do the opposite of holding weight
They hold light?
It's dark inside so of course the light is held □
Structures underground experience buoyant forces that act to push them up. Manholes (and any structure bottom, like storm inlets, pump station wet wells, etc) need special consideration since they can be partially or mostly hollow, so they have to be heavy enough to remain in place.
You know, I knew that but couldn't figure out what the opposite of weight was for some reason. Thanks for the extra learning!
Fun fact: Coffins experience this force too, and during flooding can rise up out of the ground
Ah, that explains why my grandpa was buried in a lead coffin.
He lost a lot of weight right before he died, so he was too light!
Right, so the light is actually pushed up by these buoyant forces and I guess that then also explains why it's so dark underground. Fascinating how learning some little new details about the world can sometimes make it all just click together!
But does that mean that light is actually hollow?
Well we know light is concentrated into comparatively tiny, but heavy points in the universe (stars), while dark fills the vast vacuum of space. So light in itself is likely denser than dark, ergo the light we have on earth must be forming hollow structures to remain lighter than dark when comparing by volume.
Gravity force is so strong even the candlelight can't esc- oh wait...
Technically, the walls of the manholes, in turn, support the weight of the cover and whatever load is on top of it. The entire manhole is a weight-bearing structure.
For formula one races they weld them down to stop the cars incredible downforce from sucking them up into the air. Even then they sometimes get torn up and thrown around.
Very important to take them seriously.
Manhole covers are also a common exercise for engineering students. Like designing one that can hold x amount of weight with a specific set of limitations and/or requirements.