Generally, the load requirements for parking garages are smaller than the loading for space where humans can crowd.

Cars just aren't that heavy compared to people.

A typical car park is about 2.5m wide and 6m long, so 15 sq m. A typical car weighs about 1.5 tonnes, so the load on the floor of a car park is 100 kg per sq m.

I think it would be hard to exceed that with people, without breaking the fire code.

Design loading for human occupancies come in in the range of 100psf, which is something like 500kg/m2. That's roughly one 200lb adult per 2sf.

You can pack a _lot_ of humans into a small space.

Cars are about half that, because they don't pack well.

Of course, libraries are even worse, sometimes you're looking at 300+ psf design loads there.

Last I checked, my car weighed more than me, and all my posessions, combined.

Is it also a lot bigger than you?

> Imagine how much parking would be needed for everyone in a 40-story high-rise to put their car there.

About as much as many 40-story high-rises already have underground.

I've never seen a 40-story high-rise that had enough underground parking for more than a small fraction of its occupants. They build them in places that have transit links and set the cost of parking spaces at a high enough level that most don't bother.

> About as much as many 40-story high-rises already have underground.

It isn't about where you put it, it's about how much it costs.