In the 19th century, buildings constructed around the United States, primarily in larger urban hubs, like Chicago and New York

and Detroit, began to set new modern construction height records, weighing in at 10 stories and higher.

These upwardly evolving blueprints were the consequence of new technologies and materials.

Among them, the advent of steel framing that allowed load-bearing outer walls

to be piled higher than was conventionally possible with older building methods.

And a load-bearing outer wall basically means the external skin of the building, the walls,

played a role in holding the building up. It's a structural, not just a decorative element.

Curtain walls, which are decorative, but also help create the internal, environmental

envelope of a building, but which don't hold anything up. They're not load-bearing.

We're enabled by more advanced and sturdy steel framing setups and arrangements,

and because of their non-load-bearing nature

could be constructed of lighter weight materials.

They have to hold themselves up and prevent the air and people inside the building

from escaping or falling out,

but otherwise they just have to look pretty, or at least not ugly, and allow

in light and things of that nature.

And that's a task easily managed by a wider array of materials, including glass and

various sorts of insulating substances.

The steel frame that enabled these curtain walls is what you see in modern high-rise buildings

when they're under construction, central skeletons of metal that are eventually filled in with

livable components, like walls and floors, made of metal and wood and concrete and plaster,

but which are initially just steel bars arranged in such a way that you can

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