They had to know what materials to use, which alloys, load bearing weight of the materials, how to place and configure them, etc.
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The same way we built the Eiffel tower: steel is really strong, and engineers are really good at their job.
Engineers figured out materials.
Engineering has been studied in universities since the Greeks in the west and the Ottoman Empire (maybe before) in the Middle East, and I’m not sure how long in Asia(China/india specifically, but more cultures than I want to name), but realistically several thousand years.
Many other cultures had a concept and practice of engineering, but not many had formalized university structures focused on engineering.
Many structures like The Egyptian or Mayan pyramids, Angkor Watt(as examples) are ancient and well engineered.
Computers figured out the numbers.
Computers used to be the catch all term used for a person figuring out the math of a problem. The movie “hidden figures” highlights some of these folks. In the 1960s many of these computers were women, or women of color as this job was seen as “beneath” men at the time.
So the short answer is: engineering and mathematics. All by hand. Sometimes chiseled into a stone, sometimes drawn on animal skins with charcoal. Lots of trial an error until something worked.
The Egyptians built skyscrapers 5000 years ago. And many Roman constructions required tall structures (aqueducts).
Things like math and geometry are well understood (enough for construction) thousands of years ago. The basics are well understood from a long time ago – foundation, support, load bearing etc are the realm of fairly simple models. Concrete was developed thousands of years ago and steel hundreds of years ago.
The main differences in modern construction is efficiency and refinement. A skyscraper built 100 years ago would be ridiculously inefficient and overdesigned today. Rather than 3 feet thick walls, we’d probably design them with one foot thick walls today. Rather than 1000 support columns, we’d probably use 100.
The same way the first computer was built, engineering and math. All a computer provides is a quick way to do the same math and convenient way to store and recall information.
Why would you need computers to know any of that information? How would computers even *help* with any of that?