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by CareyBot

For millennia, the only way to build to build a strong building was to pile up lots and lots of stone or brick, forming massive masonry walls that could hold up the weight of the floors and roof. This ancient approach worked well enough as long as buildings weren’t more than six stories tall or so. If they were, the lower walls had to be made impractically thick in order to carry the weight of all that masonry above them — the more stories, the thicker the walls.

By the late 19th century, when American engineers and architects began contemplating structures of 10, 15 or even more stories, the limitations of masonry construction reached a critical point. One of the tallest masonry buildings of this era, Chicago’s Monadnock Building of 1891, carried its 17 stories of brick on ground-floor walls 6 feet thick. Such a ponderous system simply wouldn’t do if tall buildings were to become practical. Fortunately, a new building material — steel — solved this problem just in time.

Steel’s earliest ancestors date back at least 4,000 years, but it was the invention of the Bessemer process in 1855 that first allowed it to be mass produced. Steel’s structural advantages were immediately obvious: Pound for pound, it was many times stronger than masonry, and just as important, it was equally strong under both compression and tension. Steel was also ductile and would bend under a heavy load rather than cracking. These attributes meant that a steel beam could support heavy loads and span long distances much more efficiently than any type of masonry, allowing even the tallest building to be supported by a relatively light "skeleton frame" of girders rather than by hundreds of tons of stone or brick.

Steel members — including the familiar though now little-used "I-beam" — were typically riveted together (and later, bolted or welded) into a cage-like structure that carried the entire building load. Because the outer walls no longer had to support the weight of the stories above, they could be very light enclosures of glass, metal or some decorative veneer — hence the term "curtain wall."

The first tall building with a load-bearing steel frame was Chicago’s 10-story Home Insurance Building, designed by William Le Baron Jenney and completed in 1885. Engineers and architects quickly followed Jenney’s lead over the next few years. So sweeping was this change that, while the aforementioned Monadnock Building had marked the apogee of load-bearing masonry construction, an addition to it built just two years later was already being framed in steel.

Meanwhile, the price of downtown land in large cities all over America was beginning to skyrocket, putting pressure on developers to pack more volume into the same amount of real estate. That meant just one thing: ever-taller buildings. By the late 1890s, the new possibilities inherent in the steel skeleton frame, spurred by rising urban real estate prices and enabled by the invention of the safety elevator, had set off a national skyscraper boom that, for better or worse, is still with us today.

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