KPDS 2002-1 Soru=93

We understand from the passage that the construction of skyscrapers only became structurally feasible -----.

Although the idea of the skyscraper is modern, the
inclination to build upward is not. The Great
Pyramids, with their broad bases, reached heights
unapproached for the next four millennia. But even
the great Gothic cathedrals, crafted of bulky stone
into an aesthetic of lightness and slenderness are
dwarfed by the steel and reinforced concrete
structures of the 20th century. It was modern
building materials that made the true skyscraper
structurally possible, but it was the mechanical
device of the elevator that made the skyscraper
truly practical. Ironically, it is also the elevator that
has had so much to do with limiting the height of
most tall buildings to about 70 or 80 stories. Above
that, elevator shafts occupy more than 25 percent
of the volume of a tall building, and so the
economics of renting out space argues against
investing in greater height.