At the turn of the last century, as the supply of real estate in America's biggest commercial centers dwindled significantly, engineers conceived a new type of building that would enable cities to grow vertically instead. To build skyward, structural weight would be removed from thick, heavy, load-bearing walls and placed instead on a lighter yet rigid metal frame composed of horizontal beams and vertical columns.

In a skyscraper, the weight of the building's floors rests on the beams, or girders, which are supported by the columns. Force moves downward through the columns to the ground, where it is dispersed over a wide solid surface called the foundation. To balance the forces of gravity, load (the weight of the structure itself), and nature (wind and earthquakes), the materials used to build skyscrapers are especially strong in both tension and compression. Early skeletal frames were made from iron. It wasn't long, however, before a much lighter and stronger metal -- steel -- became the material of choice.

Steel is mostly iron, but it contains the right amounts of carbon, manganese, and other elements mixed in to make it stronger and more resistant to rust and corrosion. Steel can be made into long slender beams or used to reinforce concrete, a material strong by itself in compression but relatively weak in tension. Today, steel-reinforced concrete is widely used in construction as a less costly -- and more fire-resistant -- alternative to steel.

Chicago's John Hancock Center, completed in 1969, illustrates how skyscraper design has evolved to utilize less material without sacrificing strength. Unlike New York's Empire State Building, which was built in the 1930s around a stiff steel backbone called a core, the Hancock Center is of tubular construction. In tube designs, a number of steel columns are concentrated along the building's perimeter, leaving its center hollow. Consequently, tube designs are much lighter. The Hancock Center builders also attached enormous diagonal braces over each of its sides. This trusslike configuration, another lightweight design element, provides additional stiffness against strong wind gusts.