The central support structure of a skyscraper is its steel skeleton. Metal beams are riveted end to end to form vertical columns. At each floor level, these vertical columns are connected to horizontal girder beams. Many buildings also have diagonal beams running between the girders, for extra structural support.
In this giant three-dimensional grid — called the super structure — all the weight in the building gets transferred directly to the vertical columns. This concentrates the downward force caused by gravity into the relatively small areas where the columns rest at the building’s base. This concentrated force is then spread out in the substructure under the building.
In a typical skyscraper substructure, each vertical column sits on a spread footing. The column rests directly on a cast-iron plate, which sits on top of a grillage. The grillage is basically a stack of horizontal steel beams, lined side-by-side in two or more layers (see diagram, below). The grillage rests on a thick concrete pad poured directly onto the hard clay under the ground. Once the steel is in place, the entire structure is covered with concrete.
This structure expands out lower in the ground, the same way a pyramid expands out as you go down. This distributes the concentrated weight from the columns over a wide surface. Ultimately, the entire weight of the building rests directly on the hard clay material under the earth. In very heavy buildings, the base of the spread footings rest on massive concrete piers that extend all the way down to the earth’s bedrock layer.
One major advantage of the steel skeleton structure is that the outer walls — called the curtain wall — need only to support their own weight. This lets architects open the building up as much as they want, in stark contrast to the thick walls in traditional building construction. In many skyscrapers, especially ones built in the 1950s and ’60s, the curtain walls are made almost entirely of glass, giving the occupants a spectacular view of their city.
Making it Functional
In the last section, we saw that new iron and steel manufacturing processes opened up the possibility of towering buildings. But this is only half the picture. Before high-rise skyscrapers could become a reality, engineers had to make them practical.
Once you get more than five or six floors, stairs become a fairly inconvenient technology. Skyscrapers would never have worked without the coincident emergence of elevator technology. Ever since the first passenger elevator was installed in New York’s Haughwout Department Store in 1857, elevator shafts have been a major part of skyscraper design. In most skyscrapers, the elevator shafts make up the building’s central core.
Figuring out the elevator structure is a balancing act of sorts. As you add more floors to a building, you increase the building’s occupancy. When you have more people, you obviously need more elevators or the lobby will fill up with people waiting in line. But elevator shafts take up a lot of room, so you lose floor space for every elevator you add. To make more room for people, you have to add more floors. Deciding on the right number of floors and elevators is one of the most important parts of designing a building.
Building safety is also a major consideration in design. Skyscrapers wouldn’t have worked so well without the advent of new fire-resistant building materials in the 1800s. These days, skyscrapers are also outfitted with sophisticated sprinkler equipment that puts out most fires before they spread very far. This is extremely important when you have hundreds of people living and working thousands of feet above a safe exit.
Architects also pay careful attention to the comfort of the building’s occupants. The Empire State Building, for example, was designed so its occupants would always be within 30 feet (ft) of a window. The Commerce bank building in Frankfurt, Germany has tranquil indoor garden areas built opposite the building’s office areas, in a climbing spiral structure. A building is only successful when the architects have focused not only on structural stability, but also usability and occupant satisfaction.