Resource: Designing the Citigroup Skyscraper

Rarely do architects and structural engineers face a problem as unusual and difficult to solve as that presented by the Citigroup Center (formerly called Citicorp Center and Citibank Center) in New York City. Like all large structures, this skyscraper, completed in 1977, is subject to forces, including wind, vibrations from city traffic, the weight and movements of occupants and equipment inside, and the force of its own weight. Unlike most buildings, however, the 914-foot-tall (279-meter-tall) Citigroup Center has no support directly under each of its four corners.

In the early 1970s, developers acquired almost an entire city block for the site of the proposed skyscraper. That's almost, because one corner of this particular block was already occupied by a church, which stood exactly where architects wanted one of the skyscraper's corners to be. The church allowed the developers to use the space above it, on the condition that a new church be built in the same spot as the old one.

In order to use the space above the church, structural engineers designed the Citigroup building so that it would stand on four stiltlike columns, one in the middle of each of its sides. Normally such a design would result in a structure far less stable than a building supported on all four corners or supported all the way around its base.

To account for the unusual placement of the building's support columns, engineers of the Citigroup Center designed an internal steel skeleton that would support the corners indirectly. In this frame, steel beams are arranged like giant Vs, with the tops of each V attached to opposite corners and the bottom attached to a column running vertically up and down the center of each side. These angled beams transfer the weight from the corners of the building to the strong vertical columns, much like a shelf bracket transfers the weight of the shelf and its contents to the wall to which it is mounted. The result of this design is a building that seems to defy gravity.

When you put columns at the corners, you have to support the centers of the sides. When you put columns at the centers of the sides, you have to support the corners. Why are these two problems so different?

Where else do you have to use triangles because the load is neither near nor between the columns (e.g., shelf brackets)?

Why might it have been important to build over the church and not just tear it down? What might have been the societal impact of tearing it down?

Why are triangles so rigid? Make some triangles and quadrilaterals out of cardboard and fasteners, and experiment with trying to change their shapes. What does what you have learned in geometry about triangles have to do with how they are used in structures?

Look around at buildings in your neighborhood. Can you find places where triangles are used to make the building more stable?