Unlike buildings with flat roofs, which rely on regularly spaced columns for support (the bigger the roof, the more columns required), domed roofs are designed to provide the maximum amount of unobstructed covered space. With no internal support columns standing in the way, domed structures are well suited as places where people congregate, such as convention centers and sports venues.
In a traditional, hemispheric dome, a series of arches intersects at the crown. Here, forces move inward toward the center, pushing the halves of each arch together and making the resulting dome rigid. The great weight of concrete material, however, creates downward and outward forces near the bottom of the dome that must be balanced by upward and inward forces to prevent the dome from collapsing. In a well-designed dome, the material from which it is built provides enough support to balance the downward force of the load. But what can be done to minimize the outward push, or tension, in the lower portion of the structure? Two things: encircle the dome’s rim with a steel cable or chain, or build heavy concrete step rings around the dome’s perimeter to keep it in compression, or pushed in.
Over time, engineers have devised new ways to manage forces in domes, employing lighter materials and using less of them. By using a smaller, self-supporting internal dome as a base, fourteenth-century engineers discovered they could build steeper, more impressive outer domes that weighed just a fraction of what the inner dome weighed. London’s St. Paul’s Cathedral and the U.S. Capitol building each have “false”p double domes, the outermost of which is little more than a shell. Engineers have also turned to new materials like iron to construct domes that are more supportive and considerably lighter than stone or concrete domes of the same size.
In the mid-twentieth century, space frames, which are assemblies of lightweight tubular steel struts, were adapted to create a model for the most efficient and economical means of enclosing large spaces: the geodesic dome. This self-supporting spherical structure has inspired the wide-spanning tension domes that have today become the design of choice for sports venues.
Questions for Discussion
- How is the room you are in like a dome? If it is not curved, is it a dome anyway?
- Use everyday materials to build a dome that will cover an object in your house, such as a plant or cake or some delicate item. What forces act on your dome? How does your structure withstand these forces?
- Use a sketch to show where tension and compression forces are operating in the domes you chose in the interactive challenge for the baseball stadium, the greenhouse, and the Capitol Building.