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" 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.