Among Washington's most memorable architectural features are domes and arches, ancient structural forms that gracefully cover monumental spaces, orchestrate loggias and arcades, or span boldly over rushing rivers.

Of course, the District is full of buildings with structural systems composed predominantly of rectilinear framing elements: vertical columns supporting horizontal beams, trusses and slabs. But it's not a city whose image is one of soaring framed towers, like Chicago, or towers and suspension bridges, like New York.

Arches, vaults and domes are structural forms found in nature. A cave can exist because its ceiling and sides approximate an arch in cross-section. Likewise, the shells of walnuts, coconuts and turtles are natural domes whose curved surfaces provide great structural rigidity.

How does an arch behave in comparison to a "post and lintel" framing system? Consider first how a lintel, or beam, works.

Envision a small stream spanned by a log. You and your companions are crossing the stream atop the log. The log bends and curves downward slightly, its point of greatest deflection occurring in the middle. A close look reveals that, in bending, the bottom of the log stretches and elongates; the top of the log simultaneously shortens and compresses.

What's happening inside the log, composed of millions of tiny wood fibers? The stretched fibers along the bottom of the log are in "tension" and are being pulled apart from one another. The fibers along the top of the log are in "compression," being squeezed together and shortened. If you and your companions are evenly spaced along the top of the log, the greatest compressive and tensile stresses will occur in the top-most and bottom-most fibers at mid-span, the point of maximum deflection.

This fundamental "beam behavior" occurs irrespective of whether a beam is made of wood, steel, concrete, stone or any other material. However, the coexistence of compression and tension within a beam requires a material that is capable of resisting compressive and tensile forces equally.

Wood and steel have this capability; brick, stone and concrete do not. Concrete, although very strong in compression, has almost no tensile strength. When stretched, it breaks immediately, explaining why steel-reinforced concrete was invented. Reinforcing rods are placed within concrete beams, slabs, columns, walls and footings wherever engineers predict that tensile forces will develop. Without tension reinforcing, all concrete structures would crack open and collapse.

The structural virtue of the vault, the arch or the dome is that all the forces in their constituent elements are compressive. Ideally, there is no bending, or "beam," action.

Pretend now that your imaginary stream is spanned by a simple bridge consisting of a circular brick arch, in lieu of the straight log. Each brick in the arch will be in compression, squeezed into an infinitesimally smaller volume by compressive forces transmitted by adjacent bricks. The greater the superimposed load, the higher the compressive forces and the tighter the arches' bricks will be squeezed together.

In effect, a vault is an extruded arch, and a dome is an arch rotated about its vertical axis of symmetry. The profiles of arches, vaults and domes are often semicircular, but they can be less than half a circle. When their profile is only a small segment of a circle, they are considered "flat." The flatter the profile, the greater becomes the outward horizontal thrust at their bases.

Arches, vaults and domes also can be configured to follow other geometric shapes such as parabolas and catenaries (the curve made by a cable freely suspended from its two ends). St. Louis' free-standing arched Gateway to the West, designed by Eero Saarinen, is a catenary arch. McDonald's twin golden arches seem to approximate catenaries as well.

Looking at Washington, you see countless examples of arched forms. The flat arches of Memorial Bridge skim gracefully across the Potomac. A short distance upriver, Key Bridge vaults across the Potomac with two rhythms. Its semicircular arcades of varying height support the roadway while sitting atop the bridge's major arched spans. Massive arches of stone-and-concrete bridges span Rock Creek and along the George Washington Parkway lining the Potomac River.

Arched windows and arcades occupy the facades of many D.C. buildings, particularly along their bases, where stores or entries demand more expressive, nobler demarcation. Sometimes, buildings have arched openings only at significant points of entry, or above such points, with rectilinear openings everywhere else. Look carefully at the selective use of arched openings on the U.S. Capitol and White House facades, on the faces of many Federal Triangle buildings or on the entrance pediment on the Museum of Natural History.

Washington, like Paris and Rome, is a city of architectural domes. Domes sit atop many of the mall's buildings, the most prominent and famous being the Capitol's. The domes of the Jefferson Memorial and the National Gallery of Art were inspired by the shape and proportions (but not the dimensions) of one of history's most famous domed buildings, the Pantheon in Rome, built nearly 2,000 years ago.

Not all the arches and domes that you see are structural; many do not use compressive, masonry arch technology. Conventionally framed buildings often contain rooms with suspended ceilings shaped like vaults or domes. Curtain walls or other non-weight-bearing walls can have arched openings cut out of them, without the arches being genuinely structural.

The dome of the U.S. Capitol, completed during the Civil War, is framed structurally by two curved shells, one above the other, made of triangulated cast iron trusswork. The visible exterior dome, as well as the interior coffered and painted ceiling domes covering the Rotunda, are actually veneers supported by the concealed iron framework.

Arches, vaults and domes always have more than structural significance. Their geometry is based on the circle and the sphere, figures considered by many cultures to be universal symbols of the ideal, of completeness, unity and perfection. Philosophers through the ages have ascribed cosmic and theological connotations to them, recognizing that they are omnidirectional, yet focal and centered. It is therefore not surprising that architects historically have used circular geometries for special, sacred places.

Standing beneath a dome, you sense immediately the power of its form, its ability to span and contain centralized space. You also comprehend intuitively its innate structural stability. But domes suggest something beyond themselves. Perhaps humans always have perceived, consciously or subliminally, inexplicable connections between the circular structures they build and the intangible universe they didn't build.