As you read this column, you may be sitting in your wood-framed house in the District, Maryland or Virginia unaware of your home's framing pattern, unsure about which parts of the house are structural and which are not.
Most wood-framed houses exhibit little visible evidence of how they are held up. Conventional "balloon" or "platform" framing allows builders to shape homes in almost any configuration, to place walls in any location, to use almost any kind of roof and to cover the exterior with almost any material.
Whether outside or inside, you get little sense of the location of major or secondary supporting beams, columns or bearing walls. Are roofs made with rafters or trusses? In which direction do ceiling or floor joists span? How has the frame been braced to resist lateral forces? Generally, the structural system of such houses is subordinated totally to the composition of volumes, rooms and finished surfaces.
At the other extreme are buildings whose structural systems are visually dominant. Primary structure components -- trusses, beams, arches, floor slabs, piers, columns -- not only are visible, but also regulate the shaping of rooms within the building along with exterior massing and facade composition. Office and apartment buildings, schools, large single-space buildings (such as factories or air terminals) and other special-purpose buildings frequently typify this design strategy.
In the District, the Federal Home Loan Bank Board building at 17th and G Sts. NW, the National Permanent building at Pennsylvania Avenue and 18th Street NW, the West German Chancery on Reservoir Road, and Metro's headquarters building at 5th and G Sts. NW overtly display the patterns of their structural systems. And the Washington Monument obelisk is the pure embodiment of "column," of compressive stresses increasing toward the earth of hewn material firmly resisting the forces of nature.
Few edifices rival the magnificent 13th and 14th century French Gothic cathedrals when it comes to expressing the marriage of structure and form.
They are tours de force in the use of powerfully articulated columns, arches, vaults and flying buttresses to constitute structural frame and enclosing shell all at once. That they were carved entirely of stone is even more remarkable.
The Gothic cathedral's structural system corresponds perfectly with the building's spaces. Structural elements define, contain and impart rhythmic modulation to church nave, side aisles, transept and choir loft. Even elaborate, decorative stone carvings do not mask the soaring, ethereal order of structure, space, mass and surface.
However, most buildings stand between the extremes of the Gothic cathedral and balloon-framed house. Their structural-system geometry is only partially apparent. Often, concealed building skeletons are revealed implicitly through patterns of windows and facade treatment. Clues to structural order may come from emerging pilasters, porches, colonnades or arcades.
Indeed, it is an ancient but timeless idea to design buildings so that their visual order derives partly from their necessary structural order. Many neoclassical buildings in Washington, from the White House to Union Station to the Pentagon, exemplify this mode of structural expression.
By contrast, the Hirshhorn Museum on the Mall has a structural system entirely subservient to its arresting, circular geometry. Despite the presence of massive piers supporting the inscrutable, truncated cylinder overhead, it is difficult to comprehend the latter's structure. In fact, structural framing is probably not significant in determining the Hirshhorn's shape, proportions or interior space configurations.
The National Gallery's East Building derives its framing patterns from a plan and massing geometry generated by the trapezoidal shape (resolved in the two triangles) of the site. Unlike the Hirshhorn, the East Building's shape responds to the city's street pattern, but the triangular patterns of its structural systems, intimately linked to the shapes of spaces inside, are not imprinted on its exterior facades.
In many Washington office buildings, an unseen force, unrelated to lot configuration or the city's street pattern, orchestrates column grid patterns: underground parking. Designing such buildings is a planning puzzle; to solve it, the architect must create a network of columns to accommodate cars and driving aisles. Zoning regulations stipulate that parking spaces be at least 9 by 19 feet (8 by 16 feet for compact cars) with access aisles at least 20 feet wide.
These requirements lead to underground garages with column grids typically forming bays measuring about 20 feet square. In turn, the garage column grid pattern dictates the office grid pattern above, because columns must be continuous vertically from roof down to foundation. Minor shifts in column placement in the above-grade office structure are sometimes accommodated in underground garages by using now-familiar, but still visually bizarre, leaning columns.
D.C. office buildings often have structural frames and facades based on "flat plate" or "flat slab" reinforced-concrete construction, ideally suited to 20-by-20-column grids. A continuous concrete slab, normally about eight inches thick, spans both directions between columns without any girders or beams below the slab.
As a result, the overall depth of the floor structure is minimized, reducing floor-to-floor dimensions. In high-density zones, the dimensional savings may add up sufficiently to allow an extra story to be squeezed in under height and zoning limits. The floor-to-floor height saving also reduces the amount and cost of the facade of the building, an economic benefit appealing to developers.
The structural systems of most buildings represent a surprisingly small percentage of overall project investment. Labor and materials to construct foundations and superstructures may be less than 10 percent of total development expense (including land, financing and fees) and rarely exceed 25 percent of all construction contract costs.
Consequently, even a substantial variation in structural-system cost may affect project capital cost by only 2 or 3 percent, making a building's structure a relatively fixed development budget component. And from a public-safety point of view, the structure is clearly not the place to scrimp or take shortcuts, because it alone keeps buildings standing and stable.
Building structural systems can be subdued, passive, invisible. Or they can influence dramatically how buildings look and how you feel about them. Even without structural expertise, your intuitive assessment of, and response to, the structure of any artifact can be both correct and compelling.
The Washington Monument looks structurally logical and comprehensible. Yet, if we wanted, we could build it upside down, poised wonderfully on its pyramidal apex, or leaning over like a giant stone sphere thrust into the landscape. But your instincts probably would tell you that something was amiss, inappropriate, perhaps even upsetting about such provocative propositions.
The structural systems of buildings can carry messages as well as forces, and it's satisfying, as well as safe, to see them transmitted properly.