Comprehending how structural systems work to support and stabilize buildings can be interesting, possibly useful and not nearly as difficult conceptually as one might believe.

To mentally concoct a simplified model of a structural framing system, imagine two folding-leg card tables stacked on top of one another and sitting outside on the ground. Pretend further that you are sitting on the lower table. The legs of the table above, your metaphoric "roof," are securely attached to the four corners of the tabletop on which you sit.

Assume it's a windless day. Everything seems motionless. Chances are that the system will remain stable as it transmits your weight (the "live" load), plus the weight of the tables themselves (the "dead" load), down to the ground.

But despite the appearance of stability and lack of motion, a close look reveals that, in fact, some movement has occurred. The thin, tubular legs of the bottom card table may be pushing their way gradually into the earth, because all the weight from above is being concentrated on four small points. To prevent the legs from settling further into the ground, you know intuitively that you must reduce the stress (pounds per square inch) between leg and ground by spreading the weight over a larger area.

A "footing" is needed. It must be large enough so that the soil below it can carry all of the weight from above without allowing the footing to settle. It also must be thick enough so that the card table's leg won't break or punch through the footing itself. Thus, a dictionary under each leg would work better than a thin piece of slate or cardboard.

Putting the footing on top of the ground might be all right in Florida or Southern California, where it never freezes. But in colder climates, freezing and thawing cycles cause the ground to "heave," to move alternately up and down. Therefore, the footing should be in the ground below the frost line (at least 30 inches below grade in the Washington area) to keep the above-grade structure from heaving also.

Even with firm footings, the card-table top on which you sit doesn't remain level. It moves, too, deflecting noticeably because of your weight. Common sense suggests that to reduce this deflection and bending, you must stiffen the whole top or decrease the dimensions over which it expands. "Beams" could be added below to strengthen the edges, and a series of "joists" could span between the stiffened edge beams. Or additional legs and footings could support the tabletop at more points from below.

The table top and its edges are analogous to the floor slab and beams in a building. They must be strong enough to carry superimposed live loads as well as their own dead load. Bending, deflection and stresses from overloading can result in excessive deformation, fracture and eventual collapse.

Assume optimistically that the card-table top, your imaginary system's "floor," is sufficiently strong and stiff. Yet the legs -- the system's "columns" -- could be bowing slightly, a phenomenon known as "buckling." It occurs when relatively slender structural members carry enough compressive force to make them bend, just like a piece of paper bends suddenly when you hold each end and push steadily toward the middle.

The remedy: Stiffen or fatten the column. The card-table leg (assume it's a metal tube) could be made of heavier-gauge steel, or its diameter could be increased, making its proportions less slender and therefore less susceptible to buckling.

Visualize again your imaginary framing system as a whole. Each part has been made strong enough to carry both live and dead loads down columns that aren't buckling to footings that aren't settling. But the wind starts blowing. Despite adequately reinforced horizontal and vertical components throughout the system, the entire frame nevertheless is deforming. Rectangles are becoming parallelograms as 90-degree angles between table tops and legs enlarge or diminish slightly.

What's happening now? The structural frame, while able to resist satisfactorily all the forces arising from the pull of gravity, still is unstable when subjected to horizontal forces that tend to push it sideways, overturn it or twist it around its vertical axis. For complete stability under all conditions of loading, including horizontal forces arising from wind or earth movement, at least one of three tactics could be employed:

First, all the joints and connections between each table leg and top could be rigidified. Think of transforming the folding card tables into Parsons tables with thick, solid legs and edges. Steel and reinforced-concrete buildings can rely on the stiffness of columns, beams and column-to-beam connections to stabilize the entire structure when the wind blows or the earth trembles.

*Then there is triangulation. A triangle is the only polygonal figure whose shape cannot change without changing the length of one of its sides; a rectangle can be transformed into a parallelogram with no change in the dimensions of its size. Therefore, rectangular frames can be rigidified by installing structural members that subdivide rectangles into triangles. In our mental model, diagonal braces attached to each side of each card table would make the whole two-story system rigid.

Buildings stabilized with this type of diagonal bracing usually do not expose their triangulation to view, instead concealing it within opaque exterior and interior walls. However, there are exceptions, the John Hancock tower in Chicago being among the tallest and most familiar. And one routinely sees triangulation all around -- in bridge trusses and transmission towers. Finally, vertical wall planes, referred to by engineers as "sheer" walls, can stabilize a potentially wobbly framing system. Imagine sheets of plywood fastened to each of the four sides of the card-table assembly. A sheet of plywood, while bending easily when forces perpendicular to its surface are applied, strongly resists deforming when forces are applied parallel to its surface. Very rigid within its own plane, it can stiffen any skeletal framing plane to which it is attached, much like diagonal bracing. Even when some of the plywood (but not too much) is cut away to make openings for windows or doors, the remaining material still can provide sufficient rigidity.

In many buildings, continuous vertical sheer walls of reinforced concrete or masonry stabilize the overall structure. They may be exterior facade walls firmly attached to or supporting the skeleton, or party walls between abutting buildings. Other sheer walls can be inside the building envelope, frequently formed by reinforced concrete enclosures rising around stairways and elevator cores.

Contrary to appearances, exterior brick surfaces seen on residential and commercial structures are not always sheer walls. The brick is frequently a veneer, and, as the name implies, it is hung or supported like a curtain over the real structural frame behind. Diagonal bracing or sheathing such as plywood nailed to studs is the source of skeletal rigidity, not the brick.

Having mentally constructed this intuitive backyard "model" of a building frame with its respective parts, perhaps now you can better appreciate what is happening when your table, your house or your office shakes a bit from time to time -- or when they don't shake.

NEXT: Special structural forms