Structural Systems in Architecture
Any building is a defiance of gravity. Since earliest times, architects have tackled the challenge of erecting a roof over empty space, setting walls upright, and having the whole stand secure. Their solutions have depended upon the materials they had available, for, as we shall see, certain materials are better suited than others to a particular structural system. There are two basic families of structural systems: the shell system and the skeleton-and-skin system.
In the shell system one building material provides both structural support and sheathing (outside covering). Buildings made of brick or stone or adobe fall into this category, and so do older (pre-19th-century) wood buildings constructed of heavy timbers, the most obvious example being the log cabin. The structural material comprises the walls and roof, marks the boundary between inside and outside, and is visible as the exterior surface. Shell construction prevailed until the 19th century, when it began to fall out of favor. Today, however, the development of strong cast materials, including many plastics, has brought renewed interest in shell structures.
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The skeleton-and-skin system might be compared to the human body, which has a rigid bony skeleton to support its basic frame and a more fragile skin for sheathing. We find it in modern skyscrapers, with their steel frames (skeletons) supporting the structure and a sheathing (skin) of glass or some other light material. Also, most houses today—at least in the United States—are built with a skeleton of wood beams nailed together, topped with a sheathing of light wood boards, shingles, aluminum siding, or the like. Skeleton-and-skin construction is largely a product of the Industrial Revolution; not until the mid–19th century could steel for beams or metal nails be manufactured in practical quantities.
Two factors that must be considered in any structural system are weight and tensile strength. Walls must support the weight of the roof, and lower stories must support the weight of upper stories. In other words, all the weight of the building must somehow be carried safely to the ground. You can get a sense of this if you imagine your own body as a structural member. Suppose you are lying flat on your back, your body held rigid. You are going to be lifted high in the air, to become a “roof.” First you are lifted by four people: One supports you under the shoulders, one under the buttocks, one holds your arms extended above your head, another holds your feet. Because your weight is therefore channeled down through four vertical people to the ground, you can hold yourself horizontally with some ease. Next you are lifted by two people, one holding your shoulders, another your feet. A lot of your weight is concentrated in the center of your body, which is unsupported, so eventually you sag in the middle and fall to the floor. Then you are lifted by one person, who holds you at the center of your back. The weight at both ends of your body has nowhere to go, nothing to carry it to the ground, and you sag at both ends.
Tensile strength refers to the amount of tensile (stretching) stress a material can withstand before it bends or breaks. As applied to architecture, it especially concerns the ability of a material to span horizontal distances without continuous support from below. Returning to the analogy of the body, imagine you are made not of flesh and blood but of strong plastic or metal. Regardless of how you are held up in the air, you can stay rigid and horizontal, because you have great tensile strength.
If you keep these images in mind, you may find it easier to understand the various structural systems we shall consider below. They are introduced here in roughly the chronological order in which they were developed. As was mentioned earlier, all will be of the shell type until the 19th century.