Jonathan Ochshorn's Structural Elements for Architects and Builders, Third Edition
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Chapter 1 – Introduction to structural design: Sectional properties

The behavior of structural elements is conditioned by the particular shapes into which these materials are formed, as well as the particular material qualities selected. Wood, steel and reinforced concrete structures can be fabricated from elements having an enormous range of strength, stiffness, size and geometric configurations, subject only to the constraints imposed by manufacturing technologies, transportation and handling, and the requirements of safety and serviceability.

In practice, though, the usual range is smaller, limited to standard shapes and sizes endorsed by industry associations. Typical standards for wood, steel, and reinforced concrete elements are described in the chapters that follow, but a few general observations can be made. For wood and steel, standard cross-sectional shapes are promulgated by industry associations for two primary reasons. First, especially for steel, there is a huge infrastructural investment in the machinery that creates particular shapes and sizes and — in spite of advances in numerical controlled manufacturing processes — it remains impractical to routinely produce custom designs. Second, properties of standard cross-sectional shapes can be easily complied and tabulated, facilitating structural design. Not only that, a whole assortment of subtle structural requirements can be verified in advance by controlling the proportions of these cross sections.

In the case of site-cast reinforced concrete, only reinforcing bars are manufactured in standard sizes. However, even in this case, the concrete must still be cast within forms, and these forms tend to be deployed in standard increments for a variety of reasons. Of course, it is always possible to create custom designed structural shapes, and some notable instances of this practice can be found. Such instances are beyond the scope of this book. Specific requirements for wood, steel, and reinforced concrete are discussed in the chapters that follow.