The availability of suitable structural materials is one of the principal limitations on the accomplishment of an experienced structural engineer. Early builders depended almost exclusively on wood, stone, brick, and concrete. Although iron had been used by humans at least since the building of the Egyptian pyramids, use of it as a structural material was limited because of the difficulties of smelting it in large quantities. With the industrial revolution, however, came both the need for iron as a structural material and the capability of smelting it in quantity.
John Smeaton, an English civil engineer, was the first to use cast iron extensively as a structural material in the mid-eighteenth century. After 1841, malleable iron was developed as a more reliable material and was widely used. Whereas malleable iron was superior to cast iron, there were still too many structural failures and there was a need for a more reliable material. Steel was the answer to this demand. The invention of the Bessemer converter in 1856 and the subsequent development of the Siemens-Martin open-hearth process for making steel made it possible to produce structural steel at competitive prices and triggered the tremendous developments and accomplishments in the use of structural steel over the next hundred years.
The most serious disadvantage of steel is that it oxidizes easily and must be protected by paint or some other suitable coating. When steel is used in an enclosure where a fire could occur, the steel members must be encased in a suitable fire-resistant enclosure such as masonry, concrete. Normally, steel members will not fail in a brittle manner unless an unfortunate combination of metallurgical composition, low temperature, and bi-or trysail stress exists.
Structural aluminum is still not widely used in civil engineering structures, though its use is steadily increasing. By a proper selection of the aluminum alloy and its heat treatment, a wide variety of strength characteristics may be obtained. Some of the alloys exhibit stress-strain characteristics similar those of structural steel, except that the modulus of elasticity for the initial linearly elastic portion is about 10,000,000 psi (700,000 kgf/cm*cm) or about one-third that of steel. Lightness and resistance to oxidation are, of course, two of the major advantages of aluminum. Because its properties are very sensitive to its heat treatment, care must be used when riveting or welding aluminum. Several techniques have been developed for prefabricating aluminum subassemblies that can be readily erected and bolted together in the field to form a number of beautiful and well-designed shell structures. This general procedure of prefabrication and held assembly by bolting seems to be the most promising way of utilizing structural aluminum.
Reinforced and priestesses concrete share with structural material. Natural cement concretes have been used for centuries. Modern concrete construction dates from the middle of the nineteenth century, though artificial Portland cement was patented by Aspidin, an Englishman, about 1825. Although several builders and engineers experimented with the use of steel-reinforced concrete in the last half of the nineteenth century, its dominant use as a building material dates from the early decades of the twentieth century. The last fifty years have seen the rapid and vigorous development of pretested concrete design and construction, founded largely on early work by Freyssinet in France and Magnel in Belgium.
Plain (unreinforced) concrete not only is a heterogeneous material but also has one very serious defect as a structural material, namely, its very limited tensile strength, which is only of the order of one-tenth its compressive strength. Not only is tensile failure in concrete of a brittle type, but likewise compression failure occurs in a relatively brittle fashion without being preceded by the forewarning of large deformations. (Of course, in reinforced-concrete construction, ductile behavior can be obtained by proper selection and arrangement of the reinforcement.) Unless proper care is used in the selection of aggregates and in the mixing and placing of concrete, frost action can cause serious damage to concrete masonry. Concrete creeps under long-term loading to a degree that must be considered carefully in selecting the design stress conditions. During the curing process and its early life, concrete shrinks a significant amount, which to a degree can be controlled by properly proportioning the mix and utilizing suitable construction techniques.
With all these potentially serious disadvantages, engineers have learned to design and build beautiful, durable, and economical reinforced-concrete structures for practically all kinds of structural requirements. This has been accomplished by careful selection of the design dimensions and the arrangement of the steel reinforcement, development of proper cements, selection of proper aggregates and mix proportions, careful control of mixing, placing, and curing techniques and imaginative development of construction methods, equipment and procedures.
The versatility of concrete, the wide availability of its component materials, the unique ease of shaping its form to meet strength and functional requirements, together with the exciting potential of further improvements and development of not only the newer pretested and precast concrete construction but also the conventional reinforced concrete construction, combine to make concrete a strong competitor of other materials in a very large fraction of structures.
In modern times, with the increased use of steel and reinforced-concrete construction, wood has been relegated largely to accessory use during construction, to use in temporary and secondary structures, and to use for secondary members of permanent construction. Modern technology in the last sixty years has revitalized wood as a structural material, however, by developing vastly improved timber connectors, various treatments to increase the durability of wood, and laminated wood made of thin layers bonded together with synthetic glues using revolutionary gluing techniques. Plywood with essentially no directional strength properties is the most widely used laminated wood, but techniques have also been developed for building large laminated wood members that for certain structures are competitive with concrete and steel.
Materials with future possibilities are the engineering plastics and the exotic metals and their alloys, such as beryllium, tungsten, tantalum, titanium, molybdenum, chromium, vanadium, and niobium. There are many different plastics available, and the mechanical properties exhibited by this group of materials vary over a wide range that encompasses the range of properties available among the more commonly used structural materials. Thus in many specific design applications it is possible to select a suitable plastic material for an alternative design. Experience with the use of plastics outdoors is limited. Generally speaking, however, plastics must be protected from the weather. This aspect of design is therefore a major consideration in the use of plastics for primary structural elements. One of the most promising potential used of plastics is for panel and shell-type structures. Laminated or sandwich panels have been used in such structures with encouraging results that indicate an increased use in this type of construction in the future.
Another materials development with interesting possibilities is that of composites consisting of a matrix reinforced by fibers or fiber like particles. Although glass-fiber-reinforced composites with a glass or plastic matrix have been used for years, they appear to have much broader possibilities for a large variety of secondary structural components. Fiber-reinforced concrete is another composite being actively studied and developed. Several experimental applications are being observed under service conditions. Experiments have been conducted with both steel and glass fibers, but most of the service experience has been with steel fibers.
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