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This chapter does not deal with the origins of aluminum or how it is refined from bauxite, although the ruin satLes Bauxde Provence in southern France are certainly worth a visit. There is an ingot of aluminum in the museum at La Citadelle des Baux as atribute to the metal that is produced from the nearby red rock, which the geologist Berthier dubbed ‘‘bauxite’’ in honor of this ancient fortress in1821. The ruins of the medieval stronghold, though, are the real attraction. We’ll defer to Fodor’s and Frommer’s on the travel tips, and to Sharpona discussion of the history, mining, and production of aluminum. Our purpose in this chapter is to discuss aluminum’s place in the families of structural metals.

 

We include aluminum with steel and reinforced concrete as a metal-based material of construction. While our basis for this grouping may not be immediately obvious, it becomes more apparent when considered in an historical context.

 

 

2.1 METAL IN CONSTRUCTION

 

We include aluminum with steel and reinforced concrete as a metal based material construction. While our basis for this grouping may not be immediately obvious, it becomes more apparent when considered in an historical context.

 

 

Prior to the development of commercially viable methods of producing iron, almost all construction consisted of gravity structures. From the pyramids of the pharoahs to the neoclassical architecture of Napoleonic Europe, builders stacked stones in such a way that the dead load of the stone pile maintained a compressive state of force on each component of the structure (see Figure 2.1). The development of methods to mass-produce iron, in addition to spawning the Industrial Revolution in the nineteenth century, resulted in iron becoming commercially available as a material of construction. Architecture was then freed from the limitations of the stone pile by structural components that could be utilized in tension as well as compression. American architect Frank Lloyd Wright observed that with the availability of iron as a construction material, ‘‘the architect is no longer hampered by the stone beam of the Greeks or the stone arch of the Romans.’’ Early applications of this new design freedom were the great iron and glass railway stations of the Victorian era. Builders have been pursuing improvements to the iron beam
ever since.

 

An inherent drawback to building with iron as compared to the old stone pile is the propensity of iron to deteriorate by oxidation. Much of the effort to improve the iron beam has focused on this problem. One response has been to cover iron structures with a protective coating.

 

Fig 2.1 Pont du Gard in southern France. An aqueduct that

the ancient Romans built by skillfully stacking stones.

 

 

The term coating may be taken as a reference to paint, but it is really much broader than that. What is reinforced concrete, for example, but steel with a very thick and brittle oating? Because concrete is brittle, it tends to crack and expose the steel reinforcing bars to corrosion. One of the functions served by prestressing or osttensioning is to apply a compressive force to the concrete in order to keep these cracks from opening.

 

While one approach has been to apply coatings to prevent metal from  rusting, another has been to develop metals that inherently don’t rust. Rust  may be roughly defined as that dull reddish-brown stuff that shiny steel becomes  as it oxidizes. Thus, the designation of ‘‘stainless’’ to those iron-based  metals that have sufficient chromium content to prohibit rusting of the base  metal in atmospheric service. The ‘‘stain’’ that is presented is the rust stain.  Stainless steel must have been a term that originated in someone’s marketing  department. The term confers a quality of having all the positive attributes of  steel but none of the drawbacks.

 

If we were to apply a similar marketing strategy to aluminum, we might call it ‘‘light stainless steel.’’ After all, it prevents the rust stain as surely as stainless steel does, and it weighs only about one-third as much. Engineers who regard aluminum as an alien material may be more favorably disposed toward ‘‘light stainless steel.’’

 

For the past century and a half, then, structural engineers have relied on metals to impart tension-carrying capability to structural components. Technical development during that time has included improvement in the properties of the metals available for construction. One of the tasks of designers is to determine which metal best suits a given application.

 

 

2.2 MANY METALS FROM WHICH TO CHOOSE

Structural metals are often referred to in the singular sense, such as ‘‘steel,’’ ‘‘stainless steel,’’ or ‘‘aluminum,’’ but, in fact, each of these labels applies to a family of metals. The label indicates the primary alloying element, and individual alloys are then defined by the amounts of other elements contained, such as carbon, nickel, chromium, and manganese. The properties of an alloy are determined by the proportions of these alloying elements, just as the characteristics of a dessert are dependent on the relative amounts of each ingredient in the recipe. For example, when you mix pumpkin, spices, sugar, salt, eggs, and milk in the proper quantities, you make a pumpkin pie filling. By adding flour and adjusting the proportions, you can make pumpkin bread. Substituting shortening for the pumpkin and molasses for the milk yields ginger cookies. Each adjustment of the recipe results in a different dessert. Whereas the addition of flour can turn pie filling into bread, adding enough chromium to steel makes it stainless steel.

 

 

2.3 WHEN TO CHOOSE ALUMINUM

 

2.3.1 Introduction

 

Today aluminum suffers from a malady similar to that which afflicted tomatoes in the eighteenth century: many people fail to consider it out of superstition and ignorance. Whereas Europeans shunned tomatoes for fear that they were poisonous, engineers seem to avoid aluminum for equally unfounded reasons today.

 

One myth is that aluminum is not sufficiently strong to serve as a structural metal. The fact is that the most common aluminum structural alloy, 6061-T6, has a minimum yield strength of 35 ksi [240 MPa], which is almost equal to that of A36 steel. This strength, coupled with its light weight (about one-third that of steel), makes aluminum particularly advantageous for structural applications where dead load is a concern. Its high strength-to-weight ratio has favored the use of aluminum in such diverse applications as bridge rehabilitation (Figure 2.2), large clear-span dome roofs (Figure 2.3), and fire truck booms. In each case, the reduced dead load, as compared to conventional materials, allows a higher live or service load.

 

Aluminum is inherently corrosion-resistant. Carbon steel, on the other hand, has a tendency to self-destruct over time by virtue of the continual conversion of the base metal to iron oxide, commonly known as rust. Although iron has given oxidation a bad name, not all metal oxides lead to progressive deterioration. Stainless steel, as noted previously, acquires its feature of being rust-resistant by the addition of chromium to the alloy mixture. The chromium oxidizes on the surface of the metal, forming a thin transparent film. This chromium oxide film is passive and stable, and it seals the base  metal from exposure to the atmosphere, thereby precluding further oxidation. Should this film be scraped away or otherwise damaged, it is self-healing in that the chromium exposed by the damage will oxidize to form a new film.

 

Figure 2.2 Installation of an aluminum deck on aluminum beams

for the Smithfield Street Bridge in Pittsburgh, Pennsylvania

Aluminum alloys are also rendered corrosion-resistant by the formation of a protective oxide film, but in the case of aluminum it is the oxide of the base metal itself that has this characteristic. A transparent layer of aluminum oxide forms on the surface of aluminum almost immediately upon exposure to the atmosphere. The next discussion on coatings describes how color can be introduced to this oxide film by the anodizing process, which can also be used to develop a thicker protective layer than one that would occur naturally.

 

Corrosion-prone materials are particularly problematic when used in applications where it is difficult or impossible to maintain their protective coating. The contacting faces of a bolted connection or the bars embedded in reinforced concrete are examples of steel that, once placed in a structure, are not accessible for future inspection or maintenance. Inaccessibility, in addition to preventing repair of the coating, may also prevent detection of coating failure. Such locations as the seam of a bolted connection or a crack in concrete tend to be places where moisture or other agents of corrosion collect.

 

Furthermore, aluminum is often used without any finish coating or painting. The cost of the initial painting alone may result in steel being more expensive than aluminum, depending on the quality of coating that is specified. Coatings also have to be maintained and periodically replaced. In addition to the direct cost of painting, increasing environmental and worker  safety concerns are associated with painting and paint preparation practices. The costs of maintaining steel, then, give aluminum a further advantage in life-cycle cost.

 

Fig 2.3 Aerial view of a pair of aluminum space frames covered

with mill finish (uncoated) aluminum sheeting

2.3.2 Factors to Consider

 

Clearly, structural performance is a major factor in the selection of structural materials. Properties that affect the performance of certain types of structural members are summarized in Table 2.1.

 

For example, the strength of a stocky compression member is a function of the yield strength of the metal, while the strength of a slender compression member depends on the modulus of elasticity. Since the yield strength of aluminum alloys is frequently comparable to those of common carbon and stainless steels, aluminum is very competitive with these materials when the application is for a stocky column. Conversely, since aluminum’s modulus of elasticity is about one-third that of steel’s, aluminum is less likely to be competitive for slender columns.

 

Strength is not the only factor, however. An example is corrosion resistance, as we noted above. Additional factors, such as ease of fabrication (extrudability and weldability), stiffness (modulus of elasticity), ductility (elongation), weight (density), fatigue strength, and cost are compared for three common alloys of aluminum, carbon steel, and stainless steel in Table 2.2.

 

TABLE 2.1 Properties That Affect Structural Performance of Metals

 

Structural Performance of	Property
tensile members columns (compression members) beams (bending members)   fasteners welded connections	yield strength, ultimate strength, notch sensitivity yield strength, modulus of elasticity yield strength, ultimate strength, modulus of elasticity ultimate strength ultimate strength of filler alloy; ultimate      strength of heat-affected base metal

 

 

TABLE 2.2 Comparing Common Structural Shapes and Grades of Three Metals

 

While cost is critical, comparisons based on cost per unit weight or unit volume are misleading because of the different strengths, densities, and other properties of the materials. Averaged over all types of structures, aluminum components usually weigh about one-half that of carbon steel or stainless steel members. Given this and assigning carbon steel a relative cost index of 1 results in an aluminum cost index of 2.0 and stainless cost index of 4.7. If initial cost were the only consideration and carbon steel could be used without coatings, only carbon steel would be used. But, of course, other factors come  into play, such as operation and maintenance costs over the life of the structure. Also, in specific applications, the rule of thumb that an aluminum component
weighs one-half that of a steel member doesn’t always hold true. For example, an aluminum component might weigh considerably less when a corrosion allowance must be added to the steel. In other cases, the low material cost of steel is offset by higher fabrication costs, such as applications requiring complex cross sections (for example, curtainwall mullions). In such cases, the cost of steel is much more than just the material cost since the part must be machined, cold-formed, or welded to create the final shape, while the costs of aluminum fabrication are almost nonexistent (the material cost includes the cost to extrude the part to its final shape).

 

Because of stainless steel’s high cost, it is used only when weight is not a consideration and finish and weldability are. In fact, when stainless steel is used in lieu of aluminum, the reason is often only concern about welding aluminum.

 

The families of structural metals, and the individual alloys within each, then, offer a wide range of choices for designers. Each recipe or alloy designation results in certain characteristics that serve specific purposes. When corrosion resistance, a high strength-to-weight ratio, and ease of fabrication are significant design parameters, aluminum alloys merit serious consideration.