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. 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.
Other Factors to Consider
Clearly, structural performance is a major factor in the selection of structural materials. 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.
Because 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 and corrosion resistance are not the only factors, however. Additional factors, such as ease of fabrication (extrudability and weldability), stiffness (modulus of elasticity), ductility (elongation), weight (density), fatigue strength, and cost should also be considered.
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, curtain-wall 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.
Building and Construction Applications
One of the largest segments of the construction market is the aluminum curtain wall industry. Beginning in earnest in the 1950s with the United Nations Building in New York City and the Alcoa Building in Pittsburgh, aluminum curtain walls began to play a significant role in modern building construction.
These walls act like large curtains hung from the building frame, serving to maintain a weather-tight envelope while resisting wind loads and transmitting them to the frame. Vertical and horizontal extruded aluminum mullions serve as the structural members. Aluminum extrusions also enjoy wide use as frames for doors and windows and in storefronts.
Recently, standing-seam aluminum roof sheeting has become a popular architectural product. Some of these standing-seam products are used as structural members to span between roof purlins.
Aluminum sheet is employed for roofing and siding for corrosive applications or for architectural appeal, as well as routine use for flashing, gutters, siding, soffit, fascia, and downspouts on buildings. Patio and pool enclosures and canopies and awnings are also frequently constructed of aluminum for its ease of fabrication and corrosion resistance.
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.
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J. Randolph Kissell and Robert l. Ferry are both registered professional engineers and founders of The TGB Partnership, a consulting firm specializing in aluminum structural design. They have been responsible for the design of over 1,000 aluminum structures.
This article is excerpted from Aluminum Structures: A Guide to Their Specifications and Design, copyright © 2002, available from John Wiley & Sons and at Amazon.com.