Building With Architectural Metals (Continued from p. 47 of the March 2009 issue of BD+C)
at the de Young
What has been dubbed “one of the most unique metal surfaces used on any building,” San Francisco’s de Young Museum of Art—designed by Swiss architectural firm Herzog & de Meuron—presents visitors with a dynamically evolving, eye-catching building skin using one million pounds of commercially pure copper.
Aiming for a surface that would emulate light passing through a canopy of trees, the architects partnered with the fabricator A. Zahner Company to create the novel skin. After creating “digital maps” from images of light passing through an actual canopy of trees, the digital information was then coded to machine processes that pushed dies to varying levels across the sheet. All in all, more than six million impressions were applied to the surface or punched through the copper plates.
The museum’s roof is also made of copper. The system is a very-low-slope copper roof where water is evacuated via concealed stainless-steel gutters within the roof system.
If, however, you want to retain copper’s original color for as long as possible, the Copper Development Association recommends the use of overhangs and sloping copper surfaces away from other materials, gutters, and drip edges. That way the weathering process which occurs when copper salts mix with rainwater can be significantly slowed. In addition, a clear coating can be applied to copper panels to provide short-term protection to weathering.
Because copper is one of the most noble metals, care must be taken to prevent it from causing galvanic corrosion when in contact with less noble metals. One strategy, says the Copper Development Association, is painting adjacent surfaces with bituminous or zinc chromate primers or paints. Taping or gasketing with non-absorptive materials also can help.
As for fastening copper with other metals, CDA’s architecture regional manager Wayne Seale recommends that the fasteners be made of copper, copper alloys, or stainless steel. Designers should also seek ways to prevent rainwater from running off the copper to other metals, particularly aluminum or galvanized-steel gutters and downspouts.
COPPER ALLOYS—AESTHETICS AND DURABILITY
Copper alloys—brass, bronze, and nickel-silver—are known for their striking aesthetics and durability. In fact, KlingStubbins’ Castner identifies copper alloys as being associated with high-end buildings. “Because of the expense involved, these alloys were associated with projects that were high quality, where cost was less of an issue,” he says.
For example, bronze—which was historically an alloy of copper, and today, copper and brass—was traditionally used for custom-designed doors and decorative features on historic public buildings, according to preservationists Gayle and Look. Even today, brass (copper plus zinc) is used to add elegance to doors, windows, elevators, and escalators. As for nickel-silver, it’s actually an alloy of copper and nickel, despite its name. “It offers a beautiful light golden tone, and although it’s one of the more expensive of the copper alloys, nickel-silver, when used correctly, offers extremely elegant color tones,” says Zahner. Historically, nickel-silver was used to contrast other metals and became quite popular in the 1920s and 1930s; it is still in use as a striking decorative material today.
Another application is nickel electroplating, which serves the dual function of a decorative finish and corrosion resistance. In fact, two-thirds of all nickel annually manufactured goes into the production of stainless steel, specifically for this purpose.
Although copper alloys are known for their weatherability, one type of corrosion is unique to brass and bronze alloys with more than 15% zinc. Called dezincification, according to preservationist Waite, the copper-zinc alloy can be dissolved when exposed to acidic or strongly conducting solutions. The effect is pronounced, leaving the metal pitted, porous, and often weakened. Appropriate preventive measures should be taken to avoid exposing copper-zinc to these solutions.
ZINC—LONGEVITY, LOW MAINTENANCE
Zinc is known for its long-lasting, low-maintenance, and corrosion-resistant traits. As with copper, says Koziol, zinc resists corrosion by forming an oxide film when exposed to the atmosphere. Zinc is typically alloyed with copper or titanium (or both) for use in architectural applications for improved strength and to reduce brittleness, he adds.
Laurent Heindryckx, technical manager of the commercial zinc roof and wall system fabricator Umicore Building Products USA, Raleigh, N.C., explains how this process works: “Zinc forms a protective layer of hydroxycarbonate when exposed to water and carbon dioxide. This natural patina protects zinc and naturally reforms when accidentally scratched. This self-healing characteristic makes zinc very attractive to building owners.” Because of these properties, zinc is also used to protect other materials, such as galvanized steel, from corroding.
Historically, zinc was used exclusively to manufacture brass, but by the late-16th century, it was recognized as its own metal in Europe, according to the Washington, D.C.-based American Zinc Association. Although zinc has been used in Europe for the past 200 years—about 85% of the metal roofs in Paris are made of zinc—it was not adopted in the U.S. until 1850.
One of the reasons why zinc is such a popular roofing and cladding choice is its longevity. Zinc roofs can last 100 years, and zinc wall applications can last as much as 150 years. The material was historically thought of primarily as a roofing material, but recently zinc has come into favor as a cladding material because of the aesthetic appeal of its patina. KlingStubbins architect Joseph Castner points to zinc’s “excellent longevity” and softness, which allows it to be worked in the field with hand tools.
At the same time, zinc falls into the price range of higher-end building materials such as slate, stainless steel, and copper. Umicore’s Laurent Heindryckx says there are two reasons why the initial installation cost of zinc roofs is higher than roofs made of painted steel and aluminum: “First, the cost of the raw material is higher per square foot. Second, the quality of the workmanship required to install long lasting products is higher, and therefore more expensive. However, the cost per square foot per year is relatively low when the service life of the metal is considered.”
In terms of environmental considerations, zinc’s long life span means that it seldom needs replacement. Although it is 100% recyclable, it only has a recycled content of about 17%, according to Heindryckx. At the same time, he notes, “less energy is used for refining and recycling zinc because of its low melting point, whereas copper and stainless steel need twice [as much] energy, and aluminum even four times more.”
While zinc is resistant to corrosion on its top side—which is why close to half the zinc produced every year is used for galvanizing steel—it is vulnerable to corrosion on its underside when exposed to moisture for a prolonged time, says Koziol. Consequently, “Venting the underside of zinc sheeting is required to prevent corrosion from the underside.” As for galvanic corrosion, zinc is not compatible with copper or iron, so zinc is usually installed with a minimum slope in order to drain water.
Aesthetically, zinc comes in matte grey, black, pigmented blue-grey, red-grey, and green-grey. “Zinc offers an interesting gray surface, but can also be enhanced to produce oxides of color tones that rival the natural minerals found in decorative stone,” says Zahner.
TITANIUM—COMPATIBLE WITH GLASS AND CONCRETE
Also lending an attractive grey appearance is titanium, a nickel alloy. “Titanium, when detailed and constructed correctly, is a beautiful, extremely thin and soft grey metal. It can also receive interference coloring to enhance its soft natural color,” says Zahner.
Due to its strength, durability, corrosion resistance, and low thermal coefficient, titanium is actually become a roofing option for commercial and institutional building owners, according to Haddock.
In fact, titanium’s coefficient of thermal expansion is half that of stainless steel and copper, and one-third that of aluminium, making it almost equal to glass and concrete in that regard. As a result, the thermal stress on titanium is very low and it is highly compatible with glass and concrete, according to The A to Z of Materials, Warriewood, NSW, an Australia-based online resource for the design and engineering community supported by a team of global materials scientists.
Spotlight: Zinc Roofing at UNC Greensboro When it came time to choose a roofing material for the Science Building and Hall for Humanities & Research Administration Building at the University of North Carolina in Greensboro, longevity and durability were high on the priority list. Consequently, the university chose zinc panels with a preweathered quartz-zinc surface. Though the product had a higher first cost than other materials, Fred Patrick, the university’s director of Facilities Design and Construction, was able to justify the expense to the university’s board of trustees. “‘Utilizing better materials that will meet our long-term goals will save money for us in the long run,’” Patrick recalls telling the trustees. As a result, he says, “The university was willing to pay a little more up front for quality materials that will meet our standards. Once they saw how products like zinc actually save us money in the long run, we got the green light.” Designed by Calloway Johnston Moore & West, Winston Salem, N.C., the building called for zinc panels that were specified with a backside coating to ensure the prevention of corrosion on the underside of the panels. The designers also chose to go with a sloped roof, as opposed to a flat roof, in the interest of longevity. In terms of blending aesthetics and durability, CJMW project manager Andy Sykes says, “Zinc is a very natural material with a self-healing characteristic that protects it from scratches and mars.” |
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Zahner adds that titanium’s low expansion and contraction characteristics allow for tight-fitting surfaces. “Like stainless steel, titanium resists oxidation in natural environments and maintains an even appearance indefinitely,” he says.
In addition, titanium has very low thermal conductivity—one-tenth that of aluminum—making it an excellent insulator. It is also relatively lightweight, enabling easier fabrication and installation and requiring less structural support, according to AZoM.
At the same time, titanium is somewhat harder to work with, says Zahner, and one of the more expensive of the architectural metals. For those reasons, it offers more potential as a surfacing material.
Monel. Another “exotic metal” with properties similar to titanium’s is monel. Although it is quite expensive, its strength and corrosion resistance make it a great application for harsh environments such as coastal areas, explains Castner.
Monel is an alloy of approximately two-thirds nickel and one-third copper. Discovered in 1905, it was more commonly in the first half of the 20th century for applications such as roofing, until it was replaced by more economic metals like stainless steel and aluminum, according to Gayle and Look.
GALVANIZED STEEL—ECONOMICAL, BEAUTIFUL
Galvanized steel and Cor-Ten steel are also viable metal options available to designers. “Galvanizing steel is an ideal way to extend steel’s life by improving its resistance to the elements,” explains McLane. “Galvanized flashing, siding, and roofing are relatively inexpensive, long-lasting products. If they are also coated or painted, their life expectancy is increased even further.”
Typically a pure zinc coating is used for galvanizing; in addition to increasing steel’s longevity, the zinc becomes part of the steel recycling circuit.
“When used correctly, hot-dipped galvanized steel is an economical choice and can provide beautiful irregular reflective surfaces,” says Zahner. “We create our own hot-dipped galvanized panels and forms that can be spectacular in appearance.”
Cor-Ten. According to Castner, Cor-Ten is a steel alloy that forms a patina on the surface to slow the rate of corrosion, such that it requires no other finish to protect the metal. “It is particularly well-suited to the drier climates where it achieves that patina finish early and then stops corroding because the environment is dry.”
In addition, it is becoming a more attractive choice for cool roof systems. However, Vlahos warns against other applications. “Cor-Ten should not be used for siding or roofing. It is more susceptible to the risk of corrosion due to improper design, fabrication, erection, and maintenance. Cor-Ten does not perform well for roofing panels or wall panels because it is susceptible to weathering and corrosion. The material will reform, resulting in perforations in the surface over time.”
METAL COMPOSITE MATERIALS
Metal composite materials, or MCMs, are defined as two metal skins sandwiching an engineered plastic core. When they were first used in construction in the early 1980s, the metal of choice was aluminum. But more recently, zinc, copper, stainless steel, and titanium are being manufactured into panels as well.
Ted Miller, of the Miller-Clapperton Partnership, says MCMs (as outlined by Section 1407 of the International Building Code) offer superior flatness over many other building materials, which can have surface imperfections that show in the finished product. “MCMs are easily formable with both computer-controlled machining centers or simple hand tools, and they can be utilized to ‘trim out’ the intersection of other major materials to form a seamless transition,” says Miller.
Metal used in composite materials offers very high strength, rigidity, flatness, and durability, says Kriner. Applications include interior and exterior wall systems, curtain walls, facades, signage, and replacing full-thickness aluminum or steel wall products.
Glossary of Metal Deterioration In addition to the well-known causes of metal deterioration—corrosion and galvanic corrosion—a number of other chemical and mechanical elements can cause the breakdown and eventual failure of metal. Historic preservation expert John G. Waite, FAIA, principal of John G. Waite Associates, New York, defines the many terms associated with metal corrosion in his book Deterioration and Methods of Preserving Architectural Metals. Here is a partial glossary: Abrasion. The erosion of the metal caused by dirt, dust, sand, rain, sleet, or hail, or by rubbing against another architectural element, or through human contact. Connection failure. Bolted or welded connections of metal elements may fail through overloading, fatiguing, or corrosion of the connectors. Creep. Deformation of soft metals under sustained stressing, sometimes under relatively high temperatures. Erosion. Can occur when protective oxide, films, or layers are removed by abrasion and the metal is exposed to corrosive agents. Fatigue. Metal failure due to repeated cyclic stresses below the elastic limit. Fire damage. Unprotected iron and steel framing members can fail rapidly when exposed to prolonged fire. Overloading. Stressing the metal beyond its yield point, causing deformation, fracturing, or failure. Stress corrosion cracking. Can occur when stresses are induced into the metal during the metalworking process and the metal is subsequently exposed to corrosion. |
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Metal composite materials are easily workable into a wide range of geometric configurations, and due to their light weight, they require less structural support than some other architectural metals.
NOVEL FINISHES—CREATING ‘UNLIMITED DESIGNS’
Although metals can be galvanized, anodized, and painted in countless number of colors, a number of novel finishes can greatly enhance the decorative qualities of metal.
For example, prints of “unlimited designs” can be prepainted onto metal sheet products, according to Kriner. Wall cladding can have stucco-facsimile textures or concrete appearances. And metal roofing is available in a wide variety of colors and granular coatings to simulate wood shake, clay tile, asphalt shingle, and slate.
Similarly, there are faux metal finishes to emulate wood grains or stone looks, according to Miller: “Many of the color finishes offer interesting metallic and prismatic looks as well as a textured.” Zahner adds colored zinc and colored and custom-textured stainless steel, as well as “angel hair” stainless steel.
In addition, copper in colors other than green and pre-weathered weathering steel are also “beautiful surfacing materials,” says Zahner.
An unusual approach to metal finishes comes from Josh Shelton, principal of el dorado inc., Kansas City, Mo. “I like to think of light as an interesting finish for different metal surfaces,” he says. “Being aware of what type of light an exterior wall panel will receive from dawn to dusk can really animate the profile of a selected metal siding. Also, integrating lighting systems into metal wall systems can achieve the same effect at night.”
MORE THAN A FEW CHOICES
Although there are many building materials to choose from and each has its time and place, metals, in Castner’s opinion, “in general are well suited as a building material because of their weight-to-strength ratios, durability, workability, and cost. In essence, great value.”
Most architectural metals also have an impressive recycling track record, as Zahner explains. “Scrap from fabrication processes, damaged material, and obsolete metal structures are readily recycled and put back to use in new metal constructs,” he says. “Can you name another material that comes close to this?”
About the authors
C.C. Sullivan is a communications consultant and author specializing in architecture and construction. Barbara Horwitz-Bennett is a writer and contributor to construction industry publications.
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