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CapabilitiesCapabilities andand LimitationsLimitations ofof ArchitecturalArchitectural :Metals: Part 1

by Catherine Houska

etal offers many attractive design possibilities. But to Muse metals safely and eco- nomically, architects, contractors, and building owners must under- stand both their capabilities and limitations. A providing good performance in one application can corrode rapidly under different cir- cumstances. Strength, weight, desired appearance, resistance, expected service life, and costs associ- ated with installation and longterm maintenance must be considered.

PHOTOS COURTESY DEVELOPMENT INSTITUTE

October 2000 The Construction Specifier 1 ADDITIONAL INFORMATION 304 and 316 and other 300-series stainless oxide (rust) layer. Corrosive substances can are not magnetic and can be still pass through the oxide film, but corro- Author strengthened by cold work. In most archi- sion rates decrease as the layer thickens.1 CATHERINE HOUSKA, CSI, is a tectural applications, stainless is used metallurgical engineer with an MBA in in the annealed or "dead soft" condition. In This uniform protective layer will not form industrial marketing. She specializes in architectural applications for metal and structural and sheet applications subjected indoors, or in arid or continuously wet is market development manager at to higher loads or impact, cold working can environments. If this layer is allowed to Technical Marketing Resources, Inc., a be an economical means of reducing form and is not disturbed, weathering consulting firm in Pittsburgh, Pennsylvania. She can be reached via weight and increasing strength. steels provide significantly better corrosion e-mail at [email protected]. resistance than carbon steels. Type 430 is used in indoor, rural, or urban MasterFormat No. locations with low corrosion potential. Weathering steels should not be used 05050—Basic Metal Materials Type 304 is used in indoor, rural, moder- where mechanical or chemical disruption and Methods ately polluted urban applications, and of the film is likely. Mechanical damage coastal applications with low humidity and could result from windborne abrasive par- Key Words temperatures. Type 316 is used for marine, ticles, rubbing, or scratching. Chlorides alloys coastal, or deicing salt exposures and in and industrial pollutants disrupt the film. oxide industrial and aggressive urban locations. Weathering steels are not suggested for corrosion In some very aggressive sites, a more corro- coastal, marine or deicing salts, or industri- 3 Abstract sion-resistant may be needed. al environments.

Metal has become increasingly popular because it provides unique design If an appropriate stainless steel is Water coming into contact with weathering opportunities, strength, and corrosion used, there is no reason to apply steel can stain other materials. Designs must resistance. Each metal family has a protective coating. A variety of drain water away from other materials and unique characteristics. Performance over time depends on the designer mechanical and colored finishes are permit the surface to dry. Elements not understanding the advantages and available. Stainless steel is available in boldly exposed to the atmosphere should limitations of each metal and taking sheet, strip, plate, bar, tube, pipe, castings, be painted. (See photo at end of article.) these into consideration during the design process. and extrusions.

Stainless steel is some- This article provides some guidelines for times mistakenly ordered determining the service environments by gage number. There where architectural metals are likely to is no standard definition perform well and where problems may of stainless-steel gage occur. Physical and mechanical properties numbers, and specifiers will be summarized and comparative should always specify the appropriate thicknesses in sample applica- desired thickness. tions will be shown. Selection Criteria: Selection Criteria: Stainless Steels Weathering Steels Stainless steels contain at least 12 percent High-strength, low- . They form a thin (invisible), weathering steels are car- protective, corrosion-resistant passive film bon steels with combined on their surface. The film self-repairs additions of up to two per- quickly if it is damaged or removed dur- cent , chromium, ing fabrication or polishing. If an appro- nickel, and phosphorus. priate stainless steel is selected, it will They are used in exterior retain its original appearance with no cor- applications and have rosion product discoloration of sur- mechanical properties rounding materials. similar to those of struc- tural carbon steels. In cli- The stainless steels most commonly used in mates with wet and dry and construction are Types cycles, they form a protec- The United States Steel Worker’s Building (formerly the IBM Building) was completed in 1963. It has Type 304 stainless steel 430, 304, and 316. Type 430 and other 400- tive, dense, adherent, cladding on the steel lattice trusses and anodized aluminum panels series stainless steels are magnetic. Types stable, corrosion-resistant under the windows. Architect: Curtis & Davis.

2 The Construction Specifier October 2000 The corrosion rate is nonoxidizing acids. Alkalis (which may dependent on the envi- leach from concrete), ammonia, and sul- ronment and protective fur compounds will cause corrosion. Fire- coating integrity or the retardant treated wood may be treated effectiveness of the with chemicals that corrode copper. cathodic protection sys- tem (e.g., ). Regular has good resistance to atmospheric exposure to moisture corrosion, alkalis, and organic acids. In some (rain, fog, high humidity potable waters and in seawater, with levels), industrial pollu- 20 percent or more zinc may suffer corrosive tion, sulfur oxides, and attack. Moist ammonia, mercury, and moist chlorides can significantly chlorine gas can cause stress corrosion increase corrosion rates. If cracking. Brasses used in architecture protection is not main- include commercial , red brass, car- tained, rust can discolor tridge brass, yellow brass, and Muntz metal. surrounding materials, and rust may undermine Nickel silvers are copper-nickel-zinc alloys more of the protective with a golden-silver luster. They were par- surface layer. Surface rust ticularly popular during the early 1900s buildup should be when architecture was in style. removed or it will retain Other names used for this alloy family are moisture, concentrate German silver, white copper, white bronze, corrosive chemicals, and Paris metal, and dairy bronze. accelerate corrosion.3 Selection Criteria: ® Selection Criteria: MONEL® is a trade name for a highly Copper and corrosion-resistant family of silver-white One Oxford Center has a painted carbon steel curtain wall and was Copper Alloys colored nickel-copper alloys. Alloy 400 is completed in 1983. Architect: Hellmuth, Obata, & Kassabaum. The natural color of cop- the generic name for the alloy used in per alloys Selection Criteria: Carbon Steel varies considerably with Carbon steels are at least 95 percent alloy content and includes with two percent or less carbon and are shades of red, gold, and classified by grade based on their carbon silver. Copper and its content. Strength and hardness increase alloys form a thin adher- with carbon content but ductility decreas- ent oxide layer or es. The grades typically used in architecture which gradually changes are very mild (0.05 percent to 0.15 percent the metal’s color. The carbon), mild (0.15 percent to 0.25 percent color will vary with the carbon), and medium (0.25 percent to alloy and may not be uni- 0.35 percent carbon) carbon steel.2 form. If moisture is pre- sent, copper can stain Carbon steels are available in the product porous materials. Detail- forms needed for architecture including ing should prevent water structural shapes, pipe, tube, sheet, and run-off onto these sur- strip. Because bare carbon steels will rust in faces. A wide range of air, they are normally protected with metal product forms are avail- coatings, porcelain enamel, or paint. able. Intricate castings Common metal coating processes include are possible. aluminized (aluminum), chromium, cop- per, brass, galvanized (zinc), Galvalume® Electrolytic tough pitch (aluminum-zinc), terne (- or tin- (99.9 percent pure) copper, zinc), and tin. The precautions discussed in used for roofing, provides the zinc alloys section also influence the life good corrosion resistance The copper details on the dome of Midtown Towers were installed in of zinc coatings. to chlorides and deareated 1908. Architect: Thomas Hannah.

October 2000 The Construction Specifier 3 Aluminum forms a pro- darker gray patina on these alloys. tective oxide film with a high level of corrosion Zinc is resistant to chloride corrosion, but resistance when clean industrial atmospheres containing sulfur metal is exposed to air. dioxide can accelerate corrosion. The only strengthens common woods that can be used in contact and thickens the film, with zinc are pine, poplar, and spruce. If zinc providing greater corro- is in contact with another metal and mois- sion protection. Either ture is present, the metals must be electrical- anodized or resinous ly insulated from each other because zinc organic coated alu- corrodes in preference to all other architec- minum is normally spec- tural and engineering metals. This effect is ified for exterior applica- so strong zinc is regularly used as a sacrificial tions. Ceramic coatings anode to protect other metals. can also be applied. Aluminum is available in Zinc and zinc coatings must be protected sheet, strip, bar, plate, from condensation and standing water. tube, pipe, forgings, cast- ings, and extrusions. There should be a free flow of air behind zinc panels to prevent condensation. If When aluminum cor- thermal insulation is used, the space rodes, the surface between the zinc and sheeting should becomes rough and be vented. Zinc should not be placed in chalky with a white or direct contact with cement, mortar, or gray film. The film color other moisture retaining materials. makes corrosion less vis- ible but there can be Extending Service Life some staining of adjacent Like all materials used in exterior environ- surfaces. Atmospheric ments, metals can accumulate dirt and Smithfield United Church of Christ is a Historic Landmark built in 1926. The elaborate metal spire is aluminum castings on a steel struc- and coastal and debris. These deposits can be corrosive. tural frame. Both are painted. It was the first aluminum spire in the deicing salts can cause The corrosion resistance of metals is world. Architect: Henry Hornbostel. corrosion. A heavy pro- improved by regular removal of corrosive tective coating is needed deposits from the surface. In environments architecture. It was very popular from if aluminum will be in contact with con- with plentiful rain, designs encouraging 1909 to the mid-1950s and is sometimes crete, masonry, plaster, or soil. natural rain cleaning can be effective in mistaken for stainless steel in restoration removing corrosive deposits, particularly if work. It continues to be used in roofing. Wood in contact with aluminum should be the surface finish is smooth. Building exte- In exterior applications, it will develop a sealed with a primer and paint. However, riors often have elements not easily washed patina ranging from light gray-green to fastenings into oak, chestnut, or western by rain. Regular cleaning to remove medium brown. It provides excellent cedar should be avoided unless the wood deposits can enhance the corrosion resis- resistance to atmospheric corrosion, is well seasoned. Under damp conditions, tance of metals, extend their service life, including relatively severe chloride levels, insulating products can release by- and keep them attractive. alkalis (which may leach from concrete), products corrosive to aluminum unless a or industrial . sealant is used. Mechanical and Physical Properties Selection Criteria: Aluminum Alloys Selection Criteria: Zinc Typical mechanical and physical properties Aluminum alloys are lightweight, silver- Zinc’s low creep strength at room tempera- of some common architectural metals are colored, and provide corrosion resistance ture and high thermal expansion coeffi- shown in Tables 1 and 2. The ASTM speci- in a variety of environments. Some alloys cient should be considered during design fications should always be used to specify used in architectural applications are 3003, of roof and wall panels. Zinc alloys with at the desired grade or alloy and its mechani- 3105, 5005, 6061, and 6063. The 3000 and least 0.05 percent and 0.10 per- cal properties. Additional information on 5000 series alloys can be strengthened by cent copper have improved creep resistance physical properties can be obtained from heat treating ("H" tempers). The 6000 and mechanical strength and weather to a metal suppliers, industry associations, and series alloys can be strengthened by cold grayer patina than pure zinc. A phosphate metals reference books such as the ASM work ("T" tempers). pretreatment can be used to produce a International Metals Handbook series.

4 The Construction Specifier October 2000 Specifying Metal Thickness determine the appropriate thickness or metals (e.g., steel versus stainless steel) can Metal strength, strength-to-weight ratios, weight for a project. Assuming the same lead to costly over or under specification. corrosion resistance, and wear resistance thickness is appropriate for two different Industry associations and manufacturers

Building Material Strength Tensile Yield Strength, Ultimate Elong. to Weight Strength, ksi (MPa) Shear in 2 in. ratio ksi (MPa) Strength (50mm), ksi (MPa) min %

Stainless steel Type 316 Annealed 257 75 (515) 30 (205) –– 35 Stainless steel Type 304 Annealed 257 75 (515) 30 (205) –– 35 Stainless steel Type 430 232 65 (450) 30 (205) –– 22 HSLA Steel, 3/4-1.5 in. (20-40 mm) 239 67 (460) 46 (315) –– 21 Structural steel (plate and bar) 261 58/80 (400/550) 36 (250) –– 23 Carbon steel commercial sheet, cold rolled 164 46 (317) 34 (234) –– 35 Aluminum Alloy 3003-H14 222 22 (150) 21 (145) 8 (55) 40 Aluminum Alloy 3105-H14 255 25 (170) 22 (150) 15 (105) 5 Aluminum Alloy 5005-H16 265 26 (180) 25 (170) 15 (105) 5 Aluminum Alloy 6061-T6 459 45 (310) 40 (275) 30 (205) 12 Aluminum Alloy 6063-T5 276 27 (185) 21 (145) 17 (115) 12 Copper 112 36 (250) 28 (195) 25 (170) 30 Red Brass 158 50 (345) 39 (270) 35 (240) –– Muntz Metal M20 178 54 (370) 21 (145) 40 (275) 45 H01 (C74500) 207 65 (450) 45 (310) 45 (310) 25 Cast 20% Leaded Nickel Silver 146 47 (324) 26 (179) –– 10 Cast 25% Leaded Nickel Silver 172 55 (379) 31 (214) –– 16 Alloy 400, cold rolled, annealed 250 80 (550) 35 (240) –– 40 Zinc-Copper-titanium alloy 84 21.8 (150) 14.5 (100) –– ––

Table 1: Typical mechanical properties for architectural metals.4

Building Material Density Lbs/ft3 Coef. Thermal Expansion Thermal Conductivity (g/cm3) 68-212 OF (20-100 OC) 212ºF(100 OC) µin/in OF (µm/mo OK) Btu·in/ft2·hr·F (W/m· OK)

Type 316 0.29 (8.0) 8.8 (15.9) 113 (16.3) Type 304 0.29 (8.0) 9.6 (17.2) 113 (16.3) Type 430 0.28 (7.8) 5.8 (10.4) 181 (26.1) Gray Iron 0.27 (7.5) 6.6 (11.9) 348(50) Carbon Steel 0.28 (7.8) 6.7 (12.1) 324 (47) HSLA Weathering Steel 0.28 (7.8) 6.7 (12.1) 324 (47) Alloy 400 0.32 (8.80) 7.7 (13.9) 151 (21.8) Copper C11000 0.321 (8.89) 9.4 (17) 2688 (387) Red Brass 0.316 (8.75) 10.4 (18.7) 1104 (159) Muntz Metal 0.303 (8.39) 11.6 (20.8) 852 (122) Nickel Silver 0.314 (8.69) 9.1 (16.4) 312 (45) Cast 20% Leaded Nickel Silver 0.321 (8.90) 9.3 (16.7) 156 (22.5) Cast 25% Leaded Nickel Silver 0.320 (8.86) 9.7 (17.5) 176.4 (25.4) Aluminum Alloy 3003 H14 0.099 (2.73) 12.9 (23.2) 1100 (159) Aluminum Alloy 3105 0.098 (2.72) 13.1 (23.6) 1190 (172) Aluminum Alloy 5005 0.098 (2.70) 13.2 (23.75) 1390 (200) Aluminum Alloy 6061 T6 0.098 (2.70) 13.1 (23.6) 1160 (167) Aluminum Alloy 6063 T5 0.097 (2.70) 13.0 (23.4) 1450 (209) Zinc-Copper-titanium alloy 0.259 (7.17) 12 (22) 727 (105)

Table 2: Physical constants and thermal properties.5

October 2000 The Construction Specifier 5 Pan Width for a Seam Recommended thickness, gage, or weight Height of 25 mm (1 in.) Stainless Steel Galvanized Steel Copper Aluminum Inches (mm) Gage Inches (mm) Oz. Inches (mm) Inches (mm)

16.75 to 20.75 0.015 (0.38) 24 0.024 (0.61) 16 0.022 (0.55) 0.032 (0.81) (430 to 530) 22.75 to 23.75 0.018 (0.46) 24 0.024 (0.61) 16 0.027 (0.69) 0.040 (1.02) (580 to 600)

Table 3: SMACNA recommendations for standing seam roofs.

Girth Galvanized Steel Copper Aluminum Stainless Steel Inches (mm) Gage Inch (mm) oz/ft2 Inch (mm) Inch (mm) Inch (mm)

≥15 (≥380) 26 0.022 (0.55) 16 0.022 (0.55) 0.032 (0.81) 0.016 (0.40) 16–20 (410-510) 24 0.028 (0.70) 16 0.022 (0.55) 0.040 (1.02) 0.019 (0.48) 21–25 (530–640) 22 0.034 (0.85) 20 0.027 (0.69) 0.051 (1.30) 0.025 (0.64) 26–30 (660-760) 20 0.040 (1.01) 24 0.032 (0.82) 0.063(1.60) 0.031 (0.80)

Table 4: SMACNA recommendations for rectangular gutters. can be a good source of guidelines. The steel are normally encased in insulating commercially available 10-m (33-ft) long Architectural Sheet Metal Manual pub- materials to limit temperature increase cable ladders uniformly loaded to simulate lished by the Sheet Metal and Air during fires. Types 316, 304, and 430 stain- Conditioning Contractors National less steel can withstand short exposure A close up of the roof. Association, Inc., (SMACNA)6 is an excel- times up to 870 OC (1600 OF). lent resource for determining appropriate metal thicknesses, gages, or weights. Five industry associations and metals Examples are shown in Tables 3 and 4. producers sponsored fire and radiation resistance testing of structural materials: Fire-Resistance galvanized carbon steel, fiberglass- Resistance to fire and thermal radiation reinforced plastic (FRP), aluminum, and damage can be an important safety consid- Type 316 stainless steel.10 The tests used eration. The metals used for architecture exhibit significant differences in strength retention at elevated temperatures even after brief exposure to high temperatures. Copper and aluminum start to experience strength reductions at fairly low tempera- tures. Aluminum alloys begin to show a reduction in strength at temperatures above 100 OC (212 OF).3 At 204 OC (400 OF), the tensile strength of copper has decreased about 25 percent and 6061-T6 aluminum has decreased by about 60 percent.7

Under continuous loading, carbon steel is usually limited to a maximum tempera- ture of 370 OC (700 OF).8,9 By the time steel reaches 500 OC (930 OF), it has lost about 30 percent of its tensile strength. Unprotected weathering steel loses about half of its tensile strength above 538 OC The Pittsburgh Mellon Arena had the largest dome in the world and largest retractable roof when it was (1000 OF).2 Both carbon and weathering completed in 1961. The roof is Type 304 stainless steel. Architect: Dahlen K. Ritchey.

6 The Construction Specifier October 2000 the weight of electrical cables. The FRP and Trademarks: aluminum ladders failed both tests. MONEL® is a registered trademark of Special Metals Corporation In the five-minute test for fire-resistance, the metals were exposed to direct flames at Galvalume® is a registered O O O O trademark of Bethlehem 1000 C to 1050 C (1832 F to 1922 F). Steel Corporation The galvanized mild steel passed the five- minute exposure requirement and reached Page 39 Photo credits: 642 OC (1188 OF). Type 316 ladder passed left photo: Two Mellon Bank Center (formerly the five-minute test. The test time was the Union Trust Building) extended to 45 minutes, and it reached 705 has copper canopies over the OC (1300 OF). The deflection of the galva- entrances. It was completed in 1917. Architect: Frederick nized steel after five minutes was 166.5 mm John Osterling (6.6 in.). The deflection of the stainless steel after 45 minutes was 80.5 mm (3.2 in.). right photo: The ALCOA Building was the first multi-story building with Radiation testing simulates heating by aluminum (anodized) radiation rather than with direct flame. curtain walls. It was com- pleted in 1953. Architect: The test was continued until the ladder Harrison and Abramovitz. temperature stabilized or structural failure occurred. The galvanized steel ladder References O 1. Harold E. McGannon, reached a stable temperature of 552 C The Making, Shaping (1026 OF) in two hours. The stainless steel & Treating of Steel,9th reached temperature stability at 556 OC Edition, 1971, United States Steel. (1033 OF) in three hours. The deflection of the stainless steel after three hours was 2. Rozen, Harold and about one-third of the galvanized steel lad- Heineman, Tom, The USX Tower has a CORTEN exterior (a weathering steel exterior) Architectural Metals for der after two hours. and was completed in 1971. The CORTEN was allowed to weather Construction, McGraw- naturally but was eventually painted because the rust was staining Hill, New York, 1996. Conclusion nearby buildings. Architect: Harrison and Abramovitz. 3. Doran, D.K., Construction When specifying a metal, it is important to Materials Reference Book, identify the specific grade or alloy and the Acknowledgements Butterworth-Heineman appropriate ASTM standards for the The author would like to acknowledge the Nickel Ltd., Oxford, 1992. Development Institute’s (NiDI) and the Specialty desired product form. Properties and cor- 4. Metals Handbook Vol. 2, 10th Edition, Steel Industry of North America’s (SSINA) assis- rosion resistance can vary significantly Properties and Selection: Nonferrous Alloys tance in preparation of this article and Pittsburgh and Special-Purpose Materials, ASM within a metal family. Specifying just stain- History & Landmarks Foundation’s assistance in International, 1990. less steel, aluminum, or copper is not ade- researching the photographed buildings. quate. The wrong grade or alloy within the Information Sources 5. Harold E. McGannon, The Making, Shaping th metal family could be supplied, and it may Additional information can be obtained from the & Treating of Steel,9 Edition, 1971, United States Steel. not meet application requirements. following industry associations. Aluminum Association Inc. 6. Architectural Sheet Metal Manual, Sheet Metal performance can vary significantly 202-862-5100 Metal and Air Conditioning Contractors th with the environment. Environments www.aluminum.org Association, Inc., 5 Edition, 1995. should be carefully reviewed to avoid metal American Iron and Steel Institute 7. Brandes, E. A. and Brook, G. B., Smithells corrosion that could be detrimental to aes- 202-452-7100 Metals Reference Book,7th Edition, www.steel.orgww.steel.orw.steel.org Butterworth-Heinemann Ltd, Oxford, 1992. thetic, building envelope, or structural requirements. The manufacturer or indus- Copper Development Institute 8. Metals Handbook Vol. 1, 10th Edition, 212-251-7200 try association should be consulted to deter- Properties and Selection: , Steels, and www.copper.org High-Performance Alloys, ASM mine if the metal, coating, thickness, or Nickel Development Institute International, 1990. weights are appropriate for the application. (stainless steel and nickel alloys) 416-591-7999 9. Elevated Temperature Properties of Construc- www.nidi.org tional Steels, Metals Handbook Desk Edition, For additional information on this American Society for Metals, Section 4-70. topic, specifically, "How to Select Metals Specialty Steel Industry of North America for Optimal Corrosion Resistance," see (stainless steel) 10. G. Waller and D.J. Cochrane, Stainless Steel 202-982-0355 or for Durability, Fire-resistance and Safety, the November 2000 issue of The 800-982-0355 Nickel Development Institute, technical Construction Specifier. www.ssina.com series 10042, Toronto, Ontario, Canada.

October 2000 The Construction Specifier 7