Selecting appropriate metals for architectural exteriors
BOCA RATON, Fla. – Sun, wind, rain, pollution and sometimes snow are just a few of the factors that affect building exteriors. Matching the material to the environment is essential. The design and exposure to other metals can further complicate the selection and specification of appropriate architectural exterior metals.
At last month’s 38th annual SMACNA convention here in Boca Raton, Catherine Houska, market development manager with TMR Stainless of Pittsburgh, presented contractors with an overview of expected metal performance in common service environments, along with comparative corrosion data, material combinations that should be avoided, physical and mechanical properties, and fire resistance test results.
In addition, a panel of architects and contractors discussed the American Institute of Architects (AIA) top 10 ways to avoid major problems on a project by addressing the critical issues, including documentation, unexpressed assumptions, budget, drawings and specs, and other unusual suspect areas that often create havoc on the job.
The top 10 “ways to screw up a job” list includes:
- Assuming everything will be fine
- Overpaying the contractor
- Losing the document war
- Expecting more than the budget allows
- Bad drawings and specs
- Failing to coordinate design
- Failing to understand your responsibilities
- Slow response time
- Ignoring schedule delays
Part of the discussion centered on the use of stainless steel, which has many advantages in architecture. It is defined as a low carbon steel which contains chromium at 10% or more by weight. According to the Specialty Steel Industry of North America, “As with any material used in building, the cost of stainless steel construction varies depending on design requirements and solutions. Costs will be higher for elaborate, unique or complex detailing than for simplified, standardized designs. Thus stainless structures can range from quite costly to most economical.”
It adds that “In recent years the cost of stainless steel components has been decreasing to the point where it is competitive with others metals.”
Stainless steel is attractive, reflective and durable. It is also 100% recyclable and is often desired by scrap dealers.
Stainless steel is increasingly finding its way into more than roofing and wall cladding applications. Houska wrote an article for the Door and Hardware Institute stating: “…stainless steel doors can be expected to remain attractive and functional for more than 100 years if an appropriate grade and surface finish are selected, and if they are properly designed, fabricated, installed and maintained.” In addition, graffiti including paint and ink can usually be removed quite easily with an appropriate solvent; refinishing is not required. Scratches are usually shallow and superficial.
Expensive, high-end lookStainless steel doesn’t always have to be associated with that shiny, futuristic look. It can be incorporated into historical structures as well. Houska wrote an article on the use of stainless steel handrails, kickplates, channels and hardware to help achieve an updated, modern look in a 100-year-old office building in Pittsburgh known as Penn Avenue Place. “The office renovation looks far more expensive than it actually cost, chiefly because of the strategic placement of high-end finishes for maximum visibility,” Houska wrote. “In addition, stainless steel’s durability will ensure that the renovation remains attractive for many years to come.”
According to Houska in an article written for The Construction Specifier, “High-strength, low-alloy weathering steels are carbon steels with combined additions of up to 2% copper, chromium, nickel, and phosphorus. They are used in exterior applications and have mechanical properties similar to those of structural carbon steels. In climates with wet and dry cycles, they form a protective, dense, adherent, stable, corrosion-resistant oxide (rust) layer. Corrosive substances can still pass through the oxide film, but corrosion rates decrease as the layer thickens.”
Stainless steel can be painted, etched, coated, selectively polished, engraved, etc. Decorative rolled finishes are produced when a stainless steel coil is passed through a set of rolls, according to the Specialty Steel Industry of North America. The finish on the rolls is transferred to the surface of the coil. One method, known as coining, uses one smooth roll and one textured roll. It imparts additional strength and scratch resistance to the finished material. Even more strength is gained through embossing, which uses rolls with interlocking patterns.
Stainless steel can be fabricated by all standard methods used for metal including casting, machining, stamping, spinning and extruding. The most widely used methods for the fabrication of architectural components of sheet and strip metal are brake forming and roll forming.
Cutting stainless steel requires special methods other than oxy-acetylene, oxy-propane, etc. Instead, some means must be used in the flame to break up the tenacious slag which forms when stainless steel is heated to the melting point. This can be done by adding a flux or metal powder to a flame cutting operation, or by ionizing a column of gas with an electric arc (called plasma.)
Plasma torches can be hand held or automated depending on the job requirements. The cut is accomplished by the use of an ionized gas column in conjunction with an electric arc through a small orifice. The resulting gas produces extremely high temperatures (approximately 30,000∞F). When this high temperature, high speed plasma stream and electric arc strike the stainless steel, the heat rapidly melts the metal and the high velocity gas blows away the molten metal. The intensely concentrated heat in combination with the high velocity gas produces a minimum heat-affected zone.
Low-current plasma arc welding, according to the American Iron and Steel Institute, was developed to obtain a more stable, controllable arc for welding thin-gauge metal. This process combines a continuously operating pilot arc within the torch and arc constriction to produce an arc that is stable at currents as low as 0.15 amp. The shielding gas provided through the gas cup should be an argon-hydrogen mixture for welding stainless steel. Shielding gas flow rates are usually in the range of 20 to 30 cfh. Since this is a higher voltage process than GTAW, a 1-2 volt change in arc length is less serious.
While often considered for its long-lasting good looks and low maintenance requirements, use of stainless steel is also being more widely applied in structural applications, roofing, plumbing, even for concrete rebar to avoid costly rebuilding on such public projects as roads, bridges, seawalls and in particularly corrosive environments such as swimming pool enclosures.
Carbon steel, copper and copper alloys, aluminum alloys, zinc and other materials were also covered during the session. Each material has its strengths and weaknesses, depending on the application. Aluminum, for instance, must have a heavy protective coating when it comes in contact with soil, concrete, plaster or masonry. Zinc alloys should come into contact only with a few common woods or metals; otherwise corrosion can be accelerated.
Houska writes: “When specifying a metal, it is important to identify the specific grade or alloy and the appropriate ASTM standards for the desired product form. Properties and corrosion resistance can vary significantly within a metal family. Specifying just stainless steel, aluminum, or copper is not adequate. The wrong grade or alloy within the metal family could be supplied, and it may not meet application requirements.”
SMACNA’s Architectural Sheet Metal Manual should be referred to when determining appropriate metal thicknesses, gages or weights.
(Catherine Houska, CSI, is a metallurgical engineer with an MBA in industrial marketing.)
Experts seek safety in future buildingsSteel is one of the strongest construction materials in the world. But in light of the national disaster that took place September 11, an article in USA Today reported that “Architects and security experts nationwide have undertaken a widespread discussion of future steps to ‘harden’ buildings against terrorists.”
The American Institute of Steel Construction Inc., the institute responsible for developing national steel building standards, announced the formation of a task force to look into the collapses of the World Trade Center towers.
According to the article, future building trends will include:
- More buildings surrounded by open space and fences to keep car bombs and other threats distant.
- Fewer vulnerable parking lots beneath buildings.
- Sealed and compartmentalized ventilation systems to prevent chemical or biological attacks.
- Thicker concrete facades and fewer windows at street level.
Furthermore, “In coming years, the ‘green building’ movement in architecture, which emphasized buildings with more windows that let in natural light, huge public atriums and airflow from outside, may suffer from an increased interest in making buildings secure.
Metal roofing: durable, good-lookingMetal roofing is becoming more and more popular. According to the Nickel Development Institute, stainless steel has the following advantages as a roofing material:
- It is highly resistant to corrosion and totally impervious to water.
- It requires no maintenance other than the removal of leaves and debris as with all roofs.
- It will last the lifetime of the structure
- It is light, around 3 kg/sq.-m. (hand-built).
- It has a high degree of safety in the event of fire.
- It can be used with other building materials and is not attacked by cement, mortars, or timber preservatives.
- It is aesthetically pleasing.
- It can be simple to lay.
- It is cost competitive.
- It has high strength and high ductility for forming.
- It can be safely colored.
- It can be soldered and welded.
In addition, it can be used as profiled sheeting, batten rolled or standing seam.
Hand built types of metal roofing require a substrate for support, and stainless steel is no exception. Within this category of roofing there are basically two systems used, commonly known as standing seam and batten roll. Preferably, the stainless steel used for hand built roofs should be in the softened condition for ease of forming.
It is recommended that for roof pitches up to 7∞ the batten roll method be used but above this roof pitch either method may be used. Aesthetically, the standing seam system provides a less conspicuous profile than the batten roll system which gives visually bold lines at the batten joints.