Lightweighting With Aluminum: The Growing Trend for Architects Everywhere
Durable and Non-Corrosive Aluminum
New manufacturing processes such as CNC machining have allowed architects to realize the benefits of aluminum as an extrudable material. One of the main benefits of aluminum is its durability. Unlike steel or cast iron, aluminum forms an oxidized layer that is impermeable to oxygen and is corrosion resistant. Steel and cast iron will rust as oxygen continues to “eat away” at the surface. Only stainless steel provides a similar protection to aluminum because of the chromium added to the steel through the manufacturing process. Aluminum is corrosion resistant and design professionals who select the correct classification of aluminum for their construction project will provide a material with great life-cycle advantages.
Cast aluminum is the material used in extrusions. This metal is composed of approximately 85 percent of aluminum along with other components. The manufacturing components of aluminum through an electro-chemical processes, determines the varied grades of aluminum. According to a report by Crane Materials International (CMI) “Aluminum is actually a very active metal, meaning that its nature is to oxidize very quickly. While a weakness for most metals, this quality is actually the key to its ability to resist corrosion. When oxygen is present (in the air, soil or water), aluminum instantly reacts to form aluminum oxide. This aluminum oxide layer is chemically bound to the surface, and it seals the core aluminum from any further reaction. This is quite different from oxidation (corrosion) in steel, where rust puffs up and flakes off, constantly exposing new metal to corrosion. Aluminum’s oxide film is tenacious, hard and instantly self-renewing.”i
The American National Standard Institute (ANSI) numbering system is used to classify aluminum alloys. ANSI numbers describe the tensile strength, corrosion resistance and workability of aluminum. The most common series of aluminum specified for use in long span or large structures are numbered between 5000 and 7000 (the strongest alloy). Structural systems generally use 6061 – T6 alloy. Architects and engineers who understand the properties of aluminum to resist corrosion will specify the appropriate material for use whether in a marine environment, which would require higher grade alloys, or when using aluminum that may be exposed to soils. The CMI report recommends that design professionals use a marine grade alloy if aluminum will be exposed to salt water and that the following best practices should be adapted for construction specifications. Design professionals should:
• Test to ensure backfills and/or native soils have a pH of 4.5-8.5, and, when possible, avoid clays or highly organic soils.
• If poorly compacted soils are anticipated or dissimilar metal contact cannot be avoided, use cathode protection. The sacrificial anode should be checked at regular intervals, and may need to be replaced every 10 to 20 years.
• Insulate contact with other materials that may have significant metal content (steel fasteners, concrete, pressure treated wood, etc.).
• Use stainless steel or aluminum fasteners. If stainless steel is not an option, use hot-dipped galvanized (HDG) fasteners. It is always a good idea to separate steel fasteners from the aluminum structure with polymer washers.
• Avoid grounding electrical circuits to aluminum structures when possible.ii
Understanding the properties of aluminum will increase the life cycle of this material and add to the durability of the building project. GSBS Architects selected an aluminum system for the canopy in front of the Salt Lake Public Safety Building in Utah. The city has an aggressive sustainability initiative and the building is designed to meet stringent environmental and energy standards. They chose aluminum for the life-cycle advantages and because aluminum provided a cost-effective, lightweight solution to the challenges of a tight public budget combined with high design aspirations.
Photo by Carla Boecklin, courtesy of CST Covers
The award-winning Salt Lake Public Safety Building is one of the first net-zero public buildings in the United States. The large aluminum leaf-shaped canopy is composed of aluminum framing and photovoltaic cells that generate 30 kilowatts of power.
Making a Net-Zero Statement With an Aluminum Canopy
The award-winning Salt Lake Public Safety Building is one of the first net-zero public buildings in the United States. This six-story, 318,501-square-foot building including the parking garage, may also achieve Platinum LEED® status in the next coming months. According to the National League of Cities Sustainable Cities Institute, “The Salt Lake City Public Safety Building will be the first public safety building in the nation to achieve a net-zero rating. The new Public Safety Building has an Energy Star Performance Rating of 100, with an 80% savings in energy use and emissions. A traditional building of this size would produce 2670 metric tons of greenhouse gases each year. The building itself will produce just 524 metric tons per year, which will be offset by solar to reach the net-zero goal.”iii
The architects designed a large canopy as the entrance to the building and as a showcase for the sustainability feature. There are photovoltaic (PV) cells on the canopy that generate 30 kilowatts of energy for the building. Three solar arrays provide electricity for this building as well as other public buildings in the city. In addition to the PV on the canopy, there is a solar array on the roof (350 kilowatts) and off site (950 kilowatts). The building’s PV arrays are designed to balance the energy use of the building. The building is carefully designed and engineered to reduce electrical consumption. The city plans to register the data with the climate registry after final testing is completed. The climate registry is a North American nonprofit collaboration that sets “consistent and transparent standards to calculate, verify and publicly report greenhouse gas emissions into a single registry.”iv
Project manager at GSBS Architects Utah Office, Jesse Allen, AIA, LEED AP B+C, commented that for the design of the entrance canopy “designers on the project wanted to communicate the sustainable intentions of the project as well as make an urban design element in the plaza. They designed the leaf-shaped canopy to contain photovoltaic cells in a pattern that would cast interesting shadows.” The designers wanted the canopy to have as thin a profile as possible, creating an environmental reference in this organic form.
The architect described the system as designed, engineered and constructed like a curtain wall. The thin membrane containing staggered photovoltaic cells is engineered with the thin profile of a building’s exoskeleton. However, this skin was lifted up to 30 feet into the air. The design team investigated numerous materials and chose lightweight aluminum for the overhead structure of the canopy. They wanted the edges to have crisp corners and clean profiles. They believed that only extruded aluminum members could meet their design requirements. An advantage to framing the photovoltaic cells with aluminum components is that they have similar rates of contraction and expansion. The photovoltaic cells are butt glazed and integrated into the geometry of the canopy. The steel frame and column supports are separated from the aluminum canopy by neoprene pads to separate the materials and prevent galvanic action.
Photo by Carla Boecklin, courtesy of CST Covers
The architects designed the leaf-shaped canopy to contain photovoltaic cells that would cast interesting shadow patterns.
Valerie Nagasawa, AIA, LEED AP, principal and project manager at GSBS, reviewed the complexities of controlling costs for the project. “There were many lofty aspirations for the project, which required significant costs to meet seismic loads, security and the net-zero rating. A lot of our work went toward selecting materials that were beautiful but met the budget.” Aluminum was a material that allowed the design team to achieve their design vision within the restrictions of the budget, environmental and aesthetic aspirations. The Salt Lake City Public Safety Building houses Salt Lake City’s Police and Fire Department Administrations, Emergency Operations Center and combined police and fire dispatch center known as SLC-911. The leafy aluminum canopy provides an exciting entrance to this sustainable public building.
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