Lightweighting With Aluminum: The Growing Trend for Architects Everywhere  

Defining a space with long span systems requires the knowledge of how to use advanced materials and engineering practices. Choosing structural aluminum to convey lightweight, floating design visions is becoming a growing trend. Architects and engineers are part of a growing movement that is choosing aluminum instead of steel as a structural material because it is lightweight while maximizing material strength. They are using aluminum in LEED® Gold, Silver and Platinum buildings as a durable, corrosion-resistant material with high life-cycle values. Aluminum structural systems contribute to the latest movement to design for resiliency and recyclability. Design professionals are designing and engineering systems constructed with extruded metal components bolted together. A building can be disassembled and recycled by using a bolted aluminum system. Aluminum is one structural material that can be forever upcycled into new entities without compromising strength or the integrity of the material.

The Benefits of Extrudable Lightweight Aluminum

One of the primary advantages of aluminum alloys is this high strength-to-weight ratio. Aluminum as a material is combined with other elements to create an alloy that is stronger than steel but with only half the weight of the same strength of steel. Extruded aluminum building components are lightweight, have great strength and can be assembled into numerous shapes to create simple or complex geometries.

Unlike steel, aluminum can be manufactured into numerous types of structural extrusions that are extremely thin yet also extremely strong. When steel is hot rolled into numerous structural shapes, the structural members are 2/3rds heavier than similar aluminum products. Lightweight aluminum components can weigh anywhere from 35% to as much as 80% less than steel, yet provide equal strength to the heavier steel components.

To create a structural shape, a die is created into which aluminum is pressed. The engineer can specify specific, unique shapes for each component. They can have highly defined exterior surfaces and inner surfaces that are shaped to meet structural requirements for bending, flexure and to meet specific loading requirements. This process allows the engineer to minimize the aluminum materials used in the extrusion while maximizing the strength of the component. “The advantage of aluminum is that it is a material that allows the engineer to put the metal where it is needed. Aluminum can be added in discreet areas where stress occurs to maximize the strength of the extrusion while minimizing the material. The efficient use of materials results in lower cost (and lower weight) structures,” notes Grace Ferretti, Global Business Development manager - Architectural Division at CST Covers.

Most architects are familiar with the use of aluminum extrusions for use in windows and storefronts. The newest trend is to expand the use of aluminum as a primary material for structures. In the past, the constraints imposed by manufacturing processes including the size of dies used for extrusions, limited the size of aluminum structural components. Architects are taking advantage of numerous advancements in extrusion technology to design taller buildings, longer arches and custom designed space frames that soar above roofs. Green architects are designing large aluminum canopies that can modify thermal environments both for buildings and to create new public places.

The reason that architects and engineers are now choosing aluminum for long span structures and space frames is because of the development of new manufacturing equipment. Computer machining and computer manufacturing have revolutionized the creation of many new construction products, enabling architects to achieve unique affordable custom designs.

The creation of new aluminum structural extrusions used in the design of many of the case studies in this article is possible because of Computer Numerical Control (CNC) machines. These machines are programmed to manufacture aluminum extrusions that can be customized to create precision shapes that allow for thinner profiles. The design of the Francis Gregory Library in Washington, D.C. is an example of how advanced engineering can take advantage of the extrudability and flexibility of aluminum in the creation of an award-winning LEED® Gold building.

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.

Upcycling Sustainable Aluminum – Life-Cycle Benefits

Eager to add to the growing data on sustainable materials, in September 2011, the Aluminum Association published “Aluminum: The Element of Sustainability.” The industry’s sustainability group researched and documented their achievements in the changes that have occurred in the production of aluminum. Primarily, the industry is proud of its achievements in the reduction of energy demand associated with primary aluminum production by 17 percent and of secondary aluminum production by 58 percent. Aluminum is a material that can be continuously recycled. Unlike many other building materials, this recycling does not cause the material to lose its strength. Rather, aluminum can be continuously “upcycled” into new products.

Upcycling is a sustainable concept that first was addressed in the late nineties as a call for the reduction of the consumption and waste in most construction practices. “Upsizing” is a book by the German engineer, educator and ecologist, Johannes F. Hartkemeyer. His concept is that materials and product should be converted into materials with greater value rather than lesser value. Architect and authors William McDonough and Michael Braungart incorporated this concept in their influential book “Cradle to Cradle: Remaking the Way We Make Things.” It is also a concept that has been integrated into the life-cycle analysis of materials that architects choose for sustainable projects.

Although not yet a common practice, some architects are beginning to explore “Design for Disassembly” as part of their design strategy. A bolted frame using aluminum components can be designed for disassembly and reuse in future structures or upcycled to make new aluminum components.

Lifting Lightweight Aluminum – Constructability

Lightweight aluminum has numerous advantages during construction. From shipping to placement, a lighter material can save time, money and greenhouse gases. According to the U.S. Department of Energy, “Highway vehicles release about 1.7 billion tons of greenhouse gases (GHGs) into the atmosphere each year—mostly in the form of carbon dioxide (CO₂)—contributing to global climate change.”^v Since aluminum weighs 1/3 less than steel, the associated transportation cost of this lightweight material is reduced. Fewer trucks trips and fewer freight loads are used to provide this structural material for aluminum projects. The International Aluminum Association (IAA) provides data on the advantages of improving sustainability in the transport sector through weight reductions from the application of aluminum. These studies have been primarily focused on the reduction of weight in vehicles. According to the IAA: “The application of aluminum in passenger vehicles and light trucks manufactured in 2006 will lead to potential savings of approximately 140 million tons of CO₂eq emissions and to energy savings of equivalent to 55 billion liters of crude oil over the life cycle of these vehicles. This is vital because the transport sector today generates about 19% of all manmade greenhouse gas emissions.”^vi Weight reductions in both vehicle construction as well as reductions in the loads that they carry contribute to environmental savings. Specifying lightweight aluminum reduces transportation costs and the associated impact on the environment.

In the past, steel has been the “go-to” material for structural systems. Steel structural components are heavier by weight per pound of each component. Heavy, structural steel requires larger, stronger cranes and moving equipment. Most structural steel is welded in place requiring a greater reliance on skilled labor and field inspections. In comparison, lightweight aluminum can be assembled and erected even on difficult sites and locations. Fabricated components can be designed to minimize bolted connections for easy assembly while maximizing strength. Large structures can be engineered and fabricated in factory-controlled conditions and assembled without complex tools.

The impact of using a lightweight assembled frame versus a steel frame is seen in the example of the Lockheed-Martin Security Entrance Gate in Fort Worth, Texas. By using an aluminum system, the design team was able to save time, labor, equipment and transportation costs.

The Expansion of Lightweight Aluminum into Large Structural Systems

The use of aluminum as a structural material is expanding with technology and the continued advances of design. Some of this expansion can be related to the new forms of computer manufacturing. Some is related to new three-dimensional building information modeling which has allowed professionals to design new structures that have complex geometries. In many cases, these structures require unique, custom manufacturing. New integrated design teams include architects, structural engineers, manufacturers, material consultants and clients who all contribute information as part of the early phases of a design project. Their insights allow the designer to choose materials that from the beginning of a project complement their design strategy. Design professionals who want a thin profile for a metal building will choose aluminum over steel because of the material benefits of aluminum. From fabrication to assembly, larger, lighter structures have become possible, due to smart engineering and the design of efficient aluminum extrusions.

Most green rating systems provide credits for resource efficiency. Architects are choosing materials that provide reduced maintenance and replacement costs over the life of a building. Using materials that reduce transportation, the emission of harmful gases and those that are renewable and recyclable is a sustainable design strategy. According to the California Department of Resources Recycling and Recovery (CalRecycle), “Building and construction activities worldwide consume 3 billion tons of raw materials each year or 40 percent of total global use (Roodman and Lenssen, 1995). Using green building materials and products promotes conservation of dwindling non-renewable resources internationally. In addition, integrating green building materials into building projects can help reduce the environmental impacts associated with the extraction, transport, processing, fabrication, installation, reuse, recycling, and disposal of these building industry source materials.”^vii CalRecycle recommends that the selection of a green building material be guided by numerous criteria. Among these are:

• “Recycled content: Products with identifiable recycled content, including post-industrial content with a preference for post-consumer content.

• Resource-efficient manufacturing process: Products manufactured with resource-efficient processes including reducing energy consumption, minimizing waste (recycled, recyclable and or source reduced product packaging), and reducing greenhouse gases.

• Reusable or recyclable: Select materials that can be easily dismantled and reused or recycled at the end of their useful life.

• Durable: Materials that are longer lasting or are comparable to conventional products with long life expectancies.”

Using these criteria as a guide for material selection, architects should carefully review the advantages of using lightweight aluminum in construction projects.

Responding to environmental goals, design professionals are minimizing the use of raw materials, specifying recycled materials and upcycling materials for use in new structures. Aluminum is recyclable and can contain recycled content to meet material goals for LEED® or other green building rating systems. The aluminum industry continues to expand the use of this durable material with a long life cycle into structural components. Lightweight aluminum is making some heavy statements for design professionals in the 21^st century.

Architect Celeste Allen Novak, FAIA, LEED AP (www.celesteallennovakarchitect.com) specializes in sustainable design and planning in Ann Arbor, Michigan. She is the author of "Designing Rainwater Harvesting Systems: Integrating Rainwater Into Building Systems."

 

CST Covers is a global design/build firm with expertise in high-strength aluminum signature solutions such as spaceframes, domes, environmental enclosures, canopies, large span, and specialty lightweight structures designed for unique eco-friendly vertical and overhead applications. www.cstcovers.com