Lightweighting With Aluminum: The Growing Trend for Architects Everywhere
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.
Photo by Paul Cockrell, Courtesy of CST Covers
The Lockheed-Martin Security Entrance Gate in Fort Worth, Texas, is composed of aluminum extrusions with an aluminum facade. This 135-foot x 26.5-foot structure provides a 106-foot clear span aluminum gateway to the complex.
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.
Photo Brett Drury, Courtesy of CST Covers
The geometric canopy was designed to reinforce this multi-faceted facade and is composed of a large aluminum frame that floats above the building.
Maximizing Lightness to Provide Solar Shade
Inspired by nature and reflective of its natural setting in D.C.’s Fort Davis Park, the Francis Gregory Library provides lessons about the use of lightweight structures in green buildings. Designed by Adjaye Associates and Weincek Associates, this public space provides light-filled reading and learning spaces for adults, teens and children. The building achieved LEED® Gold rating and the architectural firm utilized many design strategies to achieve this level of sustainability. The most prominent of these strategies is one of the main design features, a large light canopy which provides solar shade in the summer while maximizing winter sun exposure. The 125,260-pound floating canopy constructed with 448 tube framing members would weigh three times as much and provide a much thicker profile if not constructed with aluminum framing members. The aluminum canopy engineered for the Francis Gregory Library supported the vision of the architects in this LEED® Gold building in Washington, D.C.
The building is a public pavilion that welcomes the neighborhood into the park. From early sketches to the finished project, the building was designed to create a transitional space between the street and the forest. The architect provided an initial sketch and preliminary design drawings to the engineer with the intention that this diamond-shaped canopy would be constructed of steel and match the lightness of the diamond-shaped facade of the building.
The structural system of the building facade is comprised of a series of geometric quadrangles or diamonds clad in a reflective surface with a wood veneer. The geometric canopy was designed to reinforce this multi-faceted facade and is composed of a large aluminum frame that floats above the building. This metal pergola with aluminum louvers creates a shelter that complements and mimics the tree canopy beyond.
The Engineering Challenge
Many architects and engineers pride themselves on challenging gravity by creating buildings with the illusion of lightness. For this library, this illusion is created by a combination of design, the smart use of lightweight aluminum system and innovative engineering. When the engineers began the project, they quickly realized that they would be unable to engineer a steel system with the shallow profile desired by the architect. The initial sketch showed a 24-inch-deep profile of the canopy. The depth of the canopy was important to the architect for aesthetic reasons. The architect did not want the canopy to overpower the building form and to be “read” as a substantial design element. Using steel would require a much deeper profile given the cantilever planned for the structure. On the other hand, aluminum extrusions were limited by the size of dies to a maximum 12-inch depth. There are no presses in the world that can extrude 24-inch-deep aluminum tube members and very few that can supply even a maximum of a 12-inch-deep structural tube.
To complicate matters, the engineers were challenged by a design that required the 106-foot x 148-foot canopy to “hover” over the building below. At one edge of the building, the canopy had a cantilever of over 33 feet. In addition, the frame was not designed to fit within a regular geometry. The design expressed a tessellated quadrilateral frame, not quite a lamella, nor a geometry that could be supported like a space frame in one dimension. The plan view pattern of the frame was not symmetric but following a unique artistic “pattern” derived from the architect’s sketched layout and aesthetic. This design resulted in a non-symmetric form and therefore each member was unique as were the connectors/joints for all locations in the geometry and structure.
One of the main design challenges was for engineering to camber the system so the structure would be level and flat across its long spans. The canopy is not covered and the engineer studied the weight of the structure and the attached louvers for the effect of any snow or ice load on the frame and louvers to determine the dead load of the overall structure. The dead load is a combination of the 125,620 pounds of the canopy combined with the weight of the louvers, and contributions from minor snow loads and wind loads. The completed canopy is within an inch of level under its own weight.
Images courtesy of CST Covers
Two 12-inch tubes were extruded and mechanically affixed to each other to make the aluminum structural member. These are images of heavy and light strut cross sections engineered and manufactured to meet specific structural loads.
The Engineering Solutions
To accommodate the desired 24-inch profile, the engineer designed custom 12-inch-deep extrusion tubes which when stacked on each other became a 24-inch-deep tube assembly. The extrusion design process permitted the development of unique extrusions that created a properly aligned and almost invisible seam across the entire length of the canopy. These extrusions were manufactured using CNC machines and were custom designed with two different cross sections for the tubes (“heavy” and “light”) enabling the engineers to use appropriate cross sections where required for structural strength and deflections, thus being able to minimize the weight of the overall structure.
The plan area consisted of 15,806 square feet of framing to which 9,047 square feet of aluminum louvers were attached. The entire canopy with louvers was bolted together by 15,380 flange bolts varying in size from ½-inch diameter to 1-inch diameter as needed structurally. Each quadrilateral frame member was extruded in aluminum with the exact bolt hole placements for precision installation. The computer-engineered extrusions were manufactured with thicker and thinner striations of aluminum to accommodate precise structural calculations for tensile and compressive forces. Each component was precisely manufactured with very tight tolerances to minimize any “slop” in the structure. For example, the holes in the hub plates were made with only a 1/64-inch oversize tolerance for the fastener. The tube fabrication consists of drilling and tapping all holes as well as machining end cuts and copes.
Image courtesy of CST Covers
In the field, construction teams installed over 15,000 stainless steel flange bolts into the hub plates at Francis Gregory Library.
In the field, construction teams installed over 15,000 stainless steel flange bolts into the hub plates. The canopy was mounted on 22 column supports within the building. The hub plates were fabricated with shop-tapped holes in the tube assemblies requiring no hex nuts on the bolted system. Extremely accurate locations of the tapped holes in tube assemblies, as well as for louver placements was accomplished via CNC-controlled beam lines.
The vision for the canopy included the combination of a very long cantilever with limited framing depth and limited support “backspan” for the cantilever. This unique challenge was met by engineering the framing with an intentional camber upwards so that in its deflected shape from its own dead weight it would be level. This required that the weight of the canopy frame was made as light as possible. It also required that the engineers pre-camber the tubes. This was accomplished by adding metal thickness to the tubular shapes as needed.
The benefit of using aluminum was that its weight to strength ratio was very high and the system requirements for a lightweight structure could be met with this system. The engineers were also able to use the advantages of extrudable aluminum to custom design each piece to create a “force” field of strength throughout the components. The project engineers and designers were also able to take advantage of the corrosion-resistant properties of aluminum to provide durability for this exterior structure. Without the advantages of a lightweight aluminum system, the design vision of the architect for a thin floating structure would not have been realized for this award-winning library.
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