Aluminum stands tall as a structural metal—Part 2
Editor's Note: This is the second installment of a two-part article. Part I covers the properties, characteristics, and applications of aluminum as a structural metal. Part II explores the use of structural aluminum in the design of the U.S. Botanic Garden's conservatory in Washington, D.C.
While aluminum is considered by some to be a lightweight, suitable for structures no more robust than beverage cans, aluminum actually is used in many significant structures.
Aluminum's low density is, in fact, an important reason for its use. Structures often are rehabilitated or retrofitted with aluminum because of aluminum's small dead load. A good example is the Corbin Bridge over the Juniata River in Pennsylvania. This historic suspension bridge was rehabilitated with an aluminum bridge deck in 1996, increasing the load rating from 7 to 22 tons by reducing the bridge's dead load.
Aluminum also is used for large, clear bs in which minimizing the dead load represents a significant reduction in the total load the structure must support. Aluminum space frame structures routinely b more than 300 feet. The mother of these is the 415-ft. dome in Long Beach, Calif., that formerly housed the flying boat Spruce Goose.
In these applications, aluminum's high strength-to-weight ratio is a big advantage, as the material reserves its load-carrying capacity for live loads rather than squandering it holding up its own weight.
One of the first uses of aluminum in a building application was in the U.S. Botanic Garden Conservatory, built in the early 1930s in Washington, D.C. The conservatory was built from a new (at that time) aluminum-copper alloy called duralumin. Even though the copper comprised only a few percent of the alloy, it increased the alloy's strength roughly fourfold.
At the center of the garden, rolled aluminum structural shapes were riveted together to form a great glass-clad structure called the Palm House, rising 78 ft. above the conservatory floor and enclosing a footprint approximately 90 ft. long by 90 ft. wide.
Because aluminum is much lighter than the glass, it is mostly the glass that made up the dead load.
In addition, aluminum was used to frame adjacent conservatories called low houses. The inside environment was humid, while the framing outside was subjected to all the variations of the local weather, including snow.
By the early 1990s the conservatory and adjacent buildings were in poor repair. Six decades of service and exposure had deteriorated the aluminum. In fact, duralumin had proven to be the least durable of the aluminum alloys. By the 1940s other, more durable aluminum alloys were in use instead.
Designing the Replacement
The design challenge was to build the new aluminum conservatory with modern methods, but to make it look like the original structure using some of the same construction types that had been used in the 1930s. When the architect began the rehabilitation project in 1999, he called for aluminum to be used again in the conservatory framing. At first, he planned to salvage some of the original members, but later decided to replace them all instead because the original beams had deteriorated.
Not only was the original alloy no longer used in building applications, riveting had been replaced by welding and bolting.
Aluminum alloys such as aluminum-magnesium that get their strength from the alloying elements do not lose their strength when heated during welding. However, the 6061-T6 alloy that the conservatory architect specified gets its strength from a heat-treatment process. Welding weakens the heat-treated aluminum in the vicinity of the weld, called the heat-affected zone (HAZ), reducing the strength by 40 percent.
The conservatory designers had to account for the weakened state of the welded members by applying some tricks of the trade. According to "Specification for Aluminum Structures," published by The Aluminum Association, if a weld is located within 5 percent of the end of a column, the designer can treat the column as if the weld had no effect. This means welds placed at the base plate of a column have no weakening effect.
The longest aluminum beam or column that can be purchased in stock from a warehouse is 25 ft.; the longest beam available from a mill is 40 ft. This was an important consideration. The engineer had to know what those length limitations were to position the welds so that the member still was relatively efficient despite the strength reduction in the HAZ.
In the 78-ft. tall Palm House, some members are longer than 25 ft., so they had to be fabricated from two pieces shop-welded near the center. Welding at midheight is much more critical, because doing so weakens columns more than at the ends. Where there was a weld at midheight, or farther than 5 percent of the length of the beam from the end, the member had to be designed as if it were all-welded material at that lower strength.
For example, the original construction had included the frequent use of back-to-back angles to support the columns to a horizontal member. To compensate for the weakened state of the aluminum in the HAZ, the angles had to be larger or deeper than they would have been without welds in the member. Instead of using a 1/4-in. thick, 4- by 4-in. angle, a 3/8-in. thick, 4- by 6-in. angle might have been used.
Forming the Structural Members
The original aluminum members in the conservatory were hot-rolled, a process in which a hot, almost molten rectangular bar is pressed by a series of rolls into shapes such as an I beam or channel. Steel structural shapes traditionally have been formed by hot rolling. That same process had been used for aluminum also until after World War II, when a method called extruding was developed.
With extrusions, metal is formed similarly to the way toothpaste is squeezed out of a tube. Aluminum's formability makes it especially suitable for extrusion. Today aluminum structural shapes, including I beams, channels, and angles, primarily are formed by extruding. In appearance, they are indistinguishable from shapes formed by roll forming, but are more cost-effective for aluminum applications. Most of the new members in the conservatory are extruded shapes.
One notable exception was the use of roll forming for the aluminum columns that curve to meet the roof. That was a delicate process because the members had to be curved to the correct radius to join horizontal members at the top. In addition, the columns are very visible, so care had to be taken to make sure the columns were not scored or marred as they went through the rolling mills.
Reassembling the Buildings
Although almost all of the aluminum members were replaced, the corner steel columns were salvaged. Mating the new aluminum roof structure with the old steel column structure was difficult.
The whole Palm House roof had to be fabricated off-site, then lifted and set on the support structure, so it had to fit perfectly. The accuracy of the measurement and fabrication of all of those aluminum members in the roof was critical.
The old roof was measured, then refabricated. The new roof had to be measured a number of different ways to be sure that it was square across its base and that the various accumulated tolerances on all the parts or some possible misfabrications had not altered the dimensions. Then it had to be disassembled; put on trucks; and transported to Washington, D.C., where it was reassembled, mounted, and attached to the rest of the structure.
The corners of the curved pieces were fastened together with austenitic stainless steel bolts. The bolt metal had to be chosen wisely because the use of dissimilar metals can create problems with galvanic corrosion caused by differences in the electrical potential of different metals. Austenitic stainless steel is fairly strong and compatible with aluminum.
Axial Tension and Compression, Bending
Structural members in the new conservatory include members of virtually every type. Truss bottom chords in the top of the Palm House are in axial tension under snow loads; aluminum columns around the perimeter of the Palm House are in compression; and purlins between the roof trusses and throughout the conservatory act as beams, subjected to bending and shear.
Trusses expand across the center Palm House, slanting top chords on either side, with a triangulated latticing in between the top chord and the bottom chord. The bottom chord goes straight across the base of the roof.
The aluminum columns for the Palm House also were required to support bending loads as they were angled to horizontal, connecting to the top of vertical steel columns around the Palm House perimeter. The bending loads also can cause buckling because they cause compression in half of the member cross section and tension in the other. For each half, the design is similar to that for members in pure axial tension and axial compression.
In the low houses that surround the Palm House, purlins are pitched to form the roof. If snow accumulates, it puts those purlins in bending, because the snow effectively is a load that is transverse to the length of the purlin.
For prismatic members such as the bottom chord members of the Palm House trusses, if the load surpasses the capacity, rupture will occur first at bolted connections at the ends of the members, since the bolt holes reduce the cross section there, making a weak point.
This problem had to be accounted for in the design by referencing that reduced area as the cross-section member in question. For the 6061-T6 alloy, the minimum specified tensile strength is 38,000 pounds per square in. (PSI). The area of the cross section reduced by the hole was multiplied by 38,000 PSI to calculate how many pounds the member could support.
Randy Kissell is a senior partner with The TGB Partnership, 1325 Farmview Road, Hillsborough, NC 27278, phone 919-644-8250, fax 919-644-8252, e-mail firstname.lastname@example.org. TGB is an engineering firm that specializes in aluminum structural design. Kissell is the secretary of the Engineering Advisory Committee of The Aluminum Association, chairman of the ASME B96 Committee for Welded Aluminum Alloy Storage Tanks, secretary of the American Welding Society's Subcommittee on Aluminum Structures, and a member of the ASTM Light Metal Alloys Committee and the American Society of Civil Engineers Load Standards committee.
The Aluminum Association's Aluminum Design Manual contains rules for the design of aluminum members in axial tension, axial compression, and bending in the first part of the manual, called the "Specification for Aluminum Structures." It also addresses combined stress states as well as welded, bolted, riveted, and screwed connections.
The Aluminum Association Inc., 900 19th St. N.W., Washington, DC 20006, 202-862-5100, www.aluminum.org.
The U.S. Botanic Garden Conservatory, 245 First St. S.W., Washington, DC 20024, 202-225-8333, www.aoc.gov.
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