Manipulating Thermobimetals for Architectural Use Thermobimetals, first developed to keep clocks accurate, have been used since the mid-18th century for temperature sensing and temperature sensitive devices. Its uses may soon expand to a new field: the building industry. As a building skin, thermobimetals would respond to changing conditions of the building’s site without the use of computer technology. This system would enable the building to open up or close down, changing the airflow in the space. The technology, currently being researched and explored, is not yet ready to be implemented. INTRODUCTION
Imagine walking down a city street. You look at a building with a metal exterior. Then the sun comes out, and you look down. You glance up at the building for a second time when you reach the corner, and you’re suddenly aware that the metal panels that make up the skin are farther apart than they used to be. You immediately wonder if you’ve imagined the change. After all, buildings can’t move, ca they?
In fact, they can. For several decades, architects have been incorporating technology into building skin systems to create moving parts. These often involve operable shading systems which open and close to change the amount of light or heat let into the building. With the increase of naturally lit and passively heated structures (in which temperature control is accomplished by spatial design and natural ventilation instead of technological solutions), moving skin systems have become more prevalent in new building designs. Unfortunately, these technologies currently depend on electricity. Not only do they suck up huge quantities energy, but their mechanical nature means that they require regular maintenance and repair, if they remain working at all. A more sustainable solution would be one which does not require a steady input of power, but which instead powers itself. One solution is reactive material. Doris Sung, a professor of architecture at USC, postulates that the next innovation in building envelopes may be thermo bimetals, which would respond to the environment without the computers as an intermediary.
DEFINITION AND MECHANICS OF A BIMETAL
2: When laminated, this difference causes the bimetal to flex to accommodate the longer metal B and shorter metal A.  : Two types of metal, the same length at their normal state. Then, a temperature change occurs and metal B expands more than metal A, causing a difference in their lengths.  A thermo bimetal is a material: it consists of two metals laminated together. The two layers in a bimetal have different expansion coefficients, which means that that there is a difference in the amounts their volumes will change due to a change in temperature; though they begin at the same length, one of the metals will expand or contract more than the other, essentially making the metals different lengths (Fig 1). To accommodate two lengths of metal, the laminated material will flex or arc (Fig 2). When the bimetal is heated above the temperature at which it lays flat, the metal with a higher rate of expansion will be on the outside of the arc. When cooled, that same metal will be on the inside surface.
HISTORY OF BIMETALS
3: John Harrison’s Gridiron Pendulum was the first method using differential expansion to keep timepieces accurate. Pt A: How the pendulum typically looks when installed on a clock. Pt B: The Pendulum at a normal temperature. Pt C: The pendulum at a higher temperature. This shows (though clearly exaggerated) how the brass rods expand upward to counteract the downward expansion of the steel rods.  The bimetallic strip was developed in 1753 by John Harrison. Harrison, a British clockmaker, began experimenting with differential expansion rates as an answer to temperature changes making clocks inaccurate. In 1726, he developed the gridiron compensation pendulum, which depended on the differential expansion of steel and brass . The weight on the pendulum was supported by five vertical bars: three made of steel, and two of brass (Fig 3). When heated, the steel bars expand downward, lengthening the pendulum and making its period longer than a second. To counteract this, Harrison inserted two brass bars. Because they are fixed at the bottom of the outer steel bars, they will expand doubly upward, counteracting the downward expansion of the steel and bringing the period of the pendulum back to one second  (Fig 3). This concept was useful to Harrison throughout his years as a clockmaker, but he did not manipulate those metals into one bimetallic strip until later in his career .
The first bimetallic strip appeared in Harrison’s fourth generation timepiece. The strip was an alteration of the gridiron, made to be used in a typical balance-type watch. The bimetallic strip manipulated the effective length of a spiral spring which controlled the length of a second. This was the first portable watch whose mechanism effectively counteracted any temperature-induced change .
Over 200 years later, bimetallic still strips serve as temperature identifiers, temperature correctors, and safety mechanisms . We use them in clocks, thermostats, thermometers, fire detectors, automatically opening greenhouse ventilation flaps, and fire protection flaps. Though we have made good use of this fascinating, dynamic material, its inherent responsiveness gives it potential beyond its current mechanical uses. Bimetals could be the next smart material to be used in sustainably-aware architecture.
THERMOBIMETALS IN BUILDINGS
4: Doris Sung with one of her bimetal projects.  An adapted version of this thermobimetal technology could change the face – or façade – of today’s architecture. Professor Doris Sung certainly thinks so. She has been pioneering research with this material for several years. In her eyes, thermobimetal skins challenge the stagnant, unresponsive, and impermeable skins of today’s buildings; thermobimetal skins are “responsive systems that are treated as if they were extensions of … the environment,”  sensitive to its changes. They will be constantly changing, with opening and closing pores, seemingly breathing. A bimetal facade will close when temperatures cool, protecting the space within and impeding heat escapement. Conversely, it will open when temperatures rise, to allow airflow through the skin, cooling the building’s interior through convection [2, 9].
Sung has already designed and built three experimental systems thus far: “Armoured Corset,” which appeared in the 2010 Surface Design Show in London, “Waist Tightening,” built for the 2010 AIA DesCours exhibit in New Orleans, and “Bloom," currently assembled in the Materials & Applications courtyard in Silverlake (Fig 4). Looking at these experiments, it’s easy to imagine how the designs might be altered to fit a building’s needs; future designs will most likely emulate the most recent structure. The first two were installed indoors and activated by heaters. “Bloom,” on the other hand, sits in an outdoor courtyard and reacts to shifting weather conditions [2, 9]. Its larger scale and outdoor location make it more relevant to future building skins than its predecessors, especially in terms of its reactions to external conditions and structure.
There are a variety of structural systems to choose from when designing any project: compression, tensile, suspension, and pneumatic, among others. Sung’s first two bimetal projects, “Corset” and “Waist,” were suspended structures. A light-weight metal pipe, formed into a circle, hung from the ceiling above each installation. From this, string zigzagged to each of the top row panels, holding them in place. The rest of the panels wove together, which allowed them to support and stabilize the others simply by using the compression and friction that result from the woven system [2, 9, 10].
The structural system changes slightly for the newest piece, “Bloom.” Its larger size and span mean that it requires more stability to keep its structural integrity than its predecessors. For this, hybrid suspension-frame system provides rigidity; it consists of a timber ring from which a light-weight metal frame hangs. The panels then attach to this frame system and, rather than simply weaving together, are bolted to their neighbors [2, 9].
In the future, we can probably expect a frame structure, secured in the ground, to provide the stability necessary to make a thermobimetal building skin stand. The panel system will be the focus of the design, meaning that the structure will be located behind the sheet of panels, almost invisible.
The panel design encourages specific volumes and types of movement. The amount of deflection and the shape of that deflection depend on temperature, sheet size, thickness of material, and the attachment mechanism. A Flexivity Derivation equation (Fig 5) can be used to predict the movement of simply-shaped rectangular panels . After preliminary calculations, Sung experimented with a variety of thicknesses to find that the thinnest metals coiled into tightly wound spirals, and the thickest barely deformed [2, 9]. To create enough curvature to allow the passage of tactile ventilation, but not an amount that makes the panels double up on themselves, a manganese, nickel and iron alloy is kept
Formula Symbol Key
Angular Rotation (Coils)
Radius At Point of Load (Coils)
Outside Diameter (Disc)
Inside Diameter (Disc)
(T2 - T1)
5: The Flexivity Derivation (and corresponding formula symbol key), calculated for a strip of bimetal held tacked down on both ends. Symbol definitions below.  between .008 and .010”, respectively about 2-3 times the thickness of regular copy paper . Once the material and thickness were been determined, the next variable to resolve was panel shape and aggregation technique.
As previously mentioned, a simple strip shape makes it possible to calculate the curvature of the flex. This explains why, though Sung experimented with shapes of varying complexity and curvature, the first few projects were made with simple rectangular panels with skinny and long tabs. Since then, the panels have evolved to t-shapes, with the tabs curving into the body of the panel [2, 9]. This shape is sturdier than earlier versions because the integration of the tab strengthens an otherwise weak point at the junction of the tab and panel body. Of course, the shape is as equally due to the desired aesthetics as it is to the panels’ performance ; The integrated tabs of the t-shape create a larger area of overlap between panels. This makes the structure to appear as one surface, which breathes and flexes as a whole, rather than many, clearly-defined units, aggregated together.
Together, the t-shaped panels, specific alloys, and thickness of the bimetal determine the way the panel performs. These early experiments and the manipulations of the material already in place will directly affect how we perceive and use bimetals in the future. Judging from the progressions in current research, the design aesthetic will be one large surface with some texture, rather than clearly visible individual panels. The texture will appear to change as the panels open and close, giving the surface larger and smaller holes and shadows as the temperature changes. A panel aggregation system of a fairly flexible material, like this one, lends itself to curving forms. This means that the bimetal buildings could likely be amorphous, amoeba-like shapes.
It will likely be between five and ten years before thermobimetals appear in building skin systems . The system still requires much revision and refining, and will likely develop into several different approaches to use the material as a building skin. This evolutionary process may be in its very beginning stages, but we can still predict challenges that will occur in the future.
Within the next few years, material’s durability will undoubtedly be under scrutiny. Currently, wind, rain, sun, and virtually every other weather condition slowly eat away at the metal, wreak havoc on panels’ integrity; the iron-manganese-nickel alloys are not sturdy materials on their own; tests are already in progress to determine more durable hybrids . To further protect them, the panels metals could be shielded from the elements by protective coatings; if this is the case, it will be important to engineer coatings which reflect harmful UV rays, but still let heat through to the reactive material behind.
Another area of improvement may be the thermal capacity of the panel or space behind it. Metal does not thermally insulate, and the panels’ might be fairly ineffective on a day of extreme weather. One solution would be to laminate the panels with an entirely separate material that would better protect the space behind from heat or cold. Another idea, still just a wisp of thought in Sung’s mind, would be to develop the panels into bricks. The air they would contain would be an insulating barrier to the space beyond . The mechanics of this system are unclear. Perhaps, like the existing panels, they would open when warm, and shut down when cold.
Whether it occurs in the form of panels or bricks, it is clear that bimetals will be used in buildings someday. Though the system needs to go through several more design iterations before it is building-ready, it is clearly on that track and making great progress. The recent research has extended our understanding of thermobimetals, and opened doors to the use of all varieties of reactive smart materials. Hopefully, its employment in a skin system will come sooner rather than later, spurring on the low-energy and smart materials movements and providing natural temperature regulation for interior spaces: a brighter future. In the next few years, keep a look-out for a building that looks like a woven metal basket; if you spot one, hang around and try to spot its pieces flexing. The future may be sooner than we think.
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