During the second half of the twentieth century, there was a shift in how engineers thought about the artifacts they were designing. Rather than considering the design of individual components, engineers began to think in terms of the performance of the components and, more generally, how the components formed part of a larger system. This came to be known as the "systems approach" to engineering.
In the building industry, use of the word "system" became common at about the same time as the first use of computer models, and the two have come to be closely linked. A building's systems are, in effect, those aspects of the building's performance being represented or modeled in a computer. The structural system of a building, then, as represented by the computer model, may be subdivided into the vertical load-bearing system, the horizontal load-bearing system, the wind bracing system, and so on.
An engineering model of a building system has several components:
Another manifestation of the systems view in engineering was the modeling of a building project as a sequence of activities required to complete the project: what we now know as project planning. This had its origins in the Gantt chart which served the construction industry well after its introduction by the Bethlehem Steel works for use in building ships for the U.S. Navy at the end of World War I.
The Gantt chart was augmented in 1958 by the “program evaluation and review technique” PERT, developed by the U.S. Navy. PERT identifies the dependencies of activities and provides valuable “float” information and identifies activities on the “critical path” of a project.
During the second half of the twentieth century the development of a large number of new materials--notably the many hundreds of polymers, or plastics, as they are commonly called, had a major impact on building engineering. Plastics are ubiquitous in buildings--in electrical sockets, switches and insulation; window frames; clear polycarbonate (as a substitute for glass); pipes for gas and hot and cold water, sewage and rain water, as well as a host of small fixtures, fittings, and surface finishes.
Of the many new materials developed in the polymer caterory, Kevlar presented an interesting opportunity. Kevlar is as strong as steel, and just one third of its density.
It is also an electrical insulator, and it was this property that led to its being used for the upper three cables supporting the floors of the Collserola Communications Tower in Barcelona (1992), designed by Norman Foster and Partners, with Ove Arup & Partners as engineers. Kevlar was used in preference to steel to prevent lightning strikes from being conducted to the rooms containing the communications equipment.
One class of new materials that has made a particularly noticeable impact on building engineering is fiber-reinforced composites---high-strength fibers, usually of glass, carbon, or aramid (a high-strength polymer), that are embedded in a low-strength matrix, usually a polymer.
Fiber-reinforced composites are typically of low-density, and high strength and stiffness, and can be molded to suit the particular needs of different components. They were first developed in the late 1940s for aerospace applications and began to be used for civil applications during the 1960s.
Products made with these composites have included hulls for yachts and dinghies, the fuselages and wings of gliders, and tennis rackets and fishing rods. Actually, it could be argued that the building industry had developed composites much earlier, in the form of reinforced concrete, in which reinforcing steel is the "fiber" and cement is the matrix.
An even better case can be made for asbestos cement---cement reinforced with fine fibers of asbestos---which was developed in the early 1900s and widely used for making panel products. Glass fibers were found to be a suitable replacement for asbestos, and glass-fiber reinforced cement was used quite widely, especially during the 1960s and 1970s, for the manufacture of architectural cladding panels for buildings, such as the Credit Lyonnais Bank on the right.
Apart from polymers, another class of new materials that had a significant impact on the building industry is that of adhesives. The first synthetic waterproof adhesives were created during World War II. Afterwards, many new high-strength adhesives were developed that could bond nearly any two materials to one another including the highly loaded metal components used in both the aircraft and automobile industries.
Structural adhesives were used for the granite structure of the Pavilion of the Future at Expo '92 in Seville, Spain. Each voussoir of the arch, and the similar elements in the columns, is made from a number of granite struts and end plates that were bonded using an epoxy resin adhesive stronger than the granite itself. The granite elements were cut to the required degree of accuracy with a laser, and located precisely during the gluing process using stainless steel pins.
The most dramatic example of the use of new materials by structural engineers has involved a range of materials that can be used to form tensile membranes. These include fabrics woven from polyester or Teflon-coated glass fibers that have been used since the mid-1970s to create canopies and roofs with an astonishing variety of complex geometric forms. More recently, ethylene tetrafluoroethylene (ETFE) a chemical relative of the "nonstick" materials polytetra fluoroethylene (PTFE) and Teflon-has become available as a transparent or translucent film from which can be made air-inflated cushions with extremely good thermal insulation properties.
Air-inflated ETFE foil cushions form the roof of the Hampshire Health & Tennis Center shown here.
From the 1920s on, the techniques of scale model testing were developed in a wide range of research organizations, including universities, especially in the fields of aerodynamics and structural behavior. By the 1950s, model testing was an established method throughout the engineering research community, and it became increasingly available for building design engineers as the need arose, usually when a building design problem lay outside normal experience. There is no better example than the Sydney Opera House, begun in the late 1950s, to illustrate how physical scale models, in conjunction with theoretical analysis, can be used to deal with unprecedented engineering challenges.
The Danish architect Jørn Utzon had conceived of a number of curved, triangular, thin concrete shells resembling the sails of a boat in Sydney Harbor and rising to a height of some 70 meters. Not only were they very large, but they were not like any previous form of concrete shell roof, being predominantly vertical in orientation and appearing to be supported mainly on just one corner. Because the shells were vertical, the wind loads were as significant as the gravity loads, and the space they enclosed created a very unusual shape for the auditoriums within.
These design challenges were not overcome easily or quickly. Many different forms for the roof were studied over a period of five years. It was essential to find a way to find a way of creating the illusion of a thin shell, yet providing it with enough stiffness to resist the gravity and wind loads, and the appearance of standing on one corner. It also had to be possible to build.
For the first time ever, many of the geometrical and structural design calculations for a major building were performed using a computer. A 1 : 60 scale model of an early version of the structure was tested at Southampton University in England to establish the bending moments and forces in the shell. They were found to be much higher than had been anticipated, and it became clear that a thin shell as originally proposed by Utzon would not be possible. Two alternatives were considered: a steel frame covered with a concrete skin, and a concrete shell stiffened with corrugations. The latter was finally selected.
In conjunction with the structural model testing, another model of the building form was tested in a wind tunnel to establish the wind loads on the shell-both the positive pressures on the windward side of the building and the negative pressures, or suction, on the leeward side. Rotating the model allowed the experimenters to compare pressures created by wind coming from different directions.
The 1 : 100 scale model of solid wood had a series of small tubes embedded in its surface leading from the points at which the air pressure was to be measured to manometers outside the wind tunnel. The suction pressures at some locations on the leeward side were found to be over three times the pressures on the wind- ward side, and this information proved valuable in designing the fixings for the precast concrete "tiles" covering the surface of the roof. Another wind tunnel test involved fixing telltales to the model to identify the points at which the airflow separated from the surface and might generate vortices or turbulent airflow.
The acoustics of the finished auditoriums were finally assessed in a number of test performances with real orchestras and audiences. Microphones and tape recorders were used to record the decay of sound, both after firing a gun on stage (with a blank cartridge), and after getting the conductor to halt the orchestra abruptly while playing Beethoven at full power. The results of these tests were used to recalibrate the model tests and to identify a number of adjustments that could be made to the various movable reflectors to achieve the best acoustic results.
Some engineering innovation may not be visible to the naked eye or form the basis of an architectural style but may nevertheless have a profound effect on building design. One such strand of development in building engineering during the last half century has been the way buildings are designed to resist fire, and the outcome has been the creation of a new engineering discipline--fire, or fire safety, engineering.
By the end of the nineteenth century it was realized that merely replacing timber in buildings by iron or steel did not achieve the goal of fireproof construction. Iron and steel are both weakened to the point of becoming structurally inadequate when heated above about 550C. Such temperatures are reached quickly in most fires, and it was realized that the only safe solution is to ensure that the metal is protected from the source of heat.
Columns were often encased by ceramic tiles, brick, or blockwork. From the early 1900s they could also be encased with plaster applied to sheets of "expanded metal" that provide an excellent key for the plaster. Beams made of steel were usually encased partially or wholly in concrete or were protected by the hollow pre-cast concrete elements of the floor structure.
During the interwar period, fireboard products such as plasterboard began to replace the wet-trade construction, but these introduced a new problem. Boxing in the columns and beams created air gaps between the fire protection and the steel that could allow hot gases to flow through the building and contribute to the spread of fire.
This problem was the principal motivation behind the development of spray-on fireproofing products in the mid-1950s, and this in turn led to an important innovation in floor construction. For the first time it was possible to apply fire protection to floors by using profiled steel decking with a concrete topping. Corrugated steel sheeting and concrete were first used in floors in the United States in about 1890, but the thin layer of concrete acted only as a hard-wearing surface and vibration damper. Such flooring elements were not allowed in high-rise buildings because of their vulnerability in a fire. When metal decking floors were introduced in the United States in the mid-1950s, after the development of spray-on fire protection, the concrete layer was made thick enough to act compositely with the steel decking. Although it had long been realized that steel and concrete in floor structures could provide composite action, only in the 1950s were engineers allowed to rely on it in their design calculations. This resulted in a considerable savings in both weight and money, even with the cost of fire spray included.
The problem with all these methods of fire protection was that the steel had to be hidden from view. Two solutions emerged.
The scientific study of fires and their consequences had begun around the turn of the century, when experimenters began testing various materials and structural forms in controlled fires. By the 1930s most countries had established fire research institutes. Such experimentation helped establish the temperatures experienced in real fires, and the time it takes for different structural elements to heat up. The result, by the early 1950s, was that regulatory agencies began to require that different building elements be given a "fire rating"--a time for which the element must survive a hypothetical "standard fire," typically between half an hour and two hours. Fire ratings were empirical figures, based on conservative estimates, and they depended on where the element was in the building, the building's size, and its likely occupancy.
This approach, however, based on a notional time to evacuate buildings, seemed to fly in the face of common sense in some cases, such as for buildings that could be evacuated in just a few minutes, or for cases when structural steel was on the outside of a building and would never be subject to the full effects of a fire inside the building. This was the argument put forward by Mies van der Rohe when designing his Farnsworth House by reducing rooms to their essence--a series of steel-framed boxes.
Although the house probably did not violate building regulations, he anticipated objection to the use of exposed steel and put forward an argument to justify why fire protection would not be needed: it was effectively outside the building and would be adequately protected from an internal fire by the external wall. He advanced the same argument a few years later when designing Crown Hall, at the Illinois Institute of Technology, Chicago, and managed to convince the regulatory authorities of the robustness of his argument.
A precedent had been set, although initially the approach was accepted for only a limited range of building types.
One of the earliest multistory buildings with exposed columns of steel without fire protection was a five-story administrative building designed and built by the German engineering firm MAN for its own use. The external columns were 150 mm clear of the facade, through which the floor beams passed.
Permission to construct the building was granted only after exhaustive testing to verify the predicted performance in a fire.
At about the same time, the Chicago-based firm Skidmore, Owings & Merrill (SOM) was designing a nineteen-story building for Inland Steel.
This building, which opened in 1957, firmly established the use of steel without fire protection and became an icon in the history of fire engineering. It was also constructed almost entirely using welded connections rather than rivets or bolts. In the Inland Steel Building, like the MAN Building, the columns are located outside the glass curtain walls, creating a 23.5-meter-deep column-free interior space.
The use of exposed steelwork and the fire engineering approach to design was further advanced by the development of corrosion-resistant steels. The problem with putting structural steelwork on view was that this also exposed the steel to the elements. From a financial point of view, the money saved by not having to protect the steel against fire now had to be spent on protecting it against corrosion.
The steel industry responded by developing corrosion-resistant steels. Probably the best known of these is COR-TEN, which was developed in the United States. Containing about 2 percent copper and chromium, COR-TEN is much cheaper than stainless steel and, over time, forms a protective layer of oxides that is an attractive purple-brown.
The first major structure to use COR-TEN was a nineteen-story building for the John Deere company, designed by Eero Saarinen in 1956 and completed in 1964, three years after the architect's death.
Despite their architectural impact, the growing number of buildings featuring exposed steel did little to foster new, more rational methods of designing buildings to resist fires.
The first step in this direction was made in the 1950s, when the concept of a "fire load"--analogous to the concept of a statical load on a structure-was conceived. A building can be considered to respond to a fire load in much the same way as a structure responds to gravity or wind load.
A fire load is quantified as the combustion energy, measured in Megajoules (or British Thermal Units) per unit floor area. For convenience, this is often converted to the weight of timber, in kilograms, equivalent to the total combustion energy of the materials in a room. The way the combustible material burned was expressed as a graph relating temperature and time, which was used to define the form of a "standard fire."
Subsequent calculations involved analyzing the consequences of a given amount of combustion energy released into a room, which first heats the air and then the fabric and structure of the room.
In fact, this design approach was no less revolutionary than the first use of forces and stresses in structural design methods at the end of the eighteenth century.
Looking at the consequences of a fire in this manner allowed engineers to see the fire protection of a structure as a cooling problem, just as the designers of automobile engines had to devise ways of cooling engines' cylinder blocks. Indeed, the idea of circulating water through steel columns to cool them was first patented in 1884, and the idea was thoroughly investigated by research engineers in Germany, Britain, and the United States in the early 1960s.
The first major building constructed with this form of fire protection was the sixty-four-story Pittsburgh headquarters of the U.S. Steel Corporation, completed in 1971.
The eighteen external columns are made from COR-TEN plate steel up to 100 mm thick, forming 600-mm square hollow sections. Each column is 256 meters tall and contains about 92,000 liters of water and antifreeze.
This same idea was used by Ove Arup for the Centre Georges Pompidou in Paris (1971-77).
The circular stainless steel columns are filled with water. In the event of a fire, water flows around a circuit, driven by convection, and removes heat from the steel exposed to the hot gases.
The pipe for keeping the water and antifreeze at the correct level is visible at the top of each column.
Within the span of a few decades, these approaches to designing buildings to resist fires was being widely used, mainly because of the huge savings it offered to building clients, and it soon became known as fire engineering, or fire safety engineering. As the power of computers grew during the 1980s, the qualitative value of fire engineering concepts was augmented by increasingly sophisticated analytical calculations of building performance in fires. The ability to make reliable calculations of temperatures throughout a building's structural steel during the entire duration of a fire was a major step forward. Using such calculations, it could be demonstrated that at no time would the strength of the steel fall below that required to support the structural loads, which, after the evacuation of the building, would not include the weight of the occupants. Although now obvious, this latter assumption had not been permitted a decade or so earlier, and it enabled some architects to exploit the use of exposed steel both externally and internally.
The 1960s saw a renewed interest in constructing tall buildings, an interest that has continued to the present day. This was largely a result of the efforts and ingenuity of one man-the Bengal-born engineer Fazlur Rahman Khan.
Khan, together with his colleagues at SOM, challenged the conventional approach to providing wind bracing in high-rise buildings by rigidly connecting all columns and beams to create a stiff structural frame, a method that had become so costly that the construction of very tall buildings was not economically viable.
Khan's stroke of genius was to look at a tall building as a single structural entity, rather than many hundreds of elements working together, each playing a separate role. He realized the implications of looking at the facade of a building as a structural "skin." This approach was similar to that taken by the designers of aircraft. It had also become fashionable in car design in the 1960s with the development of monocoque construction, in which the chassis of a car was replaced by using the car body as a structural shell.
In buildings this idea became known as the "framed tube" concept, and it was first used by Khan in the forty three-story DeWitt-Chestnut Apartments in Chicago in 1964.
In the 100-story 344-meter John Hancock Center, Khan developed his idea of making the envelope of the building a structural element by incorporating cross bracing on a massive scale to carry the wind loads. Each braced section spans the full width of the building and extends over eighteen stories rather than floor by floor and bay by bay. The result was a saving of some $15 million.
Another of Khan's innovations was the "tube within a tube," which enabled the necessary stiffness for a tall building to be achieved with great economy of steel. In 1971-73 Khan extended his concepts of structural skin, and the macroscopic view of a structure of tall towers, by conceiving the idea of "bundled tubes" for the Sears Tower in Chicago.
At 109 stories and 443 meters, the Sears Tower was, for twenty years, the tallest building in the world.
All nine of the independent tubes, with walls of steel columns and beams, rise to the forty-ninth story; seven continue up to the sixty-fifth story, five to the eighty-ninth story, and only two to the very top.
The economy that this structural scheme achieved is remarkable. The Sears Tower is 62 meters taller than the Empire State Building, but uses only 223,000 tons of steel, nearly 40 percent less than the 365,000 tons used by the Empire State Building.
Like Fazlur Khan, the engineer Leslie Robertson took a macroscopic view when devising the wind-bracing structure for Pei Cobb Freed & Partners's 369-meter Bank of China Tower in Hong Kong, completed in 1989. The bracing in the external skin is fully triangulated in both elevation and plan, and great economy of materials was achieved despite that fact that the building was designed to withstand both typhoons and earthquakes. The result is widely held to be the most elegant of all the modern skyscrapers.
Tall buildings need to be very stiff for two reasons. One is to minimize the overall deflection, which can cause windows to crack. The other is to ensure that winds do not cause the building to sway in a manner that causes dis-comfort to the occupants. Wind-tunnel tests can be used to establish the manner in which a tower will shed vortices and, hence, the frequency and magnitude of the periodic forces that might excite oscillations.
The structural engineer then needs to ensure that the natural frequency of vibration of the proposed tower is not in the region of these forces. However, increasing the stiffness of a tower to change its natural frequency requires a lot of steel-and a lot of money. An alternative solution is to introduce a passive device that dampens oscillations if they begin to build up. A "tuned mass damper," as it is called, incorporates a large mass that can move freely but is constrained by springs and devices similar to the shock absorbers in an automobile's suspension system. In this way the vibration energy is absorbed and oscillations are not able to build up.
When structural engineer William LeMessurier proposed a lightweight steel frame for a new Citicorp Center to be built in New York, he knew that wind-induced oscillations would need to be considered. He commissioned wind tunnel tests at the Boundary Layer Wind Tunnel Laboratory at the University of Western Ontario in Canada, and the model tests revealed that the natural frequency of the proposed frame was indeed near that of the vortex-induced wind loads. The laboratory's director, Alan Davenport, recommended a tuned mass damper as the means of reducing the amplitude of oscillations. The damper designed by LeMessurier uses a 400-ton block of concrete resting on a thin film of oil.
In earthquake-prone regions such as California and Japan, the key aim of designers seeking the most economical structure is to ensure that the building will not collapse during a severe earthquake, but without designing to eliminate all permanent deformation. A diagonally braced frame cannot be used because when such frames are loaded to failure, the only possible failure mechanisms are catastrophic: fracture in tension, buckling of elements in compression, or shearing of welds and bolts.
To resist an earthquake, a tall building must be designed to deform in a non-catastrophic way, and this is usually achieved by exploiting the capacity of steel to undergo some plastic deformation. The accepted solution is to use rigid Vierendeel frames, in which the plastic deformation of the connections absorbs the kinetic energy of the earthquake; an example is the Morrison air raid shelter, constructed in 1938. The disadvantage of these frames is that the columns and beams must either be very numerous or very large. Either alternative means that buildings cannot have either large open floor plans or large windows. A solution devised in California in the 1980s was to use an "eccentrically braced frame," in which the plastic deformation is taken by short beams rather than connections; however, this does not allow an appreciable increase in open floor plans or window size.
For the Century Tower in Tokyo, Arup engineer Tony Fitzpatrick devised an eccentrically braced frame on a massive scale-two stories high and the full width of the building. The result was a twenty-story building with unprecedentedly large windows and open internal spaces.
Sometimes the sheer scale of a building using familiar materials is impressive. The steel lattice dome of the Houston Astrodome, completed in 1965, spans 196 meters and covers an area of 3 hectares. This unprecedented span led the engineers to do structural tests on a 1:100 scale model to confirm their structural calculations, and a model of similar size was tested in the wind tunnel of the McDonnell Aircraft Company in St. Louis to establish the loads that a hurricane with continuous wind speeds of 215 km per hour would exert on the building.
The roof was designed to admit about 50 percent daylight to enable grass to grow on the baseball field, but this was not a success, and artificial turf was laid after one season.
For the widest spans, tensile structures have proven to be the most versatile and economic, and no structural form has better illustrated the fruitfulness of close collaboration between engineer and architect. The pioneering structures by Frei Otto and the many engineers with whom he collaborated have inspired many engineers and architects to devise their own imaginative and visually striking tensile structures.
Following the precedent set by the Crystal Palace in 1851, international exhibitions became a regular theatre where designers have experimented with new ideas. At Expo '70 in Osaka, Japan, for example, architects Davis, Brody & Associates and the American engineer David Geiger demonstrated the first large-scale membrane roof supported by air pressure from beneath.
The membrane, spanning 142 x 83 meters, was restrained from excessive ballooning by a mesh of steel cables.
Large sports arenas, too, have provided designers with yet more opportunities for showing off the engineer's ingenuity and creativity. David Geiger's gymnastic hall, 120 meters in diameter, built for the Seoul Olympics in 1988, seems to defy common sense--it is a dome with hardly any sign of compression members. Geiger's so-called "cable-dome," devised around 1985, is one of the few practical structures that has exploited Buckminster Fuller's idea of "tensegrity."
Such structures work by employing what are, effectively, discontinuous compression members; several short compression members working together with cables in tension, not unlike the structure of the human spine. The elegance of tensegrity structures lies in the compression members being kept very short and, hence, able to resist buckling without being unduly bulky--"small islands [of compression] in a sea of tension"--as Fuller poetically described tensegrity. A particularly ingenious feature of the cable dome is that it can be erected without special lifting gear by tightening the circumferential cables one at a time, working out from the center. The membrane covering the cable dome is not part of its structural system.
In their early days, highly-stressed membrane structures were used mainly as canopies to protect the public from sun or rain. They were often temporary and had at least one side open. Since the mid-1980s, a number of designers have used them for the permanent enclosure of large spaces, such as the atriums of buildings. This always poses the problem of how to provide a flexible seal between the rigid frame of the building's facade and the more flexible membrane roof. At the Jeppesen Terminal of Denver International Airport in Colorado, the German engineer Horst Berger, who studied with Frei Otto and is one of the masters of tensile structures, used air-inflated membrane tubes up to a meter in diameter to seal the gap whose size varies along the length of the interface.
The form of the Izumo Dome of Japan, completed in 1992, was inspired by traditional Japanese paper and wood umbrellas and lampshades made of folded paper.
The dome is 140 meters in diameter, and a 1:20 scale model was built to test both the innovative method of erection and the stiffness of the extremely light weight structure under asymmetric loads. The scaled load was applied using a 7-ton weight of water.
The glulam ribs were laid radially on the ground, and alternating panels were fully stiffened using cables, struts, and the membrane itself.
The dome was erected raising the central node to its final height of 49 meters.
For many years architects had dreamed of "invisible walls," and this dream was made reality through a combination of advances in glass technology and the work of design engineers. The work of two engineers, the Irish-born Peter Rice, and Jorg Schlaich, from Germany, was especially important in achieving this goal. Rice created a 32 x 32-meter glass facade at La Villette in Paris in 1981, which set a precedent for all subsequent glazed walls. He realized that there are two key factors to keep in mind when using glass-which is rather weak in tension and very brittle to carry significant loads. The first is to ensure that the loads are precisely predictable and, especially, that no bending is applied to the panes. The second is that any shock loads, such as bird impact or the sudden redistribution of loads caused by one pane shattering, must be sufficiently dampened to prevent a domino effect, in which a failure in one pane causes adjacent panes to fracture.
At the top of each 4x4 panel, the panes are suspended from sprung supports to prevent sudden loads from being transmitted to the glass. The four-pronged connector supporting the corners of each pane, known as a "spider," contains a ball joint and rubber gasket to ensure no bending or shock loads are transferred to the glass. Both positive and negative wind loads are carried by horizontal cable trusses.
For the facade of the Foreign Office building in Berlin 14s (1999), Schlaich took a different approach. Each pane of glass is fixed to an orthogonal grid of cables and carries only its own weight.
Rather than providing additional structural elements to carry the wind loads, the facade is allowed to move under the action of wind. When wind impinges on the facade, the cables resist the load, much like the strings of a tennis racket resist the impact of a ball. To resist a strong wind, the center of a glass wall has to have the ability to deflect by some 800 mm. Such a deflection in a glazed wall sounds alarming, but such alarm can induce us to revisit our preconceived notions about the nature of glass. and to adjust them to reflect the engineer's ingenuity in overcoming the limitations inherent in the characteristics of glass.
By taking an original and radical approach to the problem, Schlaich designed a window that is 24 meters wide and 20 meters high, with a supporting structure that is no more visually intrusive than the joints between panes of glass.
When providing a roof to cover the courtyard at the Museum of the History of Hamburg, Schlaich reduced the visual impact of the structure to the absolute minimum by creating a shell roof using short steel struts stabilized in the plane of the shell by diagonal cables. The form of the shell is maintained by three sets of radial ties. To ensure the shell's stability under both statical and dynamic loads. every aspect of the geometry of the roof had to be calculated with great precision, as did the forces in the cables and ties. Similarly, the precisely calculated geometry and forces had to be faithfully reproduced in reality when the roof was constructed. Only by using the sophisticated three-dimensional structural analysis software that became available in the late 1980s could such a structure have been built.
In the 1960s a growing number of scientists had begun to study the natural environment and became increasingly concerned about the adverse impact of many twentieth century manufacturing processes and farming methods that were polluting both the atmosphere and water in rivers and lakes as well as damaging wildlife habitats and the ecosystem in general. Also of concern was the increasing rate at which natural resources were being consumed.
One text was especially influential in awakening interest in environmental issues. In 1962, Rachel Carson’s Silent Spring warned that the use of certain pesticides and removal of countryside hedgerows had caused the deaths of many birds and other wildlife and that the consequences would become even more severe unless such practices were changed.
In the 1973 energy crisis—when the cost of oil virtually tripled within a year—building designers were pressured to reduce the amount of energy that buildings use.
The one area of building design in which there has been significant change is in improving the energy efficiency of buildings; this is partly because energy use is easy to measure and because the reductions bring immediate cash benefits to building owners. Buildings specifically designed to use less energy were first considered in the 1960s but, because the cost of energy was then low in relation to other costs, these designs were rather ahead of their time and attracted relatively little interest. In the 1960s it was not uncommon for many of the services in a building to be designed and provided by the very people who made their money by selling and installing energy-consuming equipment, such as heating and air-conditioning systems, and it is hardly surprising that they did not recommend natural ventilation methods that did not need such equipment. Of course, at that time it was also still very rare for building services engineers to become involved in the early stages of designing a building, when such strategic decisions are usually made.
One of the first attempts to involve services engineers in early-stage design decisions was initiated by Arup Associates in London. This multidisciplinary building design firm had been set up by Ove Arup to realize his aspiration "total architecture."
The firm began in the 1960s with a number of laboratory buildings, notably the Mining and Metallurgy Building at Birmingham University, and was followed by a number of commercial office buildings. The result was a series of highly acclaimed buildings that incorporated more and more sophisticated ways of integrating all the engineering aspects of buildings to create a unified whole. Arup Associates' technique for structuring the different zones within the building layout became known as the "tartan grid" because of its resemblance to that plaid pattern.
At the height of the oil crisis in 1973, Arup Associates' disciplined approach to integrating the various engineering systems led them to develop a radically different approach to reducing the energy needed to run buildings. In a sense, they returned to an old idea: taking advantage of the intrinsic thermal capacity of the materials in a building to store thermal energy (heat) either as warmth in cold weather or as coolth in hot weather.
Thick mud or masonry walls had long been used in vernacular architecture to this end. For large, modern office buildings, the high thermal capacity of reinforced concrete made it the obvious choice. Concrete also has the capacity to carry large structural loads, and it can be easily manufactured in complex solid forms; thus, it can be sculpted to create the interpenetrating voids of the tartan grid through which the services pass horizontally through floor structures and vertically through risers.
Arup Associates designed their pioneering building for the Central Electricity Generating Board in Bristol, England, in 1973. In summer, cool night air passes through ducts in the concrete structure and cools it down; this thermal energy is stored as coolth. During the warmer daytime, the coolth is released from the concrete, and cools the room interiors, thus reducing the need for mechanical cooling systems and saving the energy needed to drive them. In winter, during the day, waste thermal energy generated by lights and computer equipment heats the internal air, which is passed through the concrete structure and warms it. This thermal energy is stored as warmth in the concrete overnight and released the next day, heating the air in the room spaces and reducing the demand for electric or gas fueled heating.
Since the mid-1970s, designing for energy efficiency has favored the use of reinforced concrete and masonry, which combine the necessary thermal capacity with architectural aesthetic. This mirrors the way architects and engineers since the 1930s have used materials--especially reinforced concrete and steel-to express how the structure of a building functions. This becomes a significant matter when architects are committed to delivering environmentally responsible buildings when it can be important to ensure that such buildings look energy efficient or "green."
During the 1990s and early 2000s the number of such striking and different appearances has been steadily increasing. The appearance of a naturally ventilated commercial and retail development in Harare, Zimbabwe, is dominated by the exposed concrete, tessellated to increase the effective surface area and, hence, the efficiency of heat exchange between the air and concrete fabric of the building.
The energy consumed by buildings can be considerably reduced in many climates by using natural ventilation as an alternative to mechanical ventilation.
A number of architects made a strong feature of large rotating cowls that can be turned by the wind to achieve the maximum flow of air through a building.
It is interesting to note that when this same idea was used by William Strutt and Charles Sylvester at the Derbyshire General Infirmary in 1806 the rotating cowls were kept well out of sight.
This period--the last half of te 20th century--saw a growing number of architects collaborate closely with engineers to help realize their dreams and, often, to give their buildings an appearance that somehow emphasizes the idea of new or "high" technology. This might mean simply exposing the technology underlying the building services and structure, as Richard Rogers and Renzo Piano did at the Centre Georges Pompidou in Paris. Alternatively, it might involve using exotic materials, traditionally unfamiliar in the building industry, as the Canadian-born architect Frank Gehry did at the Guggenheim Museum of Art in Bilbao by cladding the building in titanium, a material used mainly in the aerospace industry.
Several properties of titanium make it suitable for use as a cladding material: its density is about half that of steel, and it is virtually inert, which means that, unlike normal steel or aluminum, it does not corrode. Titanium can also be treated by an electrochemical process called "anodizing," which can give the surface any one of a range of different colors.
Although most good buildings arise from an effective collaboration between engineer and architect, a few people have been equally talented as both engineer and architect-for example, Owen Williams, Eladio Dieste, and Pier Luigi Nervi in the mid-twentieth century, and Santiago Calatrava in our own time.
Calatrava's statical understanding of both the stability of structures and the flow of forces and stresses through structural components has enabled him to achieve a remarkable sense of balance and poise in his structures, as well as a visual expression of the structural duty of the individual elements, both in their relative disposition and their form and cross-section.
Many architects now use the sculptural qualities of structure to striking effect. For example, at the Tokyo International Forum (1996), architect Rafael Vifioly wanted to create a huge glazed atrium some 225 meters long by 30 meters wide.
The Japanese engineers, Sasaki Planning Laboratory, devised a structure supported on just two columns, one at either end, to carry the atrium roof as well as a 60-meter-high curtain wall made of laminated glass.
The very transparency of building facades often lends them to the expression of a certain structural philosophy-for example, the organic use of tendon-like ties in the glass wall at La Villette by Peter Rice, and the glazed roof covering a street at the Hamburg Museum in Germany (1989) by the engineer Jorg Schlaich, or the gravity-defying inclined glazed roof over the reading room of the Seattle Central Library (2004), by the Dutch architect Rem Koolhaas.
Not only must this facade withstand loads arising from high winds and earthquakes, it also forms an integral part of the strategy for controlling the internal environment, which has resulted in energy savings of more than 30 percent.
At the Allianz Arena in Munich, by Swiss architects Herzog & de Meuron (2005), the facade is formed from 2,874 air-inflated ETFE cushions that not only create a remarkable appearance for a sports stadium but also change color according to circumstances.
Each of the translucent cushions can be lit from within independently to indicate which team is playing (red for FC Bayern Munchen, blue for TSV 1860 Munchen, and white for the German national team), or in combination, for dramatic visual effect.