The aforementioned case studies offer an extensive variety of green design approaches. To facilitate the discussion, this section first summarizes green design features of the examined case studies in Table 1
The next section proposes a framework to classify the salient green design features, according to major topics, such as structural efficiencies, renewable energy, façade technology, bio-climatic, biomimicry-inspired, and greeneries. These categories also engage additional buildings, not included in the aforementioned case studies.
6.1. Structural Efficiencies
6.1.1. Aerodynamic Forms
Given the increasing height of skyscrapers, structural engineers are paying more attention to aerodynamic forms and wind. For slender tall buildings, the lateral wind motion that is induced by vortex shedding is a major source of wind-induced excitation. In this phenomenon, as winds hit a building’s façade they swirl to adjacent faces creating vortices. These winds break away from the building on one side and then the other, creating what is known as the “von Kármán vortex street” effect. As each vortex breaks away, wind speed increases on the building side, lowering air pressure and increasing pull. Consequently, the building experiences a repetitive side-to-side push caused by the alteration of these vortices [22
]. Interestingly, in addition to structural benefits, aerodynamic forms could offer sustainable rewards and improve building’s iconicity. Advancements in the aerospace and automobile industries have allowed for the better analysis of aerodynamic forms of tall buildings. In the same manner, digital modeling provides architects with endless design possibilities of aerodynamic forms. It facilitates creating irregular and curvy surfaces that “confuse” and deflect the wind, which consequently reduce the required structural materials. These iterative processes could give architects and engineers opportunities to reach forms that twin structural engineering efficiency with beauty.
For example, the 30 St Mary Axe’s aerodynamic form works well with wind, reducing wind stress on the exterior and diminishing the downward wash of turbulent wind gusts (katabatic downward winds) that often disturb pedestrians and activities at the street level. This form also disperses reflected light, thereby further enhancing its environmental effects. The tower’s circular form provides 360-degree views of London, with the efficient footprint of the tower (40 m/131 ft) in diameter), maximizing the public space and natural lighting of the ground level. Further, the building’s aerodynamic form makes the tower appear less massive than a conventional rectangular block of equivalent size, with its tapered form receding from the eye so as not to overwhelm viewers at the street level.
Norman Foster also applied an aerodynamic form to The Bow in Calgary, mitigating the impact of Calgary’s strong wind, and decreasing the required structural support for the already efficient diagrid structure. Other towers that have embraced aerodynamic forms include Wuhan Greenland Center in Wuhan, China; Digital Media City Landmark Tower (unbuilt) in Seoul, South Korea; the Post Office Tower in Bonn, Germany; KfW Headquarters in Frankfurt, Germany; Strata SE1 Tower in London, UK; Devon Energy Center in Oklahoma City, OK; and Salesforce Tower in San Francisco, CA.
Remarkably, architects and engineers have crafted Shanghai Tower’s aerodynamic form to resist the typhoon-level winds common to Shanghai. To that end, the form embraces multiple strategies, including asymmetry, tapering, rounded corners, and a reduced floor plate as the tower rises. Testing scenarios and simulation were carried out to simulate typhoon-like conditions, suggesting a 120-degree twist as the optimal rotation for minimizing wind loads. The resulting form reduced the lateral loads of the tower by 24%, saving $
58 million in building materials. As such, Shanghai Tower’s aerodynamic form meets aesthetics, symbolic, and wind engineering purposes simultaneously. It symbolizes China’s powerful economic growth and reduces wind impact on the tower concurrently [14
6.1.2. Triangular Configurations
Aerodynamic forms often produce curvilinear surfaces that help to mitigate wind impact. Similarly, if properly designed, triangular configurations contribute to a tall building’s stability. Frank Lloyd Wright’s design of the proposed Mile High Tower provided an earlier example. Wright sensed that the tripod was the most stable form, given that the pressure of one side is immediately resisted by the adjacent sides. Wright’s intuition for the Mile High Tower’s form was correct. Another early example of a triangular form was the United States (U.S.) Steel Building in Pittsburgh, Pennsylvania.
The tripod-like floor plan has been employed by architect Adrian Smith in prominent buildings, including the 828 m (2717 ft) Burj Khalifa, the world’s tallest building, in Dubai, UAE, and the one-kilometer Jeddah Tower, the world’s next tallest, in Jeddah, Saudi Arabia, to be completed in 2020. In addition, Smith applied tripod design to Wuhan Greenland Center. Further, the 51-story Devon Energy Center employs a triangular form.
6.1.3. Braced, Diagrid Systems, and Exoskeleton
Given the external nature of these structural systems, they can visually communicate the inherent structural logic of a building while also serving as a medium for artistic effect. Viewed in this light, the visual appeal of these systems can be enhanced to give a tower a more distinct urban identity. This entails the creation of structural elements that are aesthetically pleasing, geometrically coherent, and that demonstrate dexterity of application in regards to a building’s composition, while also respecting the laws of physics and mechanics. In this fashion, an artistic approach can exhibit structural systems as not just purely rational features that enable the construction of tall buildings, but as important visual components that afford opportunities for creative expression.
An early system that was used to support the lateral loads of tall buildings is the braced frame, which has historically used various types of bracing elements, such as X-braces, K-braces, Chevron braces, and eccentric braces, to create distinct structural expressions. These different types of bracing are used to satisfy the criteria determined by building’s height, slenderness, architectural program, and seismic and wind load conditions in a geographic location. However, in recent years, the diagrid (diagonal grid) structural system has become increasingly popular. A diagrid system consists of series of triangles and horizontal rings of beams, which provide the structure support against gravity and lateral forces, while also making a building stiffer and often lighter than a traditional high-rise. Diagrids are usually composed of steel, because of its high strength and the ability to resist both tensile and compressive forces. The diagrid can suffer from problems of implementation due to its complicated steel joints, but recent advances in joint detailing and prefabrication are helping to address this issue. Due to their triangular configuration, diagrids can efficiently carry the shear and moment caused by lateral loads and gravity.
In regards to structural performance, both braced and diagrid systems are effective at carrying the large lateral loads of tall buildings given their three-dimensional tubular behavior by virtue of their being exterior systems. The braced tube system tends to be more efficient than the diagrid system for ultra-tall buildings with large height-to-width aspect ratios. However, when compared to braces, as alluded to before, the diagrid elements are lighter and less obtrusive. Additionally, diagrids possess greater structural flexibility than bracing systems given that diagrids can be configured, with some desirable adjustment in the modules and angle(s) of the diagonals, to meet the architectural and structural requirements. The angles of the diagrid can be adjusted to optimize structural performance, offering more possibilities of structural expression as a result.
It should not be forgotten that the bracing systems of many early tall buildings were antecedent to the diagrid. For example, Fazlur Khan’s X-braced John Hancock Center is considered to be a major milestone in the development of braced systems, serving as inspiration for later diagrid designs. However, in the 1980s, it was Norman Foster who more directly popularized the diagrid system, proposing that the Humana Headquarters should implement this structural typology [14
]. In the 2000s, Foster successfully employed the diagrid system in several buildings, including London’s iconic 30 St Mary Axe (2004) and New York’s Hearst Tower (2006) and Calgary’s The Bow (2010). 30 St Mary Axe employs an efficient steel diagrid structure comprised of triangular diagonal support beams, obviating the need for large corner columns and providing a better distribution of weight loads. The diagrid system reduced the structural steel that is normally required for a building of this size by 21% [23
]. Norman Foster also applied a diagrid system to The Bow in Calgary, mitigating the impact of Calgary’s strong wind, and decreasing the required structural support. The aerodynamic form, orientation, and diagrid system collectively reduced the required steel by 30%. The employed diagrid system is of a large-scale wherein each triangulated section unifies six stories, resulting in segments, which enhance the visual impact of the tower while avoiding a monolithic appearance [25
Earlier, Norman Foster applied a diagrid system to Hearst Tower in New York City. Completed in 2006, it received the 10 Year Award from the CTBUH, acknowledging its sustainable performance, global impact on tall buildings’ design and industry, and respect for the built heritage. Foster placed the tower atop the hollowed shell of a 1928 landmark office building. Its unique diagrid structure also saved 2000 tons of steel and used 26% less energy than a building that is constructed to normal code. Ninety percent of utilized steel was made from recycled materials [26
]. Further, developers imported merely 10% of all its materials, reducing the cost and carbon emissions. Likewise, the 103-story, 439 m (1439 ft) Guangzhou International Finance Center in Guangzhou, Guangdong, China, has applied a similar diagrid system using an aerodynamic profile [27
]. In addition to being structurally efficient, these buildings are also iconic. These towers are, in many ways, poster-children for the diagrid system, popularizing the technique and making it more palatable for the tall building industry as a whole [29
Similar to the case of braced and diagrid systems, architects and engineers have employed exoskeletons in tall buildings for providing structural stability and facilitating free-column interior spaces. In addition, exoskeletons evoke unique structural expressions. Further, the creative design of exoskeletons has offered new sustainable benefits, such as shading the façade and fostering natural ventilation. For the example, the exoskeleton of O-14 Tower stands 1 m (3 ft) away from the inner glass-walled enclosure. The space between the exoskeleton and the building’s glass surface produces a chimney effect causing hot air to rise, and hence creating an efficient passive cooling system. O-14’s exoskeleton with curvaceous, white color evoke monumental and monolithic exterior. With swerving contours, the concrete shell is perforated by 1326 openings of varying sizes that were positioned through a complex and “random” pattern, creating a lace-like effect on the building’s facade. Architecturally, the varying openings seek to attenuate the monotony of the external facade. They also provide an ever-changing sense of interior space through a fascinating interplay of natural light and shade. In addition to providing architectural and aesthetic quality, the tower’s shell serves as the prime structural component. It offers an efficient exoskeleton that frees the core from the burden of lateral forces and it creates a column-free, spacious interior of about 557 m2
]. The shell is organized in an efficient diagrid pattern that maintains the required structural support by adding material where necessary and taking away where possible. The required structural effectiveness of the shell was achieved by balancing material strength and aperture size, that is, larger openings received greater support through changes in the concrete mixture. Overall, O-14 Tower’s exoskeleton possesses a unique sculptural quality that expresses sublimity and monumentality [31
Similar to Dubai’s O-14 Tower, the COR Tower in Miami (unbuilt) features an innovative hyper-efficient exoskeleton that balances the transparency of the recessed glass curtain wall. This exoskeleton also provides insulation, shading, and natural cooling for terraces. The 25 cm (10 in) exoskeleton provides structural functionality and it serves as solar screens providing light, air, and external views. The space between the exoskeleton and the building’s glass surface also produces a chimney effect causing hot air to rise, and hence creating an efficient passive cooling system. The tower also integrates environmental technologies by incorporating rooftop wind turbines. As such, COR Tower creates a unique interplay between structural engineering, environmentally friendly design, architecture, and ecology. Finally, recent projects, such as Oasia Downtown, use the exoskeleton as a place for plants to grow and ascend the height of tall buildings [15
6.2. Bio-Climatic Design
Ken Yeang has applied bioclimatic design principles to the high-rise tower, emphasizing passive measures [32
]. He has sought to find ecologically benign ways to make built forms “greener” and more humane. As Yeang explains, “climate provides designers with a legitimate starting point for architectural expression in the endeavor to design in relation to place, because climate is one of the dominant determinants of the local inhabitants’ lifestyle and the landscape’s ecology” ([33
], p. 12). Menara Maisgani offers one of the best examples of this design approach. The building utilizing several passive design strategies. It places the building’s core off-center on the east side of the facade to shield the building from the direct rays of the “hot” sun. Made of reinforced concrete, the core provides thermal insulation, and functions as a “heat sink” that re-radiates absorbed heat during the day into the interiors at night. The core area contains elevators, washrooms, and other services, relying on natural ventilation instead of mechanical systems of heating and cooling. The design supplies natural light and views to the outside via windows, and fire escape stairways are unenclosed. Furthermore, windows facing the hot sides of the building (east and west facades) are recessed and contain protruding external aluminum sunscreens to reduce solar heat gain. These solar shading devices are effective because, at the equator, the Malaysian sun enjoys a high path in the sky. A slight protrusion of the facade substantially blocks the sun’s rays, providing significant shading. Sky gardens that have been carved out of the east and west facades provide additional shade. Areas of the facades that do not have direct solar insulation (the south and north facades) contain unshielded curtain-walled glazing to maximize natural light and external views [33
]. Operable curtain walls lie flush to these facades. Sky gardens are accessible through sliding doors, which also assist in providing natural ventilation. The circular plan facilitates natural lights to office spaces, and functions that require less light are located away from the perimeter.
Similarly, the Commerzbank in Frankfort uses an innovative “Klimafassade” to facilitate natural ventilation. Translated literally from the German as the “climate facade”, the “Klimafassade” is a custom-made double skin system that mediates the weather between the interior and exterior of the building. The outer skin comprises a solid pane of skin due to the protruded aerofoil sections. The inner skin comprises a Low-E double-glazed unit that is bottom hinged and opens inwards at the top to a maximum of 15 degrees. The 165 mm (6.5 in) ventilation cavity between the two skins houses a motorized 50 mm (2 in) venetian blind that shades and deflects solar rays [14
]. The outer skin contains slots through which fresh air can enter the cavity between the layers. Cavities contain special aerodynamic transoms that minimize the noise that is generated by air movement within. In the winter, these double facades act as a thermal barrier, allowing for solar penetration, and in the summer, these blinds provide solar control to prevent overheating. The double skin also provides acoustic protection and heats fresh air before it vents into the building. Building tenants can also open office windows on the atrium side for natural ventilation, with plants that are in the sky gardens supplying oxygen. In the wintertime, the building system shuts the sky gardens’ windows to trap heat; it reopens them during warm times to let in desirable breezes. In both circumstances, warm air rises and through the negative pressure on the leeward face of the building as the air moves across the exits, a process that removes stale air from the interior [15
]. Natural ventilation provides a healthier environment for both occupants and plants that proliferate throughout the building, including the sky gardens.
Another interesting example that addressed climate considerations is Torre Cube. The tower’s design takes advantage of Mexico’s mild and sunny climate to bring in natural ventilation and light. The building’s double skin and central atrium allow for complete natural ventilation, while the wooden-latticework placed in its outer skin reduces solar gain collectively obviating the need for air conditioning.
It is worth noting that, arguably, the first bio-climatic tower to break away from the modernist’s steel-and-glass box (see, for example, Mies van der Rohe’s skyscrapers in Chicago and New York City built in the 1950s) and that provided passive measures for sustainable design, is Frank Lloyd Wright’s Price Tower, completed in 1956 in Bartlesville, Oklahoma, USA. Price Tower employs opaque concrete walls punctuated with windows to create a greater thermal mass by reducing solar gain and insulating against the extremes of the local climate. Additionally, the tower uses louvers and fins to control solar gain and light. Its interior spaces receive natural light and ventilation, while its terraces are full of plants, creating seamless connections between indoor and outdoor environments [31
6.3. Vertical Landscaping
Throughout history, architects have claimed to connect their designs with nature. For example, Frank Lloyd Wright, Rudolf Schindler and Richard Neutra all strove to ensure a fluid relationship between indoor and outdoor spaces, human-made, and nature-made environments. In recent years, however, vertical landscaping has been promoted as a feature of bio-climatic, sustainable, or green design. This model initially followed the work of Ken Yeang who advocated “biophilic urbanism”, where the city and buildings integrate flora and fauna to promote the innately emotional affiliation of human beings to other living organisms. Yeang contended that we should build our cities by using organic, soft, and natural materials, as opposed to hard and rigid, to address environmental issue of air pollution, urban heat island (UHI), and climate change. He looked into designing the high-rise typology in the 1990s as “vertical green urbanism” and stressed improving the ecological linkages between a building and its surrounding landscape. Indeed, there is a growing recognition of the healing power of nature, and by creating physical “ecological” linkages, we foster connectivity, interaction, and mobility of a wider range of species. In the context of tall buildings, Yeang has promoted the notion of vertical landscaping to facilitate these ecological linkages [33
From a sustainability perspective, vertical landscaping is desirable because it offers a plethora of benefits, including:
Improving the environmental health of indoor and outdoor spaces by producing oxygen, filtering polluted air, dust, and reducing urban noise, thereby improving people’s comfort and productivity: A 5 m diameter canopy or 40 m2
of a vegetated wall covered with dense planting can produce the yearly oxygen requirement for one person [16
]. Also, plants sequester carbon from the atmosphere.
Enhancing aesthetics and offering a local, vernacular touch when indigenous plants are used.
Improving habitat resilience and species survival.
Bringing nature closer to the cities and softening their “urban jungle” effect.
Increasing biodiversity by attracting species such as birds, butterflies, snails, crickets, and tree frogs.
Reducing stress levels of individuals exposed to greeneries.
Potentially, providing an agricultural source (see next section).
Protecting from graffiti and vandalism.
Protecting the building from the solar load and hence reducing required energy to cool the building during summer time and reducing carbon emission.
These combined factors reduce the UHI effect and help to combat climate change.
Consequently, architects and planners increasingly integrate vertical landscaping in tall buildings. Leaders of this trend include Stefano Borie, Vincent Callebaut, Jared Moore, Jean Novel, Yansong Ma, Emilio Ambasz, and Milroy Perera, among others. Remarkably, botanist and garden designer Patrick Blanc has influenced this design direction. The plant and tree-covered towers trend is spreading worldwide, creating a new architectural design paradigm that responds to environmental problems and climate change while offering exciting aesthetics. As such, today, green roofs, sky gardens, sky courts, terraces, and vertical landscaping are among the copious greenery schemes of eco-design. In the aforementioned case studies, we find that these concepts prevail in Menara Mesiniaga, ParkRoyal on Pickering, CapitaGreen, Bosco Verticale, and Oasia Hotel Downtown. Other examples include The Met, Newton Suites, Antilia Tower, One Central Park, ACROS Fukuoka, and Tao Zhu Yin Yuan. Further, several proposed towers follow the same design scheme, including La Tour des Cèdres, Tokyo-Nara Tower, EDITT Tower (unbuilt), Sky Village, New York Tower, and Vertical Park.
The hanging garden skyscraper organizes greeneries into gardens at various levels with vertical landscaping as connectors. For example, the Antilia Tower in Mumbai employs hanging gardens that are connected with “living walls” or vertical gardens that climb to the 40th floor. The various floor plans encompass a variety of garden tiers, terraces, waterfalls, and ponds. Also, the proposed New York Tower by Daniel Libeskind features a series of sky gardens at different heights cut from a façade that provides enclosed green space and terraced balconies.
The design of the proposed Vertical Park in Mexico City resonates the hanging garden. It may compensate for the lack of urban green space, particularly in dense areas where land is scarce and expensive. The project intends to infuse the city with much-needed green space by employing sky-gardens for living and working. The Vertical Park provides a space that can be used both publicly and privately, and that also collects water and solar energy.
Interestingly, the proposed Gwanggyo Power Center integrates vertical parks that contain plantations with box hedges placed around terraces. The collective image creates a strong, recognizable, and cohesive vertical park. The park will provide pleasure to the center’s tenants and the public, while also reducing energy consumption by supplying natural ventilation and daylight. Similarly, the proposed Mile High Eco-Skyscraper’s design maintains that public parks spread out on the vertical plane. These parks are open to the outdoors (not glazed) and are large enough in scale to be visible from far away. The overall composition of these vertical pocket parks evokes an organic feel to the tower.
On the issues of maintenance, towers that incorporate trees and vegetation require significant maintenance and crews of special expertise and skills, rarely to find. As expressed by Richard Hassell this building typology requires a team of flying gardeners; a pool of laborers who are both Spiderman and gardeners. Over time, living and constantly growing vegetation may threaten façade’s integrity. Any required repair at higher altitudes entails higher costs and troubles. Further, integrated vegetation may invite undesirable insects and even snakes that would render these buildings unsafe and undesirable human habitats.
6.3.1. Vertical Courtyards
Closely related to vertical landscaping is vertical courtyards or skydecks. As described earlier, in addition to environmental benefits, vertical landscaping masks hard-looking building materials, such as steel and concrete, by adding a soft green layer. Vertical courtyards underscore vertical landscaping by adding “depth”. That is, vertical courtyards create a greater visual impact on building’s appearance by carving out significant volumes. These elements not only offer greeneries but also often facilitate natural ventilation and provide outdoor spaces that support social life in the sky. Examples of buildings that employed vertical courtyards include Oasia Downtown, National Commercial Bank, VIA 57 WEST, and Torre Cube.
6.3.2. The Vertical Farm
With a burgeoning global population that has created an ever-growing need for increased food production and housing, architects are searching for sustainable solutions that will prevent further sprawl while supplying fresh food to urban residents. This could be achieved through vertical farming in urban settings. Vertical farms could be integrated into skyscrapers as another mixed-use element, along with housing, recreation, work, and tourism. In this way, skyscrapers of the future may become small, self-sufficient cities, complete with on-site energy production and connections to mass-transit, resulting in reduced carbon footprints for both building and residents. Vertical farming would involve exploring innovative sustainable solutions to urban crises and the formation of new relationships among architects, engineers, farmers, and local communities [34
6.3.3. Balconies and Terraces
Certainly, balconies are not new design features of tall buildings, particularly residential ones. In this regard, Rem Koolhaas explains: “Without the balcony, the history of the world would have looked completely different” [35
]. Indeed, balconies play functional roles by supplying an outdoor space that residents can use for sitting and relaxing, enjoying views and fresh air, and drying clothes. In addition, balconies could be utilized for incorporating plants and even trees, as explained in a previous section on vertical landscaping. This technique allows for weaving nature in high-rises. However, a new prevailing trend suggests balconies of intriguing forms that drastically affect the resulting form of tall buildings by introducing fresh morphological expressions. In a recent article titled “How architects are changing high-rise living with amazing balconies that stand out in more ways than one” Christopher DeWolf elaborates on this notion [36
]. Advances in structural engineering allow constructing larger and deeper balconies and terraces of various forms, resulting in sculptural quality.
Aqua Tower by Studio Gang is an exemplary project of this trend. It integrates balconies of various shapes and sizes, engendering an undulating form of a “topographic expression”. Studio Gang builds on this design technique to produce exciting forms, as seen in City Hyde Park Chicago, completed in 2016. Other projects by Studio Gang that embrace this design technique include a proposal for a 40-story residential Folsom Tower in San Francisco. Jeanne Gang commented on this building by stating, “What I like about tall buildings (aesthetically) is what you do with the height, the incremental moves along the way….It’s not just the extrusion of a single form from bottom to top” [37
]. Jeanne Gang proposes also a 14-story building in Miami with varying shapes of balconies resulting in a choppy, prismatic tower. She explained that her design serves as “contemporary reinterpretation of a “Florida room”” [38
One interesting example that follows this design trend is the 46-story Regalia Condominium Tower in Sunny Isles Florida by Arquitectonic. It integrates undulating verandas to recall the adjacent ocean waves. Bernardo Fort-Brescia, the founding partner of Arquitectonica, explains that these balconies serve functional needs. He states, “Its transparent surfaces connect inside and outside, linking the occupants with the surrounding environment. Its orthogonal geometry creates elegant, serene, classical, zen-like spaces. Each floor is wrapped by a sensuously undulating terrace. The resulting walk-around veranda protects the glass surfaces from the sun, as in traditional Florida homes. It is this veranda that shapes the architecture” [39
]. The Wave Tower in Gold Cost, Australia embraces a similar design. Likewise, Beirut Terraces, as the name indicates, integrates abundance of large terraces of various sizes and design, creating dynamic visual expressions.
While manipulating the design of balconies is likely to result in new morphological expressions, it may risk losing their essential functionality. For example, varying shapes may result in dead spaces, i.e., un-utilized spaces because of being too small. For example, Aqua Tower’s balconies have odd and shallow areas that render them unusable. These shallow areas also cancel the shading benefits of balconies. In the example of Aqua Tower, facades under shallow balconies had to use different type of glass to reflect sun and heat. Further, with greater heights, wind velocity increases dramatically, making balconies uncomfortable places. Therefore, if balconies were integrated in a tall building, they should respect and support climatic considerations and not follow a preconceived form per se.
6.4. Solar Shading Devices
Balconies and terraces offer multiple functions, including shading benefits, as discussed above. However, some buildings, such as office buildings, may not need balconies. As such, shading devices could be employed. Principally, the glare caused by glass is a significant problem for both the interior and exterior of a building. Glare occurs through an extreme contrast between bright areas that receive direct sunlight and adjacent areas of darkness. Inside a building, glare often occurs in areas close to windows that receive a large amount of sunlight, for “usually a single south-facing window can illuminate up to 20 to 100 times its unit area” [19
]. Such a large amount of light may result in glare if concentrated on a small area. Additionally, unwanted heat gain occurs when sunlight enters the building on warm days, with local climate and facade orientation being the main determinants of sun control in regard to heat gain. In the last decades, the world has witnessed a significant high-rise development that embraces Western models of all-glass curtain walls, ignoring local climatic conditions. In this regard, solar shading devices play important functional roles. For example, cooling all-glass buildings is costly, both financially and environmentally, particularly in hot climates like that of Abu Dhabi, where intense sunlight causes temperatures to rise frequently above 38 °C (100 °F). In addition to functional roles, solar shading devices may offer interesting visual expressions, as follows [15
“Static” solar shading devices can address problems of glare as well as control issues of heat gain and loss. They come in different forms and shapes (e.g., sunshades, light shelves, blinds, fins, overhangs, horizontal louvers, vertical louvers, or shade cloth). These elements may have direct impact on a building’s visual expression. For example, fins can reinforce the vertical expression, while cantilevers stress a building’s horizontality.
Architects could be creative in introducing new forms of shading devices. For example, New York Times Tower integrates in its glazed facade a brise soleil consisting of 186,000 ceramic rods that link to a dimmable lighting system [14
]. Yet, the screens of ceramic rods that float in front of the clear glass curtain wall are the building’s signature. Renzo Piano, the architect of the building, has called the screens a “suncoat”—as opposed to a raincoat—that cuts the transmission of light and heat into the interior, thereby permitting the use of clear, rather than tinted glass. Interestingly, the vertical arrangement of the rods resembles lines of newspaper columns, a fitting visual expression to the function of the building, producing newspapers. Menara Mesiniaga integrates repetitive curvilinear aluminum sunscreens that give the building a unique visual expression. Finally, Doha Tower by Jean Nouvel creatively modernizes mashrabiyya. It offers a new interpretation of the traditional mashrabiyya by varying its density in response to solar orientation.
“Dynamic” solar shading devices have the opportunity to offer a better response to solar exposure and “dynamic” visual expressions. Most notably, Al Bahar Towers have modernized the traditional mashrabiyya. Unlike the static and two-dimensional traditional mashrabiyya, Al-Bahar’s mashrabiyya is dynamic and three-dimensional. In response to direct sunlight, the mashrabiyya can unfold to cover the facade, and when the sun is obscured, it can close to allow for daylight penetration. Geometrically, the mashrabiyya system follows a hexagonal pattern that simulates traditional Arabic-Islamic design. As the mashrabiyya system opens and closes, the towers always change their appearance, stimulating intriguing aesthetics. Parametric and algorithmic modeling has been used to optimize the mashrabiyya location on the facade, precluding the use of dark tinted glass, which would permanently restrict incoming light. The system provides a 50% reduction in solar gain, resulting in decreased energy consumption and CO2
In the case of KfW Headquarters, the polychromy of the facade’s narrow ventilation flaps with different hues of red, blue, and green, enhance the building’s visual aesthetics. The ventilation panels are computer controlled via the BMS, guided by environmental sensors that are placed on the DSF. The system computes when and how much the exterior panels should open to maintain a constant and even pressure within the ring, with airflow being regulated not to exceed 6 m/s (20 ft/s). As such, the color composition is dynamic, that is, the changing position and setting of the ventilation panels result in a vibrant play of color across the facade. Finally, the biomimicry-inspired Cactus Building in Doha, Qatar (unbuilt) provides an innovative example of how eco-design can mimic nature. The building’s skin is sensitive to Qatar’s hot climate and features smart shades that open and close in response to heat, thereby mimicking the natural functions of a cactus [14
6.5. Renewable Energy
Architects, environmentalists, politicians, among others, are increasingly interested in exploring ways to harness renewable energy sources that reduce reliance on fossil fuels. Wind, solar, biomass, geothermal, and hydroelectricity energy technologies generate electricity with no combustion emissions. Gradually, the costs of renewable energy are becoming much more competitive with fossil fuels. Future tall buildings may strive to become “zero energy” or even “positive energy”, so that in a year they may generate as much or more energy than they consume. In this latter case, the extra supply may be sold off to the city’s power grid [16
6.5.1. Wind Turbines
Integrating wind turbines in tall buildings may result in one of the boldest visual expression of sustainability. Functionally, wind is a renewable energy resource that can be utilized, particularly at the higher altitudes of tall buildings where wind speed is considerable. Tall buildings can be shaped to funnel these winds into zones that contain wind turbines without having a negative effect on the structure, its surroundings, and the occupants. Providing this structural profile, wind speed is amplified to produce more energy. In the case of Bahrain Towers, Pearl River Tower, and Anara Tower, among others, their shapes and forms considered harnessing wind energy.
The Bahrain Towers, in particular, were shaped to take advantage of the local prevailing winds by funneling them into three large wind turbines. Using laboratory wind-tunnel testing, Atkins was able to define the building’s tapering and angles to keep the wind source consistently strong for all three propellers. The building’s shape ensures that any wind coming at a 45° angle to either side will create a wind flow that intersects perpendicularly with the turbines. However, for practical reasons of mechanical malfunctioning and noise generation, these turbines were closed shut by building management. As a result, these turbines stand still [16
The Strata SE1 Tower in London also incorporates three 9 m (30 ft) wind turbines on the rooftop, which give the tower a distinctly “green” identity. In the case of Pearl River Tower in China, the tower was shaped to funnel the wind into two major openings in the building that contain wind turbines. Its shape also reduces lateral wind pressure. Originally, the tower was designed to be “zero-energy” (producing as much energy as it consumes). However, due to local regulations and codes, the design has been compromised, and the building is no longer energy self-sufficient [16
]. Consequently, the economic efficiency of wind turbines continues to be questionable.
Photovoltaic panels are likely to have a lesser visual impact on buildings than that by wind turbines. However, similar to wind turbines, their efficiencies continue to be questionable. Stated differently, although tall buildings feature large facades that give the promise of employing photovoltaic (PV) panels at a mega scale, the output of PV has been marginal. For example, Heron Tower in London features a facade-mounted solar PV array; it generates merely 2.5% of the building electricity demand. This is because tall buildings are energy hogs and PV technology continues to be inefficient. In addition, London’s predominantly cloudy weather makes PV the least productive [14
The 21st Century city is increasingly vertical. Many cities around the world are embracing or re-embracing the tall building as a major building typology and are attempting to make it sustainable. It is important to pay attention to the functional performance of this building typology as well as its impact on the visual aesthetics. This paper mapped out the “sustainable” design features that may affect tall building’s visual expressions. It examined dozens of projects that employed a wide-range of design approaches. Yet, all these projects are widely recognized for design excellence, and have received awards from major tall building organizations. The examination has focused on identifying sustainable design features that grant the building clear identity and make it an iconic landmark. This paper, therefore, synthesizes the concepts of sustainable design and creative artistry to facilitate a better understanding of the aesthetic developments in skyscrapers worldwide. The ultimate goal is to empower architects and engineers with design choices that result in sustainable and beautiful buildings.
The examined examples have embraced sustainable design features and technologies in different ways and to greater and lesser degrees. Some are boldly iconic, and others are subtly iconic. As such, they provide a mix of examples, yet they share two prime qualities of being green and attractive. In numerous cases, there have been sound rationales for adopting or refusing eco-design ideas. We find examples of skyscrapers that are shaped to harness wind power, others that shunt wind force to decrease lateral loads and others that have employed articulated exoskeletons to balance solar gain while providing natural ventilation. Certainly, these examples point to a new path for skyscrapers that departs from the plain, monolithic vertical extrusions of the orthogonal, air-conditioned box.
Hybrid designs embrace an eclectic approach that incorporates selected green features without adhering to any one specific theme. This option is attractive and common, providing architects with the freedom to chart their design by navigating a myriad of green options. Because some green design features are costly, the hybrid approach may allow clients and architects to work together to optimize their choices. Hence, architects need not morph, twist, or tweak their buildings to fit a particular green design theme, such as the organic or bionic. Instead, they may employ a wide-spectrum of forms that simultaneously incorporate a plethora of green design features. This paper supports and guides efforts seeking a hybrid/eclectic design approach. A summary of these design approaches is provided in Table 2
Overall, the plants and trees-covered tower offers a promising sustainable model. It represents a paradigm shift from “garden city” to “city in a garden”. This subtle but important change emphasizes the desire for a more immersive nature and a view of urban life where nature is not the “icing on the cake” element or a lonely landscaping project, rather the predominant, integrative, and ubiquitous feature of the city. Plants and trees-covered towers support biodiversity and promote flora and fauna. They offer “quiet spaces”, where tenants can enjoy peaceful contemplation, attractive views, and fresh air. In these towers, people will be able to listen to sounds of many bird species, crickets, tree frogs, katydids, and grasshoppers, while watching snails and butterflies traveling across plants. In addition to enhancing “soundscape”, these towers have the potential to enhance the “smellscape” of the city. Indeed, some vegetation provokes wonderful and attractive smell. The implementation of this model could be for the entire building or part of it, for example, a parking structure [14
Notably, Singapore has instated a citywide landscape replacement policy that mandates a minimum of one-to-one replacement of ground-level nature with vertical green elements—this, in turn, has promoted plants and trees-covered towers. What makes this possible is that Singapore has a tropical environment where growing trees and plants is relatively easier. As we have seen in this paper, ParkRoyal on Pickering and CapitaGreen are exemplary projects of this trend. These buildings have embraced genuine concepts and green design principles that resulted in interesting and eye-catching tall buildings. Nevertheless, it is important to note that ensuring a robust maintenance of greeneries is essential to sustain the intended environmental benefits. Buildings’ owners should keep out unwanted insects and animals [19
A building’s design and appearance should be determined by the collaborative decisions of the interdisciplinary team, and discretion should be used in regards to the level of boldness or the subtlety expression of sustainable design elements. Through an iterative process, and with the help of powerful computer modeling tools, design teams may decide on how specific elements, such as aerodynamic forms, structural systems (braced, diagrid, exoskeleton), vertical landscaping, renewable energy features, and shading devices can be applied to improve function and aesthetics simultaneously. Other sub-variables, such as shading, decoration, color, curtain walls, patterns, scale, structural details, lighting systems, etc. can be examined as well. In the author’s opinion, complex and iconic forms should not be used just for their novelty or fashion of a time, but also keeping in mind their cost effectiveness and rational quality that will transcend and pass the test of time.
A beautiful, sustainable architecture may have the potential to restore the original meaning of iconicity as a characterization of outstanding and timeless architecture. The forms of eco-iconic buildings stem mainly from green design, yet they are still attractive. Frank Lloyd Wright, through the influences of Louis Sullivan, once famously wrote that beautiful forms could only be created after functional needs have been satisfied. Perhaps, it is time to re-appropriate the word “iconic” for the purpose that it was originally intended; as a way of recognizing well-established architecture, which is beautiful, functional, and fitting. In the 21st Century, it is possible to create beautiful forms that embrace creative green design ideas, principles, and technologies. It is possible to twin sustainability with innovative aesthetics in designing tall buildings.
Although this paper advocates embracing sustainable design, it acknowledges that greenwashing is prevalent. Cities’ “green” agendas have been “hijacked” by industries who wish to take advantage of the new trend by converting sustainable mission into money-grubbing businesses. Industries propagate the notion that new technologies offer superior “sustainable” benefits. Stephen Mouzon reflects on this issue by stating: “Today, most discussions on sustainability focus on ‘gizmo green,’ which is the proposition that we can achieve sustainability simply by using better equipment and better materials” [40
]. Surely, integrating “smart” technology and “green” machines into our daily life is important; nevertheless, “this is only a small part of the whole equation. Focusing on gizmo green misses the big picture entirely”, according to Mouzon [40
Stephen Mouzon also offers in his recent book The Original Green: Unlocking the Mystery of True Sustainability
crucial remarks on the importance of doing less with more [41
]. He explains that sustainable design could be achieved without relying on costly technologies. He states, “In the current world of green and sustainable design, so much weight is put on technology; the adding of solar power, high tech glass, qualifying for LEED. It is all about ADDING things” [42
]. The main lesson we may learn from Mouzon’ research is how well one can do with less by sharing resources and becoming less consumptive of finite resources and more efficient in everything we do. Interestingly, “doing well with less” echoes “less is more” motto that was embraced by Modernist architects, for example, Ludwig Mies van der Rohe, whose work has a profound impact on tall buildings development.
Overall, this review paper confirms that the path to creating sustainable vertical city is arduous and long. Present practices are far from the age of the sustainable skyscraper city. Researchers need to conduct significant work at the planning, architectural, and engineering levels in the design, construction, and integration of the skyscraper into cities. Management of the building, evaluation of its performance and assessment of tenant satisfaction are also essential components to achieving more sustainable skyscrapers. “Greenwashing” or “bogus sustainability” is becoming mainstream criticisms of sustainable tall buildings. We continue to lack a solid grasp of the full implications—the physiological, psychological, social, economic, and environmental implications—of vertical living, which entail cramming greater numbers of people into smaller spaces. This requires additional research that studies the implication of integrating these urban giants in cities.
8. Future Research
This paper has focused on the sustainable design and iconicity nexus. Future research may address other important sustainability issues, such as social and economic. Many sustainable skyscrapers have claimed that their green features foster better social and public life. For example, Shanghai Tower has promised to make its wonderful skygardens accessible to the public. However, since its opening, the tower’s gardens are accessible only to its tenants. Overall, these gardens are underused. In a similar fashion, 30 St Mary Axe in London has promised to make its airy, bright top floor accessible to the public; however, it failed to do so. Although the public has access to the spacious base of the tower, this exquisite experience at the top is exclusive for tenants and their invitees. At the ground floor, the tower has an efficient footprint, maximizing the public space and natural lighting of the ground level. Much of the surrounding site has been paved to form a spacious pedestrian plaza (the size of eight tennis courts), which contains a café, tables, and chairs. However, this spacious, airy plaza lacks protection from the elements and it is underused. Importantly, future research may examine design and “sustainable behavior” nexus of tall buildings. In his 2011 book titled Fostering Sustainable Behavior: An Introduction to Community-Based Social Marketing
, Doug McKenzie-Mohr elucidates that people often do not behave “sustainably” [43
]. Therefore, if sound design and planning aim to promote sustainable buildings, it is important that people adopt “sustainable” lifestyle that reduces consumption of resources, carbon emission, and waste, and reuses and recycles materials [44
Further, future research may examine the economic, performance, and efficiency of “sustainable” tall buildings. For example, Bank of America (BoA) Tower integrates a host of high-tech sustainable features including an on-site cogeneration plant that works in concert with an ice storage system to reduce the building’s peak energy demands. It collects and recycles rainwater captured on the building’s roof. Low-flow fixtures, dual flush toilets, and waterless urinals further reduce water use. A sub-level graywater treatment plant provides water for the building’s cooling tower. BoA Tower was initially praised for being the “world’s most sustainable skyscraper”, and it received LEED Platinum certification, the USGBC’s highest rating. However, the 56-story building was recently critiqued for its high-energy consumption. According to data released by New York City (NYC) in the fall of 2012, the Bank of America Tower produced more greenhouse gases and used more energy per square foot than any other comparably sized office building in Manhattan [35