Sustainable Tall Buildings
A sustainable skyscraper is the building, whose design team have achieved a balance between environmental, economic and social issues at all stages, including design, construction, and operation. This may involve greater emphasis on different aspects at different stages in the building’s life for example, energy for building services and transport of building users and occupants and associated CO2 emissions. A sustainable tall building can also be defined as the one which emits no pollution to air, considers its land and water, and can contribute positively to the local community whilst be economically occupied throughout its time-life. However, incorporating sustainable aspects in tall buildings is a challenge.
Orientation on Plan: Daylight and Passive Solar
Tall buildings are less constrained than low-rise buildings by the shape of land plots or the layout of streets. The orientation of the building and the shape of its plan allow the maximum advantage of using daylight. The fabric of the façade and the area given to windows are also of ultimate concern both in determining the thermal insulation of the exterior walls, and in gathering light. While an individual tall building can be ideally suited to capturing the heat and light energy of the sun, a second or a third tower constructed in its shadow would be adversely affected. For example, the dark shadows cast across the Bow building, Calgary, Canada, in the morning by the surrounding buildings, may affect its running costs. However, optimizing its orientation is essential to its sustainable development.
Image credit: Solar techniques & advanced nanotechnology Tower designed by 10 DESIGN
Building Shape and the Effects of WindThe shape and profile of a tall building determines its performance in wind. Shape not only affects the loading on the structure, but also has an impact on conditions in the surrounding streets and buildings. Good aerodynamic design has a beneficial impact on the structural frame of a tower in terms of materials, and on the comfort of pedestrians at ground level. For new towers in the City, wind analysis and modeling are valuable design tools in optimizing the shape and form of the exterior. The shape of the World Trade Center Towers, Bahrain will create an accelerated airflow for the jumbo blades to generate electricity with wind turbines and to reduce the consumption of fossil fuels.
Image credit: The World Trade Center Towers, Bahrain
Floor Plate Depth and Ceiling HeightThe width of a building, usually referred to as floor plate depth, has a critical impact on its economic, social and the environmental performance. The ratio of net internal area to gross internal area can be increased with a deep floor plan. This ratio decreases with height as a greater area is given to cores in tall buildings. In terms of running costs, however, energy for lighting can be greatly reduced by the use of daylight. Useful daylight penetrates 3-6 m inside a building from the windows, and shallower floor plates maximize the use of daylight. The width of the building and the height of the ceiling also affect the level of lighting. However, businesses are now more aware of the effects of ceiling height, ventilation and distance from windows upon the output of the employees. A tall building can have fewer constraints on floor height and a narrower floor plate than a low-rise office block which can lead to an increased rentable value.
Floor Slab Thickness and Thermal MassDaylight entering a building arrives with a degree of passive solar heat gain. The structure of a building can be arranged to absorb and drive away that heat energy by its thermal mass characteristics. In principle, this involves direct absorption of heat into the floor slabs when there is sunlight and a release at cooler periods. While passive solar gains can be exploited to avoid the need for heating a buildings in winter, gains in summer can create the need for cooling. Through design, fabric energy stored in the building structure can be removed at night by natural ventilation, reducing energy consumption in cooling systems. Control over the radiant heat entering or leaving the building can be provided by external moveable shutters. Use of this type of mechanically operated shading has become more common in London, where it has replaced more conventional internal blinds.
Holloway Circus Tower Principles of Environmental Design. (Source: Pank, Girardet & Cox, 2002)
Façade EngineeringAnother area where the fabric of a building can be used to control the internal conditions is the façade. Double glazing with argon-filled cavities, triple-glazing and glass coatings can decrease energy consumption. Air tightness of the façade is a major issue for tall buildings where pressure differentials from higher winds at the top of a building can cause problems with controlling internal temperatures and draughts. Double skin facades offer several advantages, such as acting as buffer zones between internal and external conditions, and reducing heat loss in winter and heat gain in summer. Natural ventilation can be drawn from the buffer zone into the building by opening windows in the inner façade. This type of double skin glazed façade used in the RWE Headquarters, which is encased in a double glass skin. Outside is a single layer of strengthened safety glass, while the inner glazing is heat-insulated white glass, allowing daylight to be maximized. A 500mm void between the layers of glass acts as a thermal buffer.
References:1. Brundtland Report “Our Common Future” World Commission on Environment and Development, 1987.
2. Will Pank, Herbert Girardet & Greg Cox, Tall Buildings and Sustainability. Report March 2002.
3. Main picture: http://avdnowhere.blogspot.com/2011/01/gullwing-twin-wind-towers-wind-powered.html