Tall buildings emerge from two motives: density and pride. Although the motives are often mixed, it's mainly density that accounts for urban skylines like those of Hong Kong, New York, Singapore, and Toronto, and mainly pride that accounts for spectacular heights and forms like the Empire State Building, Burj Khalifa, and greater marvels yet to come.
Nick Merrick/Hedrich Blessing
There's a case to be made for pride. According to Gordon Gill, FAIA, partner at Adrian Smith+Gordon Gill Architecture (AS+GG), architects of the 3,280-foot-high Kingdom Tower now under construction in Jeddah, Saudi Arabia, a tall building can establish the identity of a place, assert the arrival of a culture on the modern stage, attract energies for growth and renewal, and thereby contribute to the vitality of a city and its inhabitants—for the long term, if the project is a good one.
Tall buildings can also inspire the imagination. Science fiction brims with tall utopias, and more than a few religions have designated the sky as home to their gods. Now the technological opportunities of building ever taller present fresh inspiration. "Most people don't realize what the sky can offer us," says Luke Leung, director of sustainable engineering at Skidmore, Owings and Merrill (SOM). At the top of Burj Khalifa, for example, the temperature is some twelve degrees cooler, and air contains 50 percent fewer particulates than at ground level. Tall buildings can bring within reach sky-sourced cooling, clean air, wind and solar energies, and even, ultimately, water.
But the costs of reaching for the sky are also high. An analysis by the Council on Tall Buildings and Urban Habitat (CTBUH) shows initial embodied energy per unit of gross floor area are generally higher for tall buildings. Operating energy also increases with height as the effort of lifting people, materials, and water to upper floors increases. Heating and cooling loads are usually higher. And the efficiency of floor plates diminishes as additional structural, egress, and elevator requirements occupy space. Although the thresholds at which each of these costs kicks in shift continually as technologies evolve, a building must still reach exceptional heights before the sky-sourced resources that could mitigate these impacts become available.
In a world of threatened nature and finite resources, the high ground for rationalizing height goes to density. The population of cities globally is growing by 200,000 people each day—that's a new city of more than a million people each week. Based on 2010 trends, the McKinsey Global Institute has predicted that China will need the equivalent of a new New York every two years, and India will need a new Chicago every year for the next 20 years. Assuming that the fuel surplus feeding these trends lasts long enough for the predictions to be incarnated, the future of building in rapidly urbanizing countries is "tall."
"The vertical city is more sustainable than the horizontal city as a concept," says Antony Wood, RIBA, executive director of the CTBUH. "To give us any chance of surviving on this planet, that's absolutely the way it's got to go."
So, from a sustainability perspective, at what point do the advantages of density outweigh the costs of height?
A study nearing completion at AS+GG, to be published in book form, uses digital models of 2,000 dwelling units in nine configurations, ranging from a single 210-story tower to 2,000 suburban houses on 1/4 lots—with each configuration on the same hypothetical land base—to compare sustainability indicators such as land use, embodied energy, operational energy, and implied emissions for infrastructure and transportation.
Overall, the study suggests that the sweet spot lies a distance from the extremes at either end—neither supertall nor single-family—but exactly where it lies depends on assumptions about building methods and materials, dwelling-unit size, transit, community infrastructure, and even the metrics of the study itself.
"What we're finding is true, in the master-planning of cities and the performance of individual buildings, is that it's the balance of systems that allows the greatest sustainability," says Gill. "When we look at our models, it's the combination that works."