ASHRAE - 90132
Design Guide for Tall, Supertall, and Megatall Building Systems
|Publication Date:||1 January 2020|
As the tall building industry moves into a new decade, it is appropriate to stand back and take stock of what we've accomplished, but also to be honest about where we have fallen short. The foreword to the first edition of this book, published in 2015, highlighted the critical issues of tall building design, especially concerning environmental footprint, in terms of both embodied and operational energy. We are duty-bound to continue to reiterate this as our highest priority. The fundamentals have not changed, though there have been some encouraging advances in the intervening five years.
First, let us review what remains the same. As always, architects and engineers determine how program, structure, and services will support each other efficiently. And, in each case, it falls to building services engineers to determine how a spatial arrangement of stacked floors will be ventilated, heated, cooled, and how it will interact with the envelope that surrounds it.
Tall buildings continue to require enormous amounts of energy to move their occupants from floor to floor, exhaust the heat they and their many electronics generate, and provide chilled air or heat to keep conditions comfortable. They require enormous skill and effort to seal envelopes and keep them airtight against the elements and to prevent unpleasant pressure changes as elevators hurtle up and down their lengths.
We are still faced with an overstock of tall buildings constructed following a protocol that dates from the 1950s-seal the building, cover it head-to-toe in glass, and air condition it 24 hours a day. For all of the advanced engineering work that goes into making these astounding structures stand up and function, from an MEP standpoint a large number of them are dinosaurs. For how iconic they are, many of these monoliths are not built for longevity.
It is commonplace to talk about a tall building's design life (or at least those of its critical constituent parts) as being 25, 50, or 100 years. It is particularly discouraging that one of the new "height records" from the past five years is that a tall building of greater than 656 ft (200 m), the Union Carbide/JP Morgan Chase headquarters building in New York, is now being dismantled for the first time. This 1960 office building fit the "dinosaur" paradigm to a tee, but it had been extensively retrofitted twice already, in 1982 and 2011, yet still the owners concluded it needed to be demolished and replaced to meet contemporary needs. We have to do better from the outset if we do not want to see buildings we construct today meet the same fate in 50 years.
Recladding in the conventional sense is enormously expensive and wasteful of materials. We should be designing not for 100 years, but for the ages, and that means designing envelopes and mechanical systems that can be replaced and improved over time as technology improves, or as human tolerance for adverse temperature conditions improves-and probably both. That's why a design guide like this is so important, as is the practice of updating it with the latest developments in subsequent editions. There is so much research, so much unrealized potential in both passive and active systems-operable façades, dynamic façades, energy-generating façades, vertically vegetative façades, "breathing" façades, etc.-that embrace, rather than resist, natural environmental conditions. Of course, a solution for a one-story building is not going to work for a 100-story building if it is simply copied upwards 100 times. But the fact that tall buildings do require special engineering in order to achieve the same levels of environmental performance as their shorter peers is not a justification for not attempting it; rather it is a call to action to elevate the practice of tall building engineering to meet this challenge.
One of the more interesting opportunities for super- and megatall buildings is the stratification of climate that exists in any one location with height. Effectively, with the extreme heights now being achieved in tall buildings, we are designing one building that cuts across several climate zones. Significant temperature differences exist between the top of the building and the street level. Tall buildings should be exploiting this for their advantage-cool air is heavier and naturally drops, warm air rises-yet most continue to extrude the same vacuumsealed façade upwards for hundreds of meters, with all of the energy expenditure that implies. Façades and building systems need to vary with height and reflect the climatic stratification that naturally occurs along their length.
This is already being done, and has been done for more than 20 years-it just hasn't been done nearly often enough. Double-skin façades like that pioneered in the 1997 Commerzbank Tower, Frankfurt, perform as environmental "switches" and provide for natural ventilation, while mediating the indoor temperature to a level that does not require 24/7 conditioning. Newer projects like the Shanghai Tower, with its large atria and communal sky gardens, take this idea to the next level, a social as well as a highly functional series of spaces.
Consider also the potential of those seemingly inexhaustible high-altitude resources- wind and solar. A handful of buildings have attempted to harness the wind and derive energy generation by directing it through sculpted gaps, towards turbines in their masses, and numerous skyscrapers now incorporate solar arrays. However, the implications of capturing wind energy at height have not been fully resolved, and is there the potential for high-rise façades to incorporate solar/thermal technologies, rather than just PV arrays.
Green walls offer a range of benefits, from increased energy efficiency to aesthetic appeal and the reduction of the urban heat island effect. The same exterior vegetation we find charming on low-slung collegiate buildings can also be exploited to great effect at height, providing significant cooling to the façade while blocking glare, enhancing the psychological well being of the occupants, and reducing impact on mechanical systems. Again, this is not a simplistic project of extrusion upwards, but it is being done, in places like One Central Park, Sydney, Oasia Downtown Hotel, Singapore, and the Bosco Verticale, in Milan, Italy.
We've also seen sophisticated mechanical, operable façades that adjust to solar conditions, such as at the Al Bahar towers in Abu Dhabi. Other cutting-edge façades, like the Doha Tower in Qatar, or the Shenzhen Energy Headquarters in China, incorporate historic building techniques to reduce thermal gain and thus improve efficiency. We should be seeing many more of these.
One of the most stunning innovations of recent years is also one of the simplest-a tall building in Tokyo, the NBF Osaki building, uses envirotranspiration to shed heat via rain-filled ceramic pipes that double as brises-soleil. In other words, it "sweats," and the microclimate surrounding it benefits from lowered temperatures. Multiply this across a whole city and the urban heat island phenomenon begins to seem less intimidating.
The superlative achievements of tall buildings in terms of height and stability have yet to be fully realized in the MEP department, but they can and must be if tall buildings are to be a positive force in urban environmental sustainability. Five years on from the first edition of this guide, it remains the case that there is much research and hard work ahead of us. Many one-off, bespoke solutions exist around the world, but there are few standardized sets of practices. In the work of our two organizations, ASHRAE and CTBUH, and in the pages before you, lie the bases for building that expertise.