New Delhi: Internationally, a building that reaches or exceeds the height of 150 metres is considered a skyscraper. Until recently, Mumbai was the only Indian city with high-rise buildings. The financial capital continues to see the highest demand for skyscrapers, as the only option to grow there is vertically. It now seems that in the coming decade, Maximum City will receive an even more cohesive skyline, with a host of projects in the race to touch the sky being constructed. The demand for high-rise buildings is certainly growing, and other cities are catching up.
Mumbai continues to have the maximum number of tall buildings approved or under construction. Development of India One - the tallest in the country - has already begun in Maximum City. It spans 126 floors and stretches up to a height of 720 metres. Apart from this, Mumbai has more than 30 such super-tall buildings ranging between the heights of 150 metres to 450 metres either at the approval stage or already under construction.
New Delhi, the capital of India, has around a dozen of such buildings coming up. They range between heights of 150-300 metres. Kolkata too is catching up with 9 such residential buildings extending to the height of 245 metres either approved or under construction.
Ahmedabad too has about 13 tall buildings which are under construction and are ranging between 200 metres to 410 metres. Hyderabad and Bangalore too are witnessing some development in construction of tall buildings for residential-commercial purpose with 2 or 3 approved projects.
All in all, this amounts to around 60 skyscrapers. Developers see such edifices as a good way to attract potential buyers - high-rise buildings are a good gambit to differentiate their offerings from the rest of the pack. However, this coin has two sides – high-rise development has its own share of demerits, too:
Ø Effect on Urban Wind
Rise in the elevation of a building increases the distance of the wind shadow and minimizes the air flow at the street level behind the building. Near high-rise buildings, the local wind speed is high even in summer. In addition, high-rise buildings tend to create a turbulent flow of the gradient wind as a result of increasing the roughness of the boundary layer surface.
Ø Increased Air Pollution
In summers, local wind speeds near skyscrapers are very high and troublesome. The ventilation conditions in the urban spaces and major streets with high vehicular traffic have significant impact on the concentration of air pollutants at the street level. The high velocity and turbulent wind at the street level results in the mixing of the highly polluted low-level air with cleaner air flowing above the urban canopy.
Ø Effect on Urban Radiation
High-rise buildings absorb direct and reflected solar radiation of surrounding low-rise buildings and convert it into heat via convection of long wave radiation. However, when buildings are of different heights, the walls of the higher buildings absorb part of the reflected and emitted radiation and block a portion of the sky, resulting in reduced solar exposure and long-wave emission from the roofs of the lower buildings.
Ø Increased Urban Temperature
Size and density of the built-up areas affect urban areas temperatures. In the congested centres of large cities, temperature levels are generally higher than in the suburbs. The largest elevations of urban temperature occur during clear and still-air nights, also called ‘Urban Heat Island'. Excessive opacity of high-rise buildings in city centres results in concentrated heat generation by high-density land use (traffic, lighting, heat exhaust) and contributes to the creation of urban heat islands.
Ø Effect on Night-Time Cooling
Nocturnal radiation is a major climatic factor that reduces atmospheric heat in urban areas located in hot, dry regions. Nocturnal radiation decreases when the density and the height of built-up urban masses increase. High-rise buildings store solar energy during the day time and release it slowly into low-speed local wind, especially at night. The vertical distance between cool winds above buildings roofs and the ground surface is long, and this results in decreased radiant cooling during the nights. Low-rise buildings that match trees heights of 12-15 meters, on the other hand, penetrate night-time ventilated cooling at the ground level and also store cool radiation through built-up urban areas.
• Tall buildings are colder in winter and hotter in summer than regular buildings, and therefore require more heating and more cooling. This is particularly true of modern glass towers. Thus, a lot of energy is required to keep these high rises functioning.
• Exterior cleaning and maintenance of a high-rise building can be very costly and dangerous. With global warming (which causes higher wind speeds) on the rise, insurance companies often refuse coverage to maintenance companies in charge of high-rise buildings at certain times of the year.
• High-rise buildings take longer to build, and due to rapid and heavy construction activity within the city, there is a heavy load on civic infrastructure.
• In-high rise buildings, the average construction cost per square foot is 20-25% higher if the building has more than 12 floors.
• Major modifications and/or renovations in a skyscraper are significantly more cost-intensive.
• If a new building has to be built on the same piece of land, the number of claimants is vastly higher.
When it comes to our largest cities, there is not much one can do about these factors – and indeed, they are accepted as a fact of life in a city like Mumbai, which must grow vertically if it is to grow at all. Unfortunately, the areas of our cities which are in the biggest need of high rise buildings are also the ones which offer the lowest scope for remedial infrastructure measures that could reduce the impact of skyscraper development.
Subhankar Mitra, Head – Strategic Consulting (West) JLL India