Skyscraper construction due for radical change

Friday, September 1, 2017

Skyscrapers command skylines around the globe, like the the Burj Khalifa in Dubai, the Shanghai Tower in mainland China, and the Makkah Royal Clock Tower in Mecca—the three tallest buildings in the world. The number of buildings that exceed 656 feet high has tripled since 2000, while construction of these towering structures is slowly evolving to meet the demands for better efficiency and sustainability.

Staff and students in the New Jersey Institute of Technology’s Masters in Civil Engineering degree program compiled an infographic as they delved into the environmental impact of skyscrapers, traditional materials governing the construction process and potential opportunities for efficiency. Here, they describe wood versus steel and the search for better materials.

Environmental Impact of Skyscrapers

It is estimated that five per cent of man-made carbon dioxide emissions worldwide come from the production and transportation of cement, a vital ingredient for concrete. A further five per cent of emissions can be traced to steel production. For every ton of steel produced, two tonnes of carbon dioxide gets emitted. This results in the annual release of approximately three billion tonnes of the greenhouse gas. Experts say that the carbon embodied in building materials will make up 45 per cent of a typical modern office black’s carbon footprint. There is a definite need to reduce this high percentage to lessen the industry’s contribution to environmental problems.

A lone elevator with traditional steel rope carrying twenty-four passengers consumes 130,000 KWh of energy per year and weighs up to 27,000 kilograms. The ropes are subjected to severe strains. On windy days, the entire building may shake and put steel-cabled elevators temporarily out of service. The traditional elevator design is simply inefficient. People have to wait for a long time just to be able to ride them. A 2010 study found that New York City office workers spent a total of 16.6 years waiting for elevators to open up and let them ride.

The Search for Better Materials

Companies are using glass with greater frequency to design the shape of skyscrapers. Glass has the advantage of lightness, visual appeal and translucence. Various coatings and construction methods have also improved insulating capacity. Many are also working to build sustainable and green-friendly wooden skyscrapers. They are using cross-laminated timber (CLT) which has excellent strength and durability. It is highly resistant to fire, moisture and decay.

Wood versus Steel

In order to solve elevator problems, some companies are working on ropeless elevators that can move vertically and horizontally. These use magnets and linear motor technology. These ropeless elevators need smaller shafts than conventional ones. With the old design, up to 40 per cent of the space is devoted to the elevators. The actual percentage will depend on the total height. Going ropeless can increase the building’s usable area by as much as 25 per cent. This is a significant increase that will allow more staff to be placed per floor. The productivity of the structure will rise because of this simple change.

This new type of elevator is going to be made from carbon composite materials which are known to be strong yet lightweight. The cabins and doors weigh about 300 kg in standard units. The switch to lighter materials will bring this down to just 50 kg or about one-sixth of the original. Other parts will stay as is, but the overall result is still a 50 per cent weight reduction over the current models. Lighter objects are always easier to lift up. The energy consumption of these elevators will drop as the motors will not have to work as hard as before.

The shaft transport capacity can also increase by almost 50 per cent. This means that elevators will be able to fit in twice as more people per trip. Waiting lines will be minimized and workers will be able to get to their offices quickly. With less trips required, the carbon footprint may also decrease by half.

Replacing Steel with Wood

Steel has traditionally been the main component of major construction projects. In 2014, however, only five per cent of the skyscrapers completed followed this old model. The majority depended on CLT. That’s 52 out of 97 buildings or 54 per cent, a significant increase to the 2013 figures, which showed that 24 out of 71 buildings or effectively one-third. CLT is composed of several layers of timber board that are bonded to form structural sheets as thick as 40 centimetres. This cross-lamination technique is what provides incredible strength and stability to the material.

For every cubic meter of concrete or blocks replaced with wood, the building’s carbon dioxide emissions decrease by up to 1.1 tonnes. Many advocate a hybrid system wherein timber and concrete beams are both used whenever appropriate. Carbon footprint reduction can reach as high as 75 per cent in this type of system when compared to the usual all-concrete structure. Wood is able to store CO2. For instance, there is a proposed skyscraper in Paris called the Baobab. It would be able to store about 3,700 tons of carbon dioxide, the equivalent of taking 2,207 vehicles off the road for an entire year.

Wood is also a capable thermal insulator. It can resist the transfer of heat 400 times better than steel and 10 times better than concrete. This means less of the material is needed to line the walls and enclose the building. Thinner walls also provide the benefit of increased usable space.

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