Skyscrapers are really tall buildings that seem to scrape the sky
Dramatic cityscape of Taipei and trees in park, Taiwan, Asia. © elwynn/Shutterstock.com.
Skyscrapers are tall buildings with many stories (floors). There is not one specific definition how tall a skyscraper is. Some people might call a 20-story building a skyscraper. Others think a skyscraper must be at least 40-50 floors.
We call them “skyscrapers,” because they are so tall they seem to scrape the sky. The tallest buildings in the world are over 2000 feet tall. For example, the Burj Khalifa in Dubai is 2717 feet tall, with 163 floors. That’s more than half a mile tall!
Before the Burj Khalifa was built, the Taipei 101 building in Taipei, Taiwan was the tallest skyscraper. The Taipei 101 is 101 floors tall above ground, with an additional five floors below ground, and is 1671 feet tall. As shown in the image, it rises well above the rest of the Taipei skyline.
Inspiration for skyscrapers came from a disaster
Up until the early 1800s, the tallest buildings anywhere were only five or six stories tall. Some inventions needed to be made before buildings could be made taller. The inspiration to create tall buildings came from an unexpected place. In 1871, the “Great Fire of Chicago” burned thousands of buildings in Chicago, Illinois. Believe it or not, this fire was blamed on a cow! Many people thought that a cow owned by the O’Leary family on DeKoven Street had kicked over a lantern and started the fire that destroyed much of the city. But no one has ever found out for sure what caused the Great Fire.
After the fire, Chicago needed to be rebuilt. There were many buildings destroyed, and those buildings had held many businesses and people. So, to rebuild Chicago, they decided to build tall buildings that could fit many people and businesses.
Some inventions were required to enable the building of skyscrapers. First, people had to figure out efficient ways to make steel - a major material for skyscrapers. Another major invention was the elevator. People probably would not have wanted to walk up and down twenty or more floors every day. A man named Elisha Otis invented the modern elevator, with a safety break, in the 1850s. Third, in order to power the elevator system - and light the building - electricity needed to be available. Electricity was made available in small areas in the early 1880s.
Skyscrapers use a steel frame to hold up the building
Left: Colorful parrot in captivity. He is looking at camera ©Gracanin Dusan / Shutterstock.com. Right: Structural steel framework © Joe Gough / Shutterstock.com.
Before skyscrapers were invented, tall buildings were built out of stone. The stone walls were what held up the building. Any openings in the stone walls made them weaker, so windows are few and small. However, skyscrapers use a steel frame that holds up the building. The frame uses steel beams (horizontal bars) and columns (vertical bars) connected together. Because the frame, not the walls, holds the building up, windows are not a problem at all.
Believe it or not, the person who came up with the idea for a steel frame got the inspiration for it from a wire bird cage. He watched his wife place a book on top of the bird cage, and saw that the cage could hold a heavy book. He realized that a similar frame might be possible for a building.
Designing skyscrapers starts with needs and use
Like any other engineering design problem, designing skyscrapers starts with needs and use. Architects and engineers work together to design the skyscraper. The architects may focus on the desired look, and feel, and the spaces in the building. They want it to look nice, and get plenty of sunlight, and be an enjoyable place for people using the building.
The engineers have to think about materials and structural soundness, so that the beautiful building doesn’t fall down. There are actually multiple types of engineers that get involved in designing a skyscraper. Skyscrapers are complicated! Engineers need to consider the site where the skyscraper will be built. For example:
What is the soil made of? How deep is the bedrock?
Does the region have strong typhoons or tornados?
Is the region in a strong earthquake zone?
Skyscrapers are designed for forces like their own weight and wind
Picture of John Hancock Center in Chicago Illinois. Photo by Joe Ravi. Image used under Creative Commons Share-Alike CC-BY-SA 3.0
Remember “dead loads” and “live loads” from the main Structures section? The dead load is the force from the structure’s own weight. The live load is the set of temporary forces on the structure at a point of time – such as the weight of a person sitting in a chair.
For a skyscraper, the dead load (its own weight) is much higher than the live load (such as the people inside). So, a lot of the design results from having to hold up the great weight of the building itself.
Wind is also a major force on a skyscraper. The wind gets stronger the higher you go in the building. Did you know that skyscrapers bend in the wind? If a skyscraper didn’t bend at all, a big wind could break it. However, if the building bends too much, a big wind could blow it over. Also, people at the top of the building might not like to experience too much movement from the wind. So, the building designers want to allow the building to move a little – but not too much.
There are different ways to keep a skyscraper from moving too much. One way is to use steel cross-bracing (called “X-bracing”) inside or outside the building. The John Hancock Center in Chicago (shown at left) uses X-bracing outside the building. Another way is to use something called “supercolumns.” Supercolumns are wider columns of metal-reinforced concrete that are placed in the corners or on the sides of the building. Another design approach is to use something called a “damper” to lessen the movement in a building. A damper is a heavy block at the top of a building that is connected to springs. When the wind blows, the building moves a bit, but the damper does not – so the springs stretch in one direction and compress in the other. The springs help pull the building back into position.
The foundation of a skyscraper depends on the ground where it will be built
There is more to a skyscraper than what you see. Skyscrapers are built on really strong foundations. Those foundations are built down into the ground. Building a skyscraper requires a lot of digging. The foundation design depends on what the ground is like where the skyscraper will be built. If the foundation is not strong enough, the building could sink or collapse!
The important thing to know for the foundation is how deep the bedrock is. The bedrock is the solid rock layer beneath the ground’s soil. If the bedrock is near the surface, then the foundation does not need to be as deep. In that case, the soil on top will be removed, the bedrock evened out, and then holes made for the building’s support columns. These holes are called “footings.” The building’s support columns are made from steel or reinforced concrete and are placed in the footings.
If the bedrock is deep, then the design is a little more complicated. The design may require some things called “piles” – which are like long columns – to be driven down deep into the earth. The piles may be steel or reinforced concrete. The design may also call for a thick slab of concrete to be built down in the earth to make a strong mat or “raft” for the building to sit on. For example, the Shanghai Tower design required both deep piles and a thick concrete mat. They used 980 piles driven down 282 feet into the ground, and then poured in more than 2 million cubic feet of reinforced concrete. The concrete mat for the Shanghai Tower is 20 feet thick!
Skyscrapers require a lot of design but they are often iconic buildings
Skyscrapers are more than “just a tall building.” When people think of certain cities, often what comes to mind are the skyscrapers the cities the cities are known for.
When the decision is made to build a skyscraper, many types of people get involved. Architects figure out how best to have the building stand out, yet fit well into its surroundings, while also meeting the needs in a beautiful way. Architects and engineers have to work together to ensure the beautifully designed building is structurally sound as well. Different types of engineers are required, too. Engineers who understand the soil and rock under the building work with structural engineers to design the foundation. Electrical engineers and mechanical engineers design some of the internal parts of the building, like the electrical systems, heating and cooling, plumbing, and more. Building skyscrapers is a complicated team effort involving many people. But look at some of the beautiful buildings that have resulted!