Not only because of the wind - on skyscrapers

The wind is the biggest engineering challenge in designing a skyscraper - a challenge that intensifies the higher the building, because the strength of the wind increases gradually as you move away from the ground

'Tall', you know, is a subjective matter. When Migdal Shalom was inaugurated in 1965, its 120 meters was a source of national pride: the tallest skyscraper in the entire Middle East. Today, the Shalom Tower does not even appear on the list of the ten tallest skyscrapers in Israel, and if we were to place it next to the 'Moshe Aviv' tower in Ramat Gan (also known by its former name, the City Gate Tower) it would probably be quite a dwarf: the Moshe Aviv tower is almost twice as tall - 235 meters. And if we were to copy the Moshe Aviv Tower, the tallest skyscraper in Israel, to Manhattan for example - no one would give it a second glance...

How, then, do you define what a 'skyscraper' is? How tall should the building be, or how many stories should it have, to be considered a skyscraper, as opposed to 'just' a multi-story building?

There are several different definitions, depending on who you ask, but there is one definition that is especially loved by building engineers, the people who have to implement the architects' fantasies: a skyscraper, according to the engineering definition, is a structure tall enough that the wind blowing on it, and not the self-weight of The structure will be the biggest challenge to be faced during the planning.
In low-rise buildings, the engineer needs to make sure that the structure is stable and strong enough to support its own weight and the weight of the people and equipment that will occupy it. In other words, the forces acting on the building are in the vertical plane, from top to bottom. Winds, on the other hand, exert horizontal forces on the building, or in other words: they push the building from the side like wind blowing on a boat's sail. These forces threaten to break the structure from its foundation and uproot it like a man trying to uproot a tree from the ground.

The wind, then, is the biggest engineering challenge in designing a skyscraper - a challenge that intensifies the higher the structure, because the strength of the wind increases gradually as you get further from the ground.

The wind, the weight and the earthquakes are major challenges, but an architect standing in front of a smooth drawing board - well, a computer screen, in practice - faces another series of additional challenges that are not obvious. The people who will live in the building are responsible for most of the challenges, which are, in principle, a serious nuisance.
Essential infrastructures such as electricity, water, and sewage pipes for example, must reach every corner of the huge structure, and this means thousands of kilometers of cables and pipes. This infrastructure is not only a maintenance nightmare in the long term, but also takes up valuable space: every few floors a mechanical center must be established to gather pumps, central electrical panels, central air conditioners, etc. Every square meter of infrastructure is a square meter on which rent cannot be charged.

The elevators are also a problem. Every floor that is added to the building also means another floor that needs to bring passengers to. If there are not enough elevators, then during rush hours, in the morning and in the afternoon, there will be unbearable queues of people waiting for the elevator. On the other hand, each shaft and each machine room with a large engine is, again, valuable space that will not be used to return the considerable financial investment of the construction and maintenance.

On top of that, stopping at every floor to pick up and drop off passengers is impractical: the frequent stops will extend the travel time from minutes to hours, especially during rush hours. The accepted solution today is the so-called 'sky-lobby': floors that are a kind of 'central stations' for the elevators. Those who want to reach the 100th floor, for example, have to change elevators on the 50th floor and so on. Even then, traveling too fast in the elevator is a problem in itself: the air pressure on the upper floors can be ten percent lower than the pressure on the bottom, and passengers will suffer severe earaches as a result of the pressure changes. This fact limits the speed of the elevator to approximately 700 meters per minute, and in particularly tall buildings there is no choice but to equip the elevators with a mechanism that maintains constant air pressure throughout.

When dealing with the force of the wind, the natural flexibility of the steel is an advantage: the structure can arch and bend a little, like a tree during a storm. The problem is that none of us likes to live in trees during a storm. Humans are very sensitive when it comes to balance, and in the high floors of skyscrapers, the wind can cause tens of centimeters of movement from side to side. Beyond the basic discomfort, such movement can lead to pressures and explosions in the pipes, doors will slam uncontrollably and window frames may be torn out of place.

The architects deal with such problems by means of reinforced concrete reinforcements in the core of the building, usually, but in cases where the reinforcements are not sufficient, more active solutions are required. In the Taiwanese 'Taipei 101', for example, a steel ball weighing 730 tons was placed in the area of ​​the 92nd floor, hanging in the air on four strong cables and four hydraulic springs that attach it to the floor. The ball and the springs form a system that restrains the movement of the building. When a wind or an earthquake shakes the building, the heavy ball stays put, more or less, because it is suspended in the air. The springs between it and the moving floor stretch and contract, part of the energy of the vibrations is converted into friction and heat, and the movement of the building decreases considerably.

And there is another challenge, which is not related to the strength of the structure or the shape of its skeleton, and yet makes building a skyscraper a risky bet.

The 'Burj Khalifa' in Dubai is currently the tallest skyscraper in the world, with a significant difference from 'Taipei 101' which held the record before it. The Burj Khalifa rises to a height of 828 meters, about 300 meters more than the Tipi 101: in other words, the difference between them is like two Azrieli towers on top of each other. It is undoubtedly an awe-inspiring engineering masterpiece: 35 people, about the number of inhabitants of a small city, can live inside it relatively comfortably. The residents of Dubai are proud of their amazing structure, and rightly so, but nevertheless, they changed the name of the structure from 'Burj Dubai', the name of their country, to Burj Khalifa - after Khalifa bin-Ziad, the president of the neighboring country, in the United Arab Emirates . why?

In 1991, the economist Andrew Lawrence published an article called 'Skyscrapers: The Towers of Poletti', in which he described the following interesting finding.

In 1907, the construction of the two tallest skyscrapers of that time was completed: the Singer Building and the Metropolitan Building in New York, and in the same year the Wall Street Stock Exchange collapsed. Construction of the Empire State Building began in 1929. Until it opened in 1931, the United States experienced the most serious economic crises in its history - the Great Depression. For several years after it was completed, the new building was called The Empty State Building, because no one had the money to rent an apartment or office in it. The late Twin Towers were opened in 1973, just before the global oil crisis. The emerging pattern is quite clear: the construction of a record-breaking skyscraper is accompanied - in a fairly high percentage of cases - by an economic crisis.

Andrew Lawrence, noticed this strange cyclical pattern, and saw it as an amusing curiosity. He wrote his article in a humorous tone: 'Folti's Towers' mentioned in the title of the article is a successful comedy series starring John Cleese. But the 'skyscraper index', as the unexpected relationship between height peaks and economic crisis peaks is known, has piqued the curiosity of many economists. Additional studies have shown that although the relationship is not one-sided - that is, there are economic crises that are not accompanied by the construction of record-breaking skyscrapers - the relationship is indeed strong and exists to one degree or another.

What is the reason for this phenomenon? The accepted theory holds that the global and local economy moves in cycles of ebb and flow. In a time of high tide, interest rates are low and the capitalists are looking for new avenues of investment - for example building a large and impressive skyscraper. This construction takes time, and when it is finished and the building is ready for occupancy - the tide has turned into a low tide, and the economy is in trouble. In other words, one of the biggest risks in building a new skyscraper is the likely possibility that it will be difficult or even impossible to cover the investment in it in a reasonable time.

This is exactly what happened to Burj Dubai. The construction of the tallest building in the world began in 2004, in a flourishing and prosperous economic climate where there was no problem in obtaining the billion and a half dollars needed for the project. In 2008, when the building was almost ready, the mortgage bubble burst in the United States and brought it, as well as considerable parts of the world, into a severe economic crisis. Dubai almost collapsed under the burden of debt... and the United Arab Emirates poured in the required funds and saved the faltering economy. The name of Burj Dubai was changed to Burj Khalifa, after the President of the Union. The luxury apartments in Burj Khalifa have been empty for many months, and the rents in the building have dropped to half of the initial estimates.

Wind, earthquakes, complex planning, inflated infrastructure, considerable economic risk... simple logic means that building a record-breaking skyscraper is, except in isolated and exceptional cases, a bad idea.

And yet, throughout the twentieth century and even today, new and higher skyscrapers are being designed and built. The Saudis, for example, are already drawing up the plans for their 'Kingdom Tower': a skyscraper about a kilometer high. Saudi Arabia, to remind you, is a huge and empty desert - so a lack of space is probably not the reason why the Saudis plan to invest twenty billion dollars in their tower. In other countries, they talk about towers with a height of one and a half kilometers. What attracts us to build higher and higher, even if there is no practical reason for it?

It seems to me that the answer can be found in the very name given to these tall buildings at the end of the 19th century, in the days when they were just beginning to grow from the ground: Skyscrapers. The desire to do what seems impossible, to dare what no one else dares, to touch the sky... this is the true motivation. Dubai, Malaysia, Taiwan and all other developing countries decorate their skylines with tall buildings to show themselves and the rest of the world that they are also on the map: that they are strong, developing, growing. The competition for the title of 'tallest skyscraper in the world' is not measured in meters or floors, but in centimeters...centimeters of the chest circumference, when every citizen of a growing country raises his head to his tall tower, puffing out his chest with pride.

I think it can be said with a high degree of certainty that as long as the laws of physics allow it, the current and future powers will continue to try and prove to each other 'who has the bigger one'. For skyscrapers, the sky is the limit.

(Ran Levy is a writer and lecturer on science and technology, and hosts the podcast 'Making history!'- A radio program about science, technology and history: )