7. PHYSICAL FORCES

SESSION 7

PART 1

Tall buildings

Watch the following short video (turn on subtitles if you wish) to learn about the problems we must face when building tall structures.

A. Now show how much you learnt about the video by playing this quiz against other students. Who will lead the scoreboard?

PART 2

Describing physical forces

Source: newslaundry.com

Read the text about physical forces and answer the questions.

There are all kinds of different forces acting on a tall structure. With very tall structures, one of the main loads you need to take into consideration is the mass of the structure, in other words its weight. Due to gravity, the mass exerts a downward load, which has to be transmitted to the ground. That downward force means the structure is in compression, especially near the bottom. The closer you are to the bottom, the more compressive force the structure is subjected to. But with tall structures, downward load compressing the structural element is only part of the problem.

Another major force acting on the structure is wind load, which is a horizontal load, exerted by air pressure against one side of the structure. Because the structure is fixed at ground level, and free at the top, that generates bending forces. And when elements bend, you have opposing forces: compression at one side, tension at the other. And at ground level, the wind effectively tries to slide the structure along the ground, and the foundations below the ground resist that. The result of that is shear force between the substructure and the superstructure. The wind generates tensile loads on the foundations of tall structures as well, as the bending action tries to pull them out of the ground on one side, like a tree being uprooted by the wind. So the foundations need to rely on friction with the ground to resist the pull-out force, just as tree-roots do. The action of the wind can also generate torsion. Sometimes you get a twisted force, when the air pressure is comparatively higher against one corner of a building, although this is less of a problem with chimneys because of their circular profile.

With very large masses of concrete, you also have to think about the forces generated by thermal movement. When concrete absorbs heat from the sun, you get expansion; as soon as the sun goes in, there is contraction. That movement can be significant over a large area, especially as the sun heats one side of the structure much more than the other.

Source: Technical English. David Bonamy. London: Pearson Longman, 2008.

B. Comprehension exercise

1. Make a list of the forces mentioned in the text. If you are in class, work in pairs to make sure you understand what they mean. If you are at home you may check them online.

Check answers

2. Label the following diagrams using your list.

Check answers

PART 3

Solar towers

How much do you know about solar towers? Watch this short video to know more about them before you read the text.

Reading: Solar Towers

Read the text and answer the following questions

The need to develop renewable energy is widely seen as a futuristic technological challenge. In reality, some of the most effective ways of harnessing horsepower from nature are based on concepts that have existed for donkey’s years. The wind turbine is an obvious example. Another – less well-known, but conceived almost a century ago – is the solar tower or solar chimney. And if the Australian company EnviroMission completes an ambitious solar tower project in the New South Wales desert, the technology could capture not just the sun’s rays but the public’s imagination worldwide. The firm is planning to construct a tower a colossal one kilometer high. If built, it will be the world’s tallest structure by a huge margin.

How does it work?

A large glass enclosure is built, with a chimney at its centre. The sun heats the enclosure, causing expansion of the air inside. At the top of the chimney, the lower temperature and lower pressure due to the higher altitude create a pressure differential known as the stack effect. This causes air to flow up the chimney. Electricity is generated by turbines at the bottom of the chimney, which are driven by the flow of air. The bigger the area of glass and the taller the chimney, the greater the airflow and the higher the generating capacity.

Source: Technical English. David Bonamy. London: Pearson Longman, 2008.

Solar tower: scheme
Source: Researchgate.net

Enviromission solar tower, New South Wales, Australia.
Source: fgould.com

QUESTIONS

1. What is a solar tower and what forces does it use?

2. What forces would act on a tower 1km high?

3. What else do you know about the stack effect? What is the French translation for that expression?