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How it works
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 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.

b What physical forces would act on a solar tower 1 km high?
c ►10.2 Su, a structural engineer specialising in the design of very tall structures, is
giving a talk to a group of engineering students. Read the talk. Which of the forces in the
box doesn’t she mention?

Su: With very tall structures, one of the main loads you need to take into consideration, clearly,
is the mass of the structure, its weight. Due to gravity, that mass exerts a downward load, which
has to be transmitted to the ground. So that downward force means the structure is in
compression, especially near the bottom. Obviously, 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 elements 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, a bit 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. You get a twisting force sometimes, when the air pressure is
comparatively higher against one corner of a building, although that’s 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's contraction. That movement can be
significant over a large area, especially as the sun generally heats one side of a structure much
more than the other. So there are all kinds of different forces acting on a tall structure.

bending; centrifugal force; compression; contraction; expansion;
friction; pressure; shear; tension; torsion/torque;

d Label the diagrams using the forces in Exercise 4c.

1 compression 2 3 4
5

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