Building is a very important human invention
that is indispensable to our daily life. The main functions
of a building are to protect us from wind and rain and provide
us a safe and comfortable environment for working and rest.
At its core, the framework of a building, that is the structure
of the house, is most important. Nowadays, safe structure
and a feeling of comfort from the residents are the basic
requirements in building architecture. We must make sure that
a building will not collapse or move excessively under great
stress or pressure. Two types of loads act on a building structure:
the vertical force and the lateral force. Vertical force refers
to the load from the weight of human, furniture or building
itself; lateral force is the load from wind or earthquake
etc.
Before introducing the building structures, we first introduce
what is “Stress” and then look into how the “Beam”
reacts under stress.
When there is load acting on a structure, stress
will be induced in each component. Stress is defined as force
per unit area. With the following simple example, you will
have a better understanding of the concept.
Example:
There is a load of 200,000 kilograms shared by 4 pillars.
So each pillar has to bear 50,000 kilograms. If the area of
each pillar is 10,000 square millimeters, the stress can be
calculated as follows:
Stress = force / area
=
(50,000 kg )
/ 10,000 
=
500,000 N / 0.01 
=
50 MPa
Furthermore, the maximum stress that a material
can bear is called the material strength. In the above case,
the structure will stand firm if the material strength is
over 50MPa. If the pillar has to bear the same load with a
material strength reduced to 25MPa, the area of a pillar has
to increase to 20,000 square millimeters. The application
of the two mechanics concepts, stress and material strength,
is very extensive and very common in our daily lives.
The
following is another simple example in our daily life: if
you sting a needle on your finger tip, you get bleeding because
the stress from the small needle head acting on your finger
exceeds the strength of your skin. On the other hand, if a
wooden column is pressed on your finger tip, you may not get
bleeding as the stress does not exceeds the skin strength
due to the smaller stress exerting on the finger as a result
of increasing the area of the wooden column.
Beam, or girder, is the prism-like component
placed laterally in the walls in the building. When loading
is applied on the upper surface of the beam, the beam will
bend. The upper side of the beam is under compression while
the bottom is under tension.
The following is an experiment of a simulated
beam under stress:
in
the experiment, the long ruler represents a beam, the book
represents the load. Hold the long ruler with both hands.
Shorten the distance between your hands so as to reduce the
ruler’s span to an appropriate level and then put the
book in the middle of the ruler. You will not observe obvious
positional shift (bending) on the ruler. Then hold a longer
ruler with its ends. Put the same book in the middle of the
ruler again. You will notice that the middle of the ruler
sinks obviously. This shows that the beam’s span is
directly proportional to its vertical displacement: a longer
beam experiences a larger displacement as the stress it bears
is increased. How can we solve the problems of positional
shift and stress when we need to use a beam of long span?
In fact, the beam’s displacement and stress will decrease
if we increase the thickness of the beam.
Hong Kong is an international metropolis with
high buildings all around. However, in places of low population
densities, buildings are commonly of two to three levels.
According to the different purposes of the buildings, the
materials for constructing the buildings, their shapes, their
characteristic nature would be different. In Europe and America,
buildings are generally made of timber because it is cheap,
light, easy to link up and has a high availability. However,
the strength of timber is low and is easy to deform under
stress or pressure; so timber cannot be used in tall buildings.
If you really want a material with high strength, steel bars
may do. Steel reinforcing bar, a material of high strength
and high resistance to deformation, is suitable for constructing
tall buildings and long bridges. However, steel bar is expensive.
It also requires a heat and corrosion resistant protective
cover, thus it increases the cost for employing it as the
construction component.
(Building made of timber)
(Steel reinforcing bar)
Nowadays in Hong Kong, most buildings are made
of steel-reinforced concrete. What is the advantage of using
this material for construction? Concrete is a material which
can bear a very high pressure but is weak in bearing tension.
We mix up these two materials to form steel-reinforced concrete,
which offsets one's weakness. The concrete in steel-reinforced
concrete can bear pressure while the steel lines can bear
tension. In addition, it is cheaper than steel bars. It not
only can apply directly in the working place, but also can
be made into components for combination in advance. It is
convenient and has a good flexibility.
As mentioned above, a building structure has to bear both
the vertical force and the lateral force. The vertical force
acting on a building passes from the floor to the beam, then
to the pillar, and finally to the base. On the other hand,
the framework has to bear the lateral force at the same time.
A higher building suffers a greater lateral force, and therefore
a tall building has to face a much greater lateral force than
the vertical force. If a shear
wall is added to the structure of a high
building, the building will be able to bear a greater lateral
force.
(Bank of China Tower)
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(Hong Kong and Shanghai Bank Building)
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Buildings having forty to fifty levels (or more)
often require special designs. We are going to give some examples
of famous commercial buildings in Hong Kong (Refer to the
film). Looking at the structure of Hopewell Centre in Wanchai,
we may find that the distances between the pillars are reduced,
while the beams are thickened to increase the structural efficiency.
However, when the structural efficiency maximizes, like Jardine
House which looks like a box with many small holes on it,
it may affect the view as the window size has to be reduced.
Another frequently used design is the super frame. Most forces
acting on the building are delivered to large super frame
components. For example, Hong Kong and Shanghai Bank Building
and Bank of China Tower employ this design. In this kind of
design, most of the external forces are taken by the super-sized
structural components in the building structure.
(Hopewell Centre)
We have introduced a lot of information on
building in this article. Does it increase your knowledge
in buildings? Try to pay attention to the buildings around
you in daily lives and observe their structural features to
see whether it matches what you have learnt here.
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