5
Structures
114
5.
Structural conditions
In order to get the students thinking about the conditions which a
structuremust meet, we can get them to suggest which conditions
they think are necessary. We can use this opening question as a
warm-up activity.
Bear inmind that this section introduces three extremely important
concepts for the design of structures: stability, resistance and
rigidity. It is a good idea to emphasise that they do not necessarily
act together, but are independent of each other and all are vitally
important.
Usually, the only condition which links these structures instinctively
is resistance. Extreme examples can be given: a bendy bridge that
is unbreakable, a fish tank that withstands the load exerted by the
pressure of the water very well without becoming at all deformed,
but which breaks at the slightest knock, etc.
In addition to the three basic requirements which every structure
must meet, we can also mention the importance of the economic
viability of the project and its aesthetic appearance.
It is worth spending some time explaining what the centre of
gravity of a body is. For this purpose students can use a book,
biro or other object which is unstable in a vertical position. The
reason why a body keeps it balance is because its centre of gravity
is vertically aligned inside the bearing surface. The larger the base
which supports it, the more stability it has because it can move its
centre of gravity without leaving the base. When the body leaves
the base, it becomes unstable and falls over.
Before seeing how we can make sure that a structure meets the
three conditions, we need to ask a new question to make the
students think about their designs in terms of these three aspects:
stability, resistance and rigidity.
5.1.
How can we make a structure stable,
resistant, and rigid?
❚
Stability
: If we look at the four pictures, we can see the
shapes which fulfill this first condition: increase the size of
the base, embed the lower part in the ground, centre the
weight of the object in its lower part and make the structure
tauter.
❚
Resistance
: This can be explained as the relationship between
the material which the structure is made of, the quantity which
it uses and its adaptation to the stress which it must bear with
its resistance.
It is also a good idea to introduce the students to the concept
of breaking strain and to show it in a graph that shows the
relationship between stress and deformation.
❚
Rigidity
: to make structures rigid we must consider the shape
of the parts, the joints and the triangulation.
Shape of the parts:
We can make use of the pictures of
different structural designs to explain which are the most
suitable for each type of stress.
Joints:
The joints between bars, whether they are made of
metal or wood, are never completely rigid. In structural terms,
they are regarded as joints and it is necessary to use shapes
which are not deformed by stress despite the possibility of the
joint turning. The exercise with the ice cream sticks and paper
fasteners can serve as such a good example of this.
Triangulation:
The triangle does not guarantee that the
structure will keep its shape (in fact, the bars lengthen or
shorten depending on different stresses). It is its ability to
change its shape freely that is important. We can see this in the
triangular brackets that are used to reinforce the metal joints
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5. Structures
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5.
STRUCTURAL CONDITIONS
There are three conditions for a structure to work well:
❚
Stability
: the capacity of a structure to remain upright and not fall over. A
structure’s
centre of gravity
must be centred over its base and close to the
ground for it to be more stable.
❚
Resistance
: the capacity of a structure to bear the tensions that it’s subjected
to without breaking. A structure’s resistance depends on its shape and the
material used to build it.
❚
Rigidity
: all bodies deform slightly when a force is applied to them, but this
deformation must not prevent the structure from
fulfilling
25
its function.
Understand
25.
Look at the pictures? Which
structure do you think is more
stable?Whichone is less stable?
Why?
26.
Which of the structures is the
oldest one in history? Why?
Analyse
27.
Look at the photo.
Where is the tower’s
centre of gravity? Give
reasons foryouranswer.
The centreofmassofanobject is thepoint aroundwhich theweight is evenlydistributed
The communications tower in
Montjuïc
.This leaning tower is
designed so that theweightof the
toppart restson thebase
Resistance
❚
Resistance depends on the material used to build the structure, the quantity of
material used and the shape of the structure.
❚
All materials have a
tensile strength
. This is the maximum force per unit of
surface area that they can bear without breaking. Examples of the strongest
materials are steel, stone, cement, wood and plastic.
1.
Widen
itsbase.
3.
Centre
theweight in
the lowestpartand/or
add extraweight.
4.
Brace
27
it so itdoesn’t
fallover (withbraceswe
widen the radiusof the
base).
2.
Bury
26
thebottom section in the
groundanduse strong foundations.
Rigidity
For structures to have
rigidity
and to avoid deforming, we need to consider
three basic aspects: shape, joints and triangulation.
❚
We choose a
shape
on the basis of the stress it bears.
◗
Compression
: we use
thick elements
, often hollow, to avoid
buckling
28
.
◗
Traction
: we can make the section bigger and
tighten
29
the elements that
traction is acting on.
◗
Bending
: we make the
breadth
30
bigger and use more material on the top
and bottom of the elements.
Apply
30.
Imagine a structure made from four ice-cream sticks, joined with
paper fasteners or screws.
a)
What would happen if you pressed on two of the corners?
b)
Ifwe reinforce the square structurewithadiagonal stick,would it still
be possible to deform it by pressing on those two opposite corners?
c)
Could we reinforce the structure with thread or wire? How many
pieces would be necessary?
Analyse
31.
Look at the photo of the model
bridge. How does it solve the
problems of stability, resistance and
rigidity?
Evaluate
32.
Now that you have studied all the different aspects of a structure,
check your design for your bridge. Draw a sketch on a scale of 1:2 or
1:3 of your model.
Hollowbeams canbear compressionwell
I andTbeams arebest for resistingbending
Toavoid
buckling
wehave to
make sure the load-bearing
elementsarenot too thin.
Understand
28.
Why
is
buckling
dangerous
in
a
structure?
Understand
29.
Look at the cables that
cross each other in the
photo. What do you
think they contribute
to the structure?
❚
To make
rigid joints
between the
parts of a concrete structure, we use
steel
rebar frameworks
. In metal
structures, we use
corbelling
. This
makes the welding area and the joint
bigger.
❚
To prevent deforming, we can use
triangular structures
or add
cross-
bracing
using crossed cables.
❚
For a structure to serve its
purpose, it must be stable,
resistant and rigid.
Key concept
25
fulfil
:doorachieve something that is
expected
26
bury
:put into theground
27
brace
: make stronger with cables,
woodormetal
5.1.
How can we make a structure stable, resistant
and rigid?
Stability
To give a structure stability we can:
28
buckling
: bend and break under
pressure
29
tighten
: make tighter or less easy to
move
30
breadth
:widthordistance fromone
side to another
Metal structure
corbelling
