A presentation for our 3rd ESO technologies students

1. Unit 4
DEPARTMENT OF TECHNOLOGIES 3rd ESO

2. INDEX
1. Forces and stresses
2. Structures and loads
3. Types of structures
4. Elements of structures
5. Structural conditions

4. What is a force?
In nature we can find different forces

5. What is a force?
A force is an influence that can deform a
body or change its state of movement or
rest.
 When the force deforms the body we
say it has a static effect
 When the force moves the body we
say it has a dynamic effect

6. How can we define a force?
We have to specify its intensity, direction
and the point from which it is acting.

7. What is an stress?
Try to understand: What happens when
you apply different forces to your fingers?
When we apply one or more external
forces to a body an internal tension
appears. That tension is called stress.

8. Types of stresses
TRACTIONCOMPRESSION
SHEARTORSIONBENDING

9. Compression
Pillars and arches
support vertical loads,
so they tend to shrink.
They are subjected to
compression.
A pushing force tries to
squash or shorten the
object.
Opposing forces move
inwards towards one
another along the same
trajectory and opposite
directions.

10. Traction
Tie rods in suspended
structures are
subjected to traction.
A pulling force tries to
stretch or lengthen the
object.
Opposing forces move
outwards away from the
body along the same
trajectory but in opposite
directions.
or Tension

11. Bending
Both the central beam in
a bridge and the pole
support loads that tend
to bend them, so they
are subjected to
bending.
A flexing force tries to
deform the object.
Normally, there are two
forces that are separated
by a certain distance and
a third force acting in the
opposite direction.

12. Torsion
The axles of two
friction wheels are
subjected to
torsion when the
wheels are turning.
In this strange
tower, each cable
pulls the pillars in
one direction. The
pillars will thus be
subjected to
torsion.
A force tries to twist or
turn the object.
The forces try to turn the
ends of the body in
opposite directions.

13. Shear
In cutting tools, each
blade exerts a force in
opposite directions.
The paper is subjected
to shear.
A screw on the wall supports
the load hanging from it an the
vertical reaction of the wall in a
small pieces of material. It’s
subjected to shear.
or Cutting
A sliding force tries to cut
through the object.
The forces are very close
together pulling in
opposite directions, like
two scissor blades.

14. Types of stresses
TRACTIONCOMPRESSION
SHEARTORSIONBENDING

15. Think to analyse…
These are the main elements in a
suspension bridge. What efforts affect to
each part?
Board / Beam
Tie rods
Tower
Pillar

17. See, think and wonder

18. What is a structure?
But, what is exactly a structure?
What are structures for?
What would happen if you did not have a
skeleton?
Both living beings and the great number
of objects that have been designed and
created by human beings can be
understood as structures.

19. What is a structure?
A structure is a set of elements in a body that
are intended to resist the effects of the forces
that act on it.
A structure prevents a body from breaking or
becoming too deformed.

20. Types of structures
Natural Manufactured

21. What is a load?
Natural and artificial structures are created to
support loads. Loads are forces that can
deform a body or make it move.
Types of loads:
 Fixed Loads: They always have the same
value.
 Variable loads: Those that can change as
time passes or due to natural disasters.

22. Think to analyse…
What are the loads that the human body
can be subjected to?

23. Think to analyse…
Locate and classify the following loads
acting on a building:
 Weight of walls, floors
and roofs
 Weight of people and
furniture
 Weight of the snow
 Earth pressure
 Weight of gardens
 Earthquake loads

24. Functions of the structures
Supporting loads
Resisting external
forces
Providing shape
Protecting the
inside
Do you think one structure can do more than one
function at a time? Think of an example.

25. Functions of the structures
Supporting their own
weight and the weight of
elements laying on them.
Example: the pillars (columns) hold the
weight of the beams. The beams
support the weight of floor slabs.
Resisting external
forces like the wind
pressure, the vibration
caused by traffic, the
impact of waves…
Example: the walls of a dam that holds
water.

26. Providing shape to an
object, machine or
construction.
Examples: the metal frame of a tent
gives shape to a textile. The
fuselage of an airplane wing.
Protecting the internal
elements of an object,
building or machine.
Example: the
outer case of a
vacuum cleaner
protects the
internal pieces
and circuits.
Functions of the structures

28. Types of structures
Structures of machines and
objects:
- Laminar or case structures
- Frame structure
--E
Construction structures:
- Massive - Lattice
- Triangulated - Suspended
- Pneumatic - Shell

29. Structures of machines and objects
Laminar or case
structures: formed by
thin resistant sheets.
Examples: video game controller
case, car’s bodywork, hull of a
boat…
Frame structures:
formed by bars, tubes or
strips united to form an
skeleton.
Examples: a bicycle frame, a ladder, an
umbrella, a chair…

30. Massive structures
Massive structures are built by stacking rock or other
materials, or by carving. They are solid heavy and
very sturdy.
The use of wooden or Stone lintels allow to create
windows or open spaces.
Examples: walls of dams, pyramids in Egypt, ancient temples…

31. Vaulted structures
Vaulted structures are built with arches, vaults and domes.
The arch is a self-supporting element that can resist
compression without mortar or cement.
A vault is a series of arches placed side by side with the
spaces between them filled in.
Examples: coliseums, pantheons, cathedrals…

32. Lattice structures
Lattice structures are built by steel or reinforced
concrete bars joined to form a rigid grid.
Each part of the structure serves a different purpose.
Examples: housing buildings Prevents the
pillars from sinking
into the ground

33. Triangulated structures
Triangulated structures are compose of metal or wood
bars that form triangles. They can cover large spans.
They are very light, versatile and strong.
When used horizontally to cover large spans can be
divided into trusses and triangulated beams.
Examples: electricity towers, bridges, roof trusses…

34. Suspended structures
Suspended structures are formed by cables called tie
rods, connected to supports from which the structure
hangs.
Their light weight allows to cover large spans.
Examples: suspensión bridges, stadiums, tents…

35. Shell structures
Shell structures are built by a three-dimensional
curved plate whose thickness is small compared to
the rest of the dimensions.
They are very resistant because they can bend.
Bends and folds are use to strengthen them.
Examples: roofs of large buildings…

36. Pneumatic structures
Pneumatic structures use compressed air inside
them, which, when they expand, stretch the plastic
outer surface to create large enclosed spaces.
They are light and can easily be dismantled
Examples: funfair attractions, field hospitals…

38. Basic elements
Foundations
Massive elements
placed underground
that support
compression efforts.
Tie beam
Thin bars that
support tension or
compression.
Walls
Massive elements
that support
compression efforts.
Beams
Horizontal elements that
support bending in the
center and shear in the
extremities.
Pillars
Slim elements that
support
compression efforts.

39. Basic elements
Walls
Massive elements
that support
compression efforts.
Arches
Curve elements that
transfer the loads
the hold to the
supports on their
extremities.
They are subjected
to compression.

40. Basic elements
Beams
Horizontal elements that
support bending in the
center and shear in the
extremities.
Foundations
Massive elements
placed underground
that support
compression efforts
Tie rods
Cables can only bear
traction stresses, but
they can adapt their
shape to different loads.
Towers
Massive elements
that support
compression efforts.

42. Structural conditions
For a structure to work well it needs to meet
three conditions:
 Stability: The capacity of a structure to
remain upright and not tip over
 Resistance: The capacity of a structure to
bear the tension that is subjected to without
breaking it
 Rigidity: The capacity of a structure to keep
its form when a force is applied to it

43. Center of gravity
The stability of an object depends on the
position of its center of gravity. The center of
gravity is the point where the weight of an
object is considered to be concentrated.
An object is stable if its center of gravity is on
top of its base.
Stable
object
Unstable
object

44. How to achieve stability?
 Adding mass to its base
 Using anchoring systems
 Burying its bottom section underground

45. How to achieve resistance?
 Adding more material
 Choosing the best materials to support them,
taking into account the efforts that will act on
the structure.
 Choosing a proper shape, taking into
account the efforts that will act on the
structure.

46. Choosing materials
WOOD
STEEL
BENDING
TRACTION
MINERALS COMPRESSION
resist
they combine
and form
COMPRESSION
BENDING
TRACTION
REINFORCED
CONCRETE
resists
WOOD
PLASTICS
ALUMINUM
resist
SMALLER
EFFORT
than steel, minerals,
concrete and wood
resist

47. Choosing shapes
With the objective of resisting different types
and intensities of efforts, steel is industrially
shaped into different types of profiles.

48. How to achieve rigidity?
 Using welding joints
 Triangulating the structure: Diagonal
supports are used for triangulation
 Making sure the structure has a shape that
is not too thin