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Unit 5. Structures
What are we going to learn in this unit? 5.1 What is a structure?
5.2 Examples of examples
5.3 Stresses
5.4 Structural elements
5.5 Resistant structures
5.6 Stable structures
Final activities
5.1 What is a structure? A structure is a set of elements that are
intended to resist the effects of the forces (loads) that act on it. A structure prevents a body from breaking or becoming too deformed.
5.1 What is a structure?
Structures throughout history Over time new materials have changed how we construct things.
5.1 What is a structure?
The first structures were made with wood, then they also used stone. Nowadays we use concrete and iron
House made of stone
Sydney Opera House
Manmade structures
Skyscraper under construction (Madrid)
Manmade structures
Electricity pylon
Manmade structures
Tower crane
Manmade structures
Clifton Suspension Bridge (Bristol)
Forces Loads Stresses o A Force is something that can deform, a
body (static effect) or change its state of motion (dynamic effect)
o The forces that act on a structure are called loads o Fixed or permanent loads Don’t vary over
time
o Variable loads are occasional and changeable
o Loads act on the bodies causing internal tensions. These tension are called stresses
Fixed Loads: weight
Variable Loads External forces - weather conditions, for example the wind…
Forces Loads Stresses
STRESS
TENSION COMPRESSION
BENDING
TORSION
SHEAR
You have a small rope like this one and you hold it by the ends. Try to stretch it by applying opposite forces. What do you think it will happen?
Traction is an effort that makes an object longer
Tension is the stress which tends to pull things apart (try to stretch the object they
are acting upon)
TOWER
CRANE
To counteract this force the structure of the crane must apply an equal and opposite force.
The weight of the load has the tendency to pull it downwards
Suspension bridges use thick steel cables to hold the bridge platform up so that cars can drive over it"
Golden Gate (San Francisco, USA)
Alamillo Bridge (Seville, Spain)
Gateshead’s Millennium Bridge (New Castle, England)
Tension oElements that are being stretched are said to be in tension
oTension: Stress which tends to push materials apart.
If you hold a bar of clay with both hands and apply oppossing forces like in the drawing. What do you think will happen?
Compression: Stress that tends to push materials together. Forces try to flatten or compress a body.
The greater the value of the forces the
shorter the object gets.
10
Columns, Pillars and load-bearing walls are subject to compression stress
They have to hold the weight of the part of the building above them.
Compression stress is the stress that tries to push materials together.
Materials must be chosen to resist these forces in order to make a successful structure
A. Parthenon (Athens)
B. Tent
C. Chair
D. Base
COMPRESSION
TENSION
Bending
A body is subject to a bending force when it receives one or more forces that have the tendency to bend it.
When a force applied in the centre of the
beam increases to a higher value the beam tends to bend.
Bending in Beam Bridges
A beam bridge needs to be stiff. It needs
to resist twisting and bending under load.
Wings of a plane
Shear Stress
The forces act like two scissors blades, moving in opposite directions, as they try to cut an object.
When you twist a towel with your hands after having a bath, the clothing is under torsion stress. As you can see the forces exerted are clockwise and counter clockwise.
Torsion Stress: twists objects
5.2 Stresses Which stresses are applied in the following
examples?
A B
C
D
E
F
5.2Stresses • Which stresses are applied in the following examples?
A B
C
Image Structure Effort
A Cable Traction
B Motorbike Compression
C Guide rope Traction
5.2 Stresses
D
E
Image Structure Effort
D Column Compression
E Bridge Flexion
• Which stresses are applied in the following
examples?
5.2 stresses
Image Structure Effort
F Nut Torsion
G Rivet
Shear
F G
•Which stresses are applied in the following examples?
5.2 Stresses • Which stresses are applied in the
following examples?
G
Image Structure Effort
A Bar/spring
B Bar/spring
C Spring
D Spring
E Bar/spring
F Bar/spring
G Bar
5.2 Stresses • Which efforts are applied in the
following examples?
G
Image Structure Effort
A Bar/spring compression
B Bar/spring compression
C Spring Traction
D Spring Traction
E Bar/spring compression
F Bar/spring compression
G Bar Flexion
Exercise pag 107 Nº 6
COMPRESSION
TENSION
BENDING
TORSION
SHEAR
Structural conditions Stable: Capacity to remain upright and not tip over
Resistant: Capacity to bear the tensions that it’s subjected to without breaking
Rigid: the deformation of an object must not prevent the object from fulfilling its function
Structures
must be resistant and
stable…
5.4 Resistant structures As we know a structure has to be resistant
A resistant structure keeps its shape
when forces are applied.
5.4 Stable structures The structures have to be stable.
A structure is stable when it keeps its position when forces are applied.
How can we make a structure stable?
Adding mass to its base
Bracing it
Burying its bottom section underground
Lowering its center of mass
The center of mass is where all the weight is concentrated
He has the mass centre evenly distributed
How can we make a structure stable?
How can we make a structure stable?
The c.d.g. (center of gravity) should fall perpendicularly on the base of support. Otherwise the structure will fall over
How is the centre of gravity maintained in the following situations?
Lowering the gravity point.
Increasing support
Placing triangles
Increasing the weight at the base.
Stretching his legs
How can we make a structure stable?
How can we make a structure resistant?
Resistance depends on:
The material used
The quantity of material used
The shape of the structures
The triangle is the only geometric shape that cannot be deformed by applying forces to any angle.
How can we make a structure rigid?
TRIANGULATION • Basic shape for creating
large rigid structures
5.4 Resistant structures
Triangulation makes it possible to transform shapes into a combination of triangles and therefore makes them non-deformable.
5.4 Resistant structures
Add bars to these structures to form triangles and make them non-deformable
5.4 Resistant Structures
Add bars to these structures to form triangles and make them non-deformable
5.4 Resistant structures
The use of triangular structures is very interesting because they are very resistant, light and easy to construct.
crane pylon
5.3 Structural elements
These are the most common elements that can be found in a structure
• Foundations • Columns • Beams • Arches • Ties (cables)
Foundations
Foundations are the base of any building and they are situated under the ground and made of concrete
concrete
Columns of Pillars • Vertical structural element.
• Designed to resist compression stresses.
Support beams
or arches
Decorative elements
BEAM • Horizontal structural element
• Designed to resist bending
• Support gravitational forces
TIE
• Long structural element (ropes, chains, cables).
• Designed to resist only tension
ARCH
• Curved structural element.
• Designed to transmit stresses to the pillars.
Keystone
Arch: Provides resistance to structures by distributing the effort of the structure.
5.6 Types of artificial structures
Massive: a great concentration of material predominates
5.6 Types of artificial structures
Vaulted : Arches, vaults and domes are important holding and supporting elements
Arches vaults domes
5.6 Types of artificial structures
Lattice: made up by timber wood, steel or concrete intersecting elements
The structural
elements are the girders, pillars or columns, and foundations.
Foundation
Column
Girder
Forces
5.6 Types of artificial structures
Triangulated: Made up by linking many triangles forming flat or spatial networks.
crane Oil rig pylon
5.6 Types of artificial structures
Suspended: They are held by cables
5. Revision activities effort
A door knob
An arch
A pen tip when you write
A screwdriver when screwing
the cable of a suspended bridge
A lintel
Scissors
the cord of a blind
Pedal axes
Girders on a bridge
5. Revision activities effort
A door knob TORSION
An arch COMPRESSION
A pen tip when you write COMPRESSION
A screwdriver when screwing TORSION
the cable of a suspended bridge TRACTION
A lintel FLEXION
Scissors SHEAR
the cord of a blind TRACTION
Pedal axes TORSION
Girders on a bridge BENDING
5. Revision activities
Name the effort in each of the following objects
A C B
C D E
5. Revision activities
Which of the following structures will support efforts best? Why?
5. Revision activities
Which structure is more stable?
5. Revision activities
Which of the following shapes is non-deformable? What can we do to make the other shapes non-deformable?
5. Revision activities
Which of the following shapes is non-deformable? What can we do to make the other shapes non-deformable?
5. Revision activities
Indicate the stresses that the following parts of the crane are subjected to.
Tirantes
Base
Working arm
mast
cables
Base
cables
cables
cables
cables
cables
cables
cables
Working arm
cables
cables
Working arm
cables
cables
Base
Working arm
cables
cables
Base
Working arm
cables
cables
Base
Working arm
cables
cables
Base
Working arm
cables
weight
5. Revision activities
Working arm cables Base mast Weight
Effort FLEXION TRACTION COMPRES
SION COMPRES
SION TRACTION
mast
Base
Working arm
cables
weight
5. Revision activities
Draw this bridge and indicate where the main efforts are.
5. Revision activities
Draw this bridge and indicate where the main efforts are.
Compresion Compresion
Flexion
Flexion
5. Revision activities
Draw this bridge and indicate where the main stresses are.
5. Revision activities
Draw this bridge and indicate where the main efforts are.
Compression
Traction Traction