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