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Structures and Forces

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Title: Structures and Forces


1
Structures and Forces
  • Unit D

http//www.pbs.org/wgbh/buildingbig/educator/act_i
ndex.html
2
Learning Outcomes
  • Recognize and classify structural forms and
    materials.
  • Interpret and evaluate variation in the design
    structures that share a common function.

3
Every object that provides support is a structure!
4
  • Structural Strength Refers to a structures
    capacity to hold itself up, as well as any weight
    added to it.

5
  • Structural Stability Refers to a structures
    ability to maintain its position even when it is
    acted on by a force.

6
  • Structures have definite size and shape, which
    serve a definite purpose or function
  • Every part of the structure must resist
    forces(stresses such as pushes or pulls)
  • Forces can potentially damage its shape or size.

7
3 basic structural forms
  • Solid/Mass Structure
  • Frame Structure
  • Shell Structure

8
(No Transcript)
9
Natural Structures
  • Not made by people
  • Occur naturally in the environment

10
Manufactured Structures
  • Built by people
  • Many are modeled after natural structures

11
Solid/Mass Structures
  • Can be made by, piling up or forming similar
    materials into a particular shape or design.
  • It is held in place by its own weight, losing
    small parts often has little effect on the
    overall strength of the structure

12
  • Formed from a solid piece (or solid combination
    of pieces) of some strong material.
  • Solid structure has little or no space inside and
    relies on its own mass to resist the forces that
    act on it.
  • Usually stronger than either frame or shell
    structures.

13
Natural Mass Structures
14
Manufactured Mass Structures

15
Is this a solid/mass structure?
16
A Layered Look
  • Mass structures are not always solid, but are
    layered and have hollowed out areas for specific
    functions

17
A Layered Look
  • Sandbag Wall Structures are mass structures that
    are layered. They prevent flooding and have 4
    key elements to avoid failure
  • Must be heavy enough to stay in place
  • Must not be too heavy to compact the earth
    unevenly below it
  • Must be thick enough so it cannot be pushed out
    of place
  • Must be anchored firmly

18
Frame Structures
  • Have skeleton of strong materials, which is
    filled and covered with other materials,
    supporting the overall structure
  • Most of the inside part of the structure is empty
    space.

19
Frame Structures
  • Load-Bearing Walls these are walls that support
    the load of the building

20
Frame Structures
  • Partition Walls these are the walls that divide
    up the space in a building

21
Frame Structures
  • Most common construction choice because
  • Easy to design
  • Easy to build
  • Inexpensive to manufacture

22
Frame Structures
  • Can be arranged in 2 dimensions (door/fence).
  • 3 dimensions (music stand/house).
  • Lighter than solid/mass structures
  • Use less material

23
Golf Ball Bridge
  • We will complete next week IF things go
  • well?

24
Shell Structures
  • Keep their shape and support loads, even without
    a frame, or solid mass material inside
  • Use a thin, carefully shaped, outer layer of
    material, to provide their strength and rigidity
  • Spreads forces throughout whole structure (every
    part of structure supports only a small part of
    the load)

25
Shell Structures
26
Shell Structures
  • Flexible Structuresare also shell structures but
    are more flexible rather than rigid

27
Shell Structures
  • They are completely empty so they make great
    containers
  • Their thin outside layer means they use very
    little materials

28
Shell Structures
  • Problems
  • A tiny weakness or imperfection on the covering
    can cause the whole structure to fail
  • When shell is created from hot or moist
    materials, uneven cooling can cause some parts to
    weaken other parts from pushing or pulling on
    nearby sections
  • Flat materials are hard to form into rounded
    shell shape
  • Assembly of flexible materials is very precise,
    so that seams are strong where the pieces are
    joined

29
Check Reflect
  • Page 268, s 1-4

30
Mix and Match
  • Football Helmets
  • Shell structure (to protect head)
  • Frame structure attached in front (to protect the
    face)

31
Mix and Match
  • Hydro-electric Dams
  • Mass structures with frame structure inside to
    house generators

32
Mix and Match
  • Airplanes
  • Frame structure
  • skin that acts like a shell (giving it added
    strength to resist stresses and making it light
    weight and flexible)

33
Mix and Match
  • Domed Buildings
  • Combines shell and frame construction

34
Mix and Match
  • Warehouses
  • Often built with columns to support the roof
    (frame) and concrete blocks (mass structure)
    which stay in place because of their weight

35
  • Topic 1 Review
  • Page 268, Check Reflect
  • s 1, 2, 3, 4

36
  • Name the 3 main structural forms?

37
  • Remember that structures do not always fit into
    one form and can be combination structures.
  • The 3 main structures are shell, frame, and
    solid (mass).

38
What is this example a combination of?
39
An umbrella is a combination of a shell and a
frame.
  • And this?

40
  • The balloon is an example of a shell, and the
    basket is an example of a shell and a frame!

41
How can you tell if a wall is a load bearing wall?
  • Link

42
  • For the 3 structural forms, provide examples from
    the natural environment, and the human-made
    environment.
  • Try not to use examples that have been mentioned
    in the text.

43
Structural Form Examples from Nature Examples that are Human-made
Solid (mass) Tree trunk Human Brain Hockey Puck Hockey Stick
Frame Spider Web Honeycomb Bus stop shelter Swing Set
Shell Snail Shell Turtle Shell Domed Roof Canoe
44
  • Large human-built solid structures are often made
    from brick, concrete, mud, or stone.
  • Why do you think these materials are used to
    construct solid structures?

45
Brick, concrete, mud, stone
  • Solid structures need to be made of materials
    that can support large loads.
  • The mass of these materials allows them to resist
    the forces acting on them more effectively than
    materials that have little mass.

46
  • Why is it an advantage that birds winds are
    hollow and not solid? Or a supporting skeleton
    outside the body?

47
  • The advantage of a birds wing bones, and the
    outer skeleton of invertebrates such as sea stars
    and lobsters, is that both types of structures
    provide strength without adding much weight.
  • Exoskeletons also provide outer protection and
    support.

48
Topic 2- Describing Structures
  • Most structures have several functions
  • Supporting (its own weight)
  • Containing (substances)
  • Transporting
  • Sheltering
  • Lifting
  • Fastening
  • Separating
  • Communicating
  • Breaking
  • Holding

49
Aesthetics
  • The study of beauty in nature
  • Best structural designs look good or are
    aesthetically pleasing
  • Aesthetics are accomplished by shape, texture,
    colour, type of material, and simplicity of the
    repeated pattern in a design.

50
Safety
  • All structures are designed and built within an
    acceptable margin of safety
  • Usually, structures are designed with a built-in
    large margin of safety
  • YouTube - Construction and Building Inspectors

51
Cost
  • Adding extra strength to a structure will
    increase the cost, as well as using more highly
    skilled workers and better materials.
  • Good design is a compromise between a reasonable
    margin of safety and reasonable cost
  • Totally unexpected events will cause even the
    best (well designed) structures fail (ie. World
    Trade Center Towers)
  • YouTube - Moore Releases Plan to Cut School
    Construction Costs
  • YouTube - Shipping Containers Recycled as Homes

52
Materials
  • Properties or characteristics of the materials
    must match the purpose of the structure
  • Example You dont want to build a boat out of
    paper!

53
Composite Materials
  • Made from more than one kind of material
  • Two types of forces act on a composite material
  • Tension (pulling)
  • Compression (pushing)
  • YouTube - Composite Materials

54
Layered Material
  • Layers of different materials are pressed and
    glued together, combining the properties of the
    different materials
  • Layers are called laminations
  • Examples of layered materials include car
    windshields, drywall, and plywood
  • YouTube - Bent Plywood Manufacturing

55
Woven or Knit Materials
  • Spinning or twisting, looping or knotting fibres
    together gives materials added strength.
  • Looms are used to weave two or more pieces of
    yarn together in a criss-cross pattern
  • Pressing, gluing, melting, and dissolving also
    combine materials to gain strength
  • YouTube - Weaving on Primitive Hand Loom

56
Choosing Materials
  • It is always important to weight the advantages
    and disadvantages of various materials
  • Factors to consider
  • Cost- with cheaper materials perform its function
    over a length of time?
  • Appearance
  • Environmental Impact
  • Energy efficiency

57
Joints Fastening Structures
  • Mobile Joints joints that allow movement
  • YouTube - Movement of the Joints

58
Rigid Joints
  • Rigid joints do not allow movement.

59
Rigid Joints
  • Fasteners
  • Ie. Nails, staples, bolts, screws, rivets, and
    dowels
  • Holes made in structure actually weaken the
    structure
  • One fastener allows movement when parts are
    pushed or pulled
  • More than one fastener will make a more rigid
    joint (but will also weaken it more)

60
Rigid Joints
  • Interlocking Shapes
  • Fit together because of their shape
  • Ie. Lego, dovetail joints, and dental fillings

61
Rigid Joints
  • Ties
  • Fasten things together
  • Ie. Thread, string, and rope

62
Rigid Joints
  • Adhesives
  • Sticky substances hold things together
  • Ie. Hot glue, drying glue, and epoxy resin

63
Rigid Joints
  • Melting
  • Pieces of metal or plastic can be melted together
  • Ie. Welding, soldering, and brazing

64
  • Topic 2 Review
  • Page 321
  • Check Reflect
  • 1, 2 3

65
What would happen if ligament material in the
human body were replaced by bone material?
66
  • If ligament material were replaced by bone
    material, the flexibility needed to allow for
    motion would be seriously limited/hampered as the
    bone material is hard (calcified).

67
Most sports injuries involve damage to joints
such as ankles, knees, and wrists. Why do you
think this is so?
68
  • Many sports injuries are the result of ligaments
    being overextended. The rapid movement required
    for these activities results in the tissue being
    damaged.

69
True or False? The different layers of materials
found in plants are needed only to make the
plants strong. Explain your answer.
70
  • False! The layers of materials found in plants
    can also be used to transport water and nutrients.

71
Topic 3
  • Mass versus Weight
  • How are weight and mass different?
  • To understand the differences we need to compare
    a few points
  • 1) Mass is a measurement of the amount of matter
    something contains, while Weight is the
    measurement of the pull of gravity on an object.

72
2) Mass is measured by using a balance comparing
a known amount of matter to an unknown amount of
matter. Weight is measured on a scale.
73
3) The Mass of an object doesn't change when an
object's location changes. Weight, on the other
hand does change with location.Video
74
(No Transcript)
75
Mass is measured in kgWeight in measured in
newtons (N)
  • 1 kg 10N
  • (actually its 9.8N, but 10 is an easier number
    to estimate)

76
Force and Weight
  • An elephant has a mass of 5000 kg.
  • On the moon would the mass of an elephant change?
  • What does change?
  • When does the mass change?

77
Types of Forces
  • Fg force of gravity (determine Fg mg)
  • Fa Force applied (Force pushing or pulling the
    object)
  • FN Force Normal
  • Ff force friction (resistance on object)

78
Force Diagram
  • Force Normal
  • Force Friction Force Applied
  • Force Gravity

79
Describing Force
  • To describe a force you need direction and its
    size.
  • You lift a 50 N box upward.
  • A 15 N book falls down on your foot.

80
Force Diagrams
  • If there is a force being strongly applied to an
    object then the arrow will be longer.
  • Draw a force diagram for the following

81
  • Pushing a box
  • Pull up
  • Tug of war
  • An elevator at rest
  • An elevator going to the top floor
  • An elevator going to the main floor

82
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83
(No Transcript)
84
(No Transcript)
85
(No Transcript)
86
(No Transcript)
87
Topic 4 Forces, Loads, and Stresses
88
Tension the pulling force
  • It stretches materials.
  • Examples rope bridges, telephone wires, tents,
    suspension bridges, inflated stadium domes, steel
    cables supporting a full elevator, and hair when
    someone yanks on it

89
Compression the pushing force
  • It squashes materials
  • Examples pyramids, telephone poles, arch
    bridges, elephant legs, tree trunks, and your
    little brother when you sit on him

90
Tension and Compression
  • When a load is placed on a beam, as above, the
    top half of the beam shortens in compression. The
    bottom half lengthens in tension.

91
Torsion
  • The twisting of an object

92
Shear Force
  • Bend or tear a material by pressing different
    parts in opposite directions at the same time
  • Example scissors

93
Dead or Alive?
94
Dead Loads
  • Dead loads do not move.
  • The structure always has to support them. They
    are, well... "dead".
  • Dead loads walls, beams, arches, floors,
    ceilings.

95
Live Loads
  • Live loads are the things a structure supports
    through regular use.
  • Like "live" things, these loads can change and
    move.
  • Live loads snow, rain, people, cars, furniture,
    wind.

96
  • Forces of Wonder
  • http//www.wonderville.ca/v1/home.html
  • Compression and Tension of Arches (6 min)
  • http//www.youtube.com/watch?vUYtIFM1ek_M
  • Geodesic Domes (18 min)
  • Video Link

97
Topic 5 How Structures Fail
  • Failure can occur if the force is too strong for
    the structures design or if the force is acting
    on a vulnerable part of the structure.

98
Levers
  • A device that can change the amount of force
    needed to move an object. When a force is
    applied to the effort arm, a large force, which
    can move the structure. This can be intentional-
    like when a crowbar is used to move a heavy rock,
    or it can be unintentional- like when a gust of
    wind knocks down a flagpole.

99
Material Failure
  • Shear- minor weaknesses in a material can cause
    failure because the particles move farther apart
    and are less attracted to each other. This can
    be cause by compression.
  • YouTube - House Payload

100
Material Failure
  • Bend or buckle- compression can also cause a
    material to bend and buckle- like a pop can that
    is stepped on. To prevent this, reinforcements
    stringers and ribs- are used to strengthen the
    structure.
  • YouTube - Boomer Bridges Are Falling Down i-35
    Bridge Collapse

101
Material Failure
  • Torsion- twisting can cause material failure.
    When sections of the structure slide past each
    other the structure and crack or break in two.
    When the twisting action makes the structure
    unusable (not broken) is has failed because it
    has lost its shape.
  • YouTube - Tacoma Bridge Disaster

102
Making Use of Stresses
  • Buckle Car bumpers are designed to buckle in a
    collision as the metal fails, it absorbs some
    of the energy of the impact, which protects the
    occupants of the vehicle.
  • YouTube - Lexus bumper

103
Making Uses of Stresses
  • Shear Shear pins are used in outboard motors to
    prevent failure of the motor (when the propeller
    gets tangled in weeds), a shear pin breaks and
    the propeller becomes disengaged with the motor
    and gears.

http//www.youtube.com/watch?vd-TJw52PGYs
104
Making Use of Stresses
  • Twist Spinning wheels twist cotton or wool
    fibres so they lock together making them strong
    enough to make cloth.
  • Controlled twisting can also be useful in hair
    braids, ropes and telecommunication cables.

http//www.youtube.com/watch?vyrrJLAXwUBU http/
/www.youtube.com/watch?vuejRaFfrOZgfeaturerelat
ed http//www.youtube.com/watch?vkKXca2l0RPofeat
urerelated
105
Metal Fatigue
  • Metal fatigue is the phenomenon leading to
    fracture under repeated or fluctuating stress.
    Fatigue fractures are progressive beginning as
    minute cracks and grow under continued stress.
    The particles in the metal move further apart and
    have less attraction to each other. When a crack
    develops it weakens the metal and can eventually
    fail even if a small force is applied.

106
Topic 6 Designing with Forces
  • 3 key methods to help structures withstand forces
    are
  • Distribute the load
  • Direct the forces along angled components
  • Shape the parts to withstand the specific type of
    force acting on them.
  • Build A Tipi

107
Flying Buttresses
  • Columns on the outside of a structure that
    connect to the building near the top and are used
    to support the outer walls in much the same way
    that two sides of an arch support each other
  • http//www.pbs.org/wgbh/buildingbig/educator/act_m
    ini_arch.html

108
Strengthening Structures
  • All materials have limitations.
  • Materials can be strengthened or weakened as they
    are made.
  • Ex. Concrete can be very strong, but if the
    proportions are incorrect, the resulting concrete
    can crumble and fail, however it does not have
    very good shear or torsion strength

109
Strengthening Structures
  • Shear forces can be fatal in metal if the shear
    strength is not analyzed when the metal is
    manufactured.
  • The cooling process can eliminate almost all
    defects if it is done properly.

110
Using Frictional Forces
  • Force of friction resists movement between two
    surfaces that rub together.
  • A brick wall is held together and kept evenly
    spaced with mortar, which helps to create large
    friction forces between each brick.

111
Using Frictional Forces
  • Friction is also important in frame structures.
  • The friction between the nail and the wood keeps
    the nail in place and the joints solid.
  • Different types of nails provide differing
    amounts of friction.
  • Windmill activity

112
Topic 7 Stable Structures
113
  • The collapse of a structure can occur when the
    external forces cause the structure to become
    unbalanced.
  • To design stable structures, engineers need to
    know what features of a leaning object determine
    whether it will tip over or stay balanced.

114
Centre of Gravity
  • The specific point where all of the mass of the
    structure is evenly distributed around.

115
  • The force of gravity acts on all parts of the
    structure and if all parts are evenly distributed
    around the center of gravity, then the structure
    will be stable.

116
Unbalanced Structures
  • To determine whether a structure is unbalanced
    locate the centre of gravity and draw a line
    directly down toward Earth.
  • If the arrow points to a solid foundation, the
    structure is balanced.
  • If the arrow falls beside the foundation the
    structure is not!
  • Check for yourself! Page 333

117
Firm Foundation
  • Foundation upon which the structure is built must
    be STABLE. Especially if it is moist because
    compressive forces may cause the structure to
    tip.
  • Take into account soil types and formations to
    avoid cracks in foundation and walls!

118
Find Something Solid
  • Foundations can be constructed on solid bedrock
    or pilings (large metal, concrete or wood
    cylinders)
  • Pilings are used when the soil is loose and too
    deep.
  • Some lightweight structures do not have to rest
    on the bedrock or have to have a foundation that
    goes down very deep, because the ground doesnt
    freeze.

119
Make A Solid Layer
  • Road builders always pack loose surface soil
    before paving to create a solid base for the
    asphalt or concrete.
  • Easy to fill in cracks because they can replace
    the soil with a solid packed layer of gravel.

120
Spread the Load
  • Spreading the weight of a structure over a large
    area allows the ground to support only a small
    amount of weight.
  • (Lying on a bed of nails, crowd surfing)
  • This is why building are often constructed on
    many shallow piling rather than a few.
  • Footings are used under a foundation wall to
    disperse the weight of the wall.
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