Structures and Forces - libvolume3.xyzlibvolume3.xyz/civil/btech/semester6/geotechnical...a frame,...
Transcript of Structures and Forces - libvolume3.xyzlibvolume3.xyz/civil/btech/semester6/geotechnical...a frame,...
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Structures and Forces
Unit 4
http://www.pbs.org/wgbh/buildingbig/educator/act_index.html
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• 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.
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Natural Structures
• Not made by people
• Occur naturally in the environment
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Manufactured Structures
• Built by people
• Many are modeled after natural structures
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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
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Natural Mass Structures
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Manufactured Mass Structures
•
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A Layered Look
• Mass structures are not always solid, but are
layered and have hollowed out areas for
specific functions
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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
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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.
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Frame Structures
• Load-Bearing Walls: these are walls that
support the load of the building
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Frame Structures
• Partition Walls: these are the walls that
divide up the space in a building
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Frame Structures
• Most common construction choice because:
– Easy to design
– Easy to build
– Inexpensive to manufacture
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Golf Ball Bridge
• We will complete next week IF things go
well☺
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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)
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Shell Structures
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Shell Structures
• Flexible Structures:are also shell structures
but are more flexible rather than rigid
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Shell Structures
• They are completely empty so they make
great containers
• Their thin outside layer means they use very
little materials
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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
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Mix and Match
• Football Helmets
– Shell structure (to protect head)
– Frame structure attached in front (to protect the
face)
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Mix and Match
• Hydro-electric Dams
– Mass structures with frame structure inside to
house generators
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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)
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Mix and Match
• Domed Buildings
– Combines shell and frame construction
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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
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•Topic 1 Review
•Page 268, Check &
Reflect
#s 1, 2, 3, & 4
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• Name the 3 main structural forms?
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• 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).
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What is this example a
combination of?
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An umbrella is a combination of
a shell and a frame.
• And this?
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• The balloon is an example of a shell, and
the basket is an example of a shell and a
frame!
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How can you tell if a wall is a
load bearing wall?
• Link
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• 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.
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Structural Form Examples from
Nature
Examples that are
Human-made
Solid (mass) 1. Tree trunk
2. Human Brain
1. Hockey Puck
2. Hockey Stick
Frame 1. Spider Web
2. Honeycomb
1. Bus stop shelter
2. Swing Set
Shell 1. Snail Shell
2. Turtle Shell
1. Domed Roof
2. Canoe
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• 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?
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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.
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• Why is it an advantage that bird’s winds are
hollow and not solid? Or a supporting
skeleton outside the body?
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• The advantage of a bird’s 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.
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Topic 2- Describing Structures
• Most structures have several functions:
– Supporting (its own weight)
– Containing (substances)
– Transporting
– Sheltering
– Lifting
– Fastening
– Separating
– Communicating
– Breaking
– Holding
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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.
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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
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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
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Materials
• Properties or characteristics of the materials
must match the purpose of the structure
– Example: You don’t want to build a boat out of
paper!
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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
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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
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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
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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
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Joints: Fastening Structures
• Mobile Joints: joints that allow movement
• YouTube - Movement of the Joints
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Rigid Joints
• Rigid joints do not allow movement.
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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)
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Rigid Joints
• Interlocking Shapes
– Fit together because of their shape
– Ie. Lego, dovetail joints, and dental fillings
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Rigid Joints
• Ties:
– Fasten things together
– Ie. Thread, string, and rope
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Rigid Joints
• Adhesives:
– Sticky substances hold things together
– Ie. Hot glue, drying glue, and epoxy resin
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Rigid Joints
• Melting:
– Pieces of metal or plastic can be melted
together
– Ie. Welding, soldering, and brazing
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•Topic 2 Review
Page 296 in
Science Focus 7
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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.
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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.
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3) The Mass of an object doesn't
change when an object's location
changes. Weight, on the other
hand does change with location.
http://www.youtube.com/watch?v=grWG_U4sgS8
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Mass is measured in kg
Weight in measured in newtons
(N)
1 kg = 10N
(actually it’s 9.8N, but 10 is an
easier number to estimate)
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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?
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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)
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Force Diagram
Force Normal
Force Friction Force Applied
Force Gravity
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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.
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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:
– An elevator at rest
– An elevator going to the top floor
– An elevator going to the main floor
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• Pushing a box
• Pull up
• Tug of war
• Elevator
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Topic #4
Forces, Loads, and Stresses
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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
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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
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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.
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Torsion
• The twisting of an object
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Shear Force
• Bend or tear a material by
pressing different parts in
opposite directions at the
same time
• Example: scissors
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Dead or Alive?
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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.
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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.
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Forces of Wonder http://www.wonderville.ca/v1/home.html
Compression and Tension of Arches (6 min) http://www.youtube.com/watch?v=UYtIFM1ek_M
Geodesic Domes (18 min) http://video.google.ca/videoplay?docid=11839832124301510
77
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Topic 5: How Structures Fail
• Failure can occur if the force is too strong
for the structure’s design or if the force is
acting on a vulnerable part of the structure.
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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.
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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
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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
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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
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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
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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?v=d-TJw52PGYs
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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/wat
ch?v=yrrJLAXwUBU
http://www.youtube.com/wat
ch?v=uejRaFfrOZg&feature
=related
http://www.youtube.com/wat
ch?v=kKXca2l0RPo&featur
e=related
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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.
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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
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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_mini_a
rch.html
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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
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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.
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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.
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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
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Topic 7: Stable Structures
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• 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.
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Centre of Gravity
• The specific point where all of the mass of
the structure is evenly distributed around.
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• 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.
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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
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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!
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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 doesn’t
freeze.
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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.
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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.