1. Grade 7 Unit 4 Topic 1 Types of Structures

2. An Overview Structure: An object with a definite size and shape, which serves a purpose or function. The parts of a structure have a specific arrangement that remains the same. Function: (of a structure or object) its main purpose.

3. An Overview – cont. To perform its function, every part of a structure must resist forces and loads that could change its shape or size. Force: Stresses such as pushes or pulls. Load: the weight carried or supported by a structure.

4. Types of Structures There are so many different structures that it is difficult to form a definition that fits them all. Instead of defining structures scientists, engineers, and designers group structures into classifications based on key factors.

5. Classifying Structures It is much more efficient to classify structures based on key common features such as: The Structure’s origin Its Function Its Form Its Design The Materials and parts it made of How it is held together.

6. The Structure’s Origin One common way to classify structures is to group them according to their origin. That is, are they natural or manufactured objects. Natural Structures Natural Structure: an object or structure not made by people.

7. The Structure’s Origin–cont. Natural Structures –cont. They have a definite shape. They can be made of many parts They could have simple or complex patterns They can serve many purposes. They can be from either the living or non-living part of the natural world. As both a feather and a sand dune can be considered structures.

8. The Structure’s Origin–cont. Manufactured Structures Manufactured Structures: An object or structure that is made by humans. Many manufactured objects are modeled after natural structures.

9. The Structure’s Design Structures can also be classified by the way they are built also known as the structure’s design. Design: the shape and size of a structure and the materials of which it is composed.

10. The Structure’s Design–cont. The 3 key types of designs are: Mass Structures Frame Structures Shell Structures

11. The Structure’s Design–cont. 1. Mass Structures: Mass Structure: A structure, natural or manufactured, that is made by the piling up of materials; examples include a pyramid a mountain, and even a cake.

12. The Structure’s Design–cont. 1. Mass Structures – cont.: Advantages of mass structures include They are held firmly in place by their own weight. If small pieces are worn away or broken it usually makes little difference. Mass structures are not always completely solid. Due to their large size and weight, mass structures must be carefully designed.

13. The Structure’s Design–cont. 1. Mass Structures – cont.: There are 4 main ways a Mass Structure can fail: The mass of the vertical sections may not be enough to resist the forces and loads put upon it. The mass structure itself may be so heavy that the ground underneath is pressed down unevenly causing instability.

14. The Structure’s Design–cont. 1. Mass Structures – cont.: 3.The structure may not be thick enough or fastened tightly together, so parts of it are dislodged, breaking the structure apart. 4.The structure may not be anchored firmly enough to the ground possibly causing it to tip over if too much force is put on the top of the structure.

15. The Structure’s Design–cont. 2. Frame Structures: Frame Structure: A type of structure in which a skeleton of materials supports the weight of the other parts.

16. The Structure’s Design–cont. 2. Frame Structures – cont.: Frame structures allow most of the inside of the building to be empty space. There are 2 types of walls in frame structures: Load bearing walls – which hold up, and are part of, the frame structure. Partition walls – these walls simply divide up the open space.

17. The Structure’s Design–cont. 2. Frame Structures – cont.: The level of complexity in frame structures varies from simple exposed frame structures (like ladders and spider webs) to more complex objects that have added parts to their frames (such as covering materials and subsystems).

18. The Structure’s Design–cont. 2. Frame Structures – cont.: The frames can be hidden or exposed They are relatively easy to design and build They are relatively inexpensive to construct.

19. The Structure’s Design–cont. 2. Frame Structures – cont.: All frame structures must overcome similar problems: How should the parts be fastened together? How do you make them strong without using too much material? What shape or bracing should they have so that they will not bend or fall down?

20. The Structure’s Design–cont. 2. Frame Structures – cont.: Common design problems in frame structures are further complicated when the building needs to be: Lightweight – such as a tent Very tall – such as communications towers Large complicated projects – such as bridges and buildings These challenges are often addressed using anchors, bracing, and low ranges of tolerance for parts.

21. The Structure’s Design–cont. 3. Shell Structures: Shell Structure: A type of structure that obtains its strength and rigidity from a thin, carefully shaped outer layer of material and that requires no internal frame; examples include an igloo and an egg.

22. The Structure’s Design–cont. 3. Shell Structures – cont.: Some shells, like those of a balloon, or parachute, are flexible. Shell structures have 2 very useful features: They are completely empty They use very little building material

23. The Structure’s Design–cont. 3. Shell Structures – cont.: The shape of the shell spreads the forces, experienced by the shell, through the whole structure, which results in each part supporting only a small portion of the load. This need to spread out the forces causes certain construction difficulties.

24. The Structure’s Design–cont. 3. Shell Structures – cont.: Problems facing builders of shell structures include: Tiny weaknesses can cause the whole structure to fail. Uneven cooling or drying can cause the whole structure to fail. The rounded shape can make fitting flat material difficult. Assembling flexible materials into a shell is also tricky.

25. The Structure’s Design–cont. Mix and Match: Many structures combine designs. For example: Hydroelectric Dams are mass structures with large rooms that use frames to support the space. Airplanes are built on a metal frame that is further strengthened by the metal skin that acts as a shell.

26. The Structure’s Design–cont. Mix and Match: Many structures combine designs. For example: 3.Domed Buildings usually used shell construction for the dome and frame construction for the rest of the building. 4.Warehouses are often a combination of frame structures, with thick walls of concrete that act as mass structures.

27. Grade 7 Unit 4 Topic 2 Describing Structures

28. The Structure’s Function Function is what the object or structure is supposed to do. Most structures have several functions. One very important function of any structure is to support its own weight.

29. The Structure’s Function-cont. Structures do more than just support loads. They also: Contain Transport Shelter Lift Fasten Separate Communicate Break hold

30. The Structure’s Function-cont. Designers have a hard time creating structures that perform all of their functions equally well. In order to guarantee that a structure can perform its functions, designers work to a set of specifications that give precise, measurable standards that their structure must meet.

31. The Structure’s Aesthetics Aesthetics: A branch of philosophy that studies the principles of beauty; the properties of an object that make it pleasing to the senses. The structure’s aesthetic quality is very important, since the best designs look good.

32. The Structure’s Aesthetics-cont. The structure’s aesthetic appeal does not only require the designers to carefully choose materials methods used to make a structure but also the structures: 3.shapes and arrangement 4.textures 5.colours

33. The Structure’s Aesthetics-cont. Above all engineers and architects try to keep their designs simple. More often then not, clean designs look better than over-complicated, busy ones. Designers therefore adhere to the KISS principle.

34. Designing for Safety Margin of safety: the need for something built or manufactured to perform as expected for a long time, so that people’s safety and health are not at risk. In a structure, a margin of safety would ensure that the structure has extra strength to support more load than normal.

35. Designing for Safety-cont. Almost all structures are built with a margin of safety. An example of margin of safety would be how roofs in Canada are designed to support enormous weights, so that large amounts of accumulated snow will not collapse the roof.

36. Balancing Safety with Cost Making structures stronger usually makes them more expensive. Adding material, using stronger materials, and / or using skilled craftsmen are all factors that will increase the strength of a structure, but will also increase the cost.

37. Balancing Safety with Cost-cont. While designers strive to allow a comfortable safety margin for any conditions they can imagine, there are occasions where things may happen to a structure that are totally unexpected. Even well-designed structures can fail due to unexpected conditions.

38. Materials Choosing which building materials to use in a structure is another important design decision. The properties or characteristics of a material must match the purpose of the structure.

39. Materials-cont. Properties: the characteristics of materials; every material has it own unique set of properties; examples of properties include: Colour Odour Density

40. Materials-cont. Combinations of different materials are often required to give a structure the properties it needs. Three common methods of creating combination materials are: Composite Layering Weaving or Knitting

41. Materials-cont. 1. Composite Materials -Composite: of materials, made up of several different materials, with different properties, to fulfill a specific purpose. -Different composites have different properties based on what they are made of and how they are made.

42. Materials-cont. 1. Composite Materials-cont. -Reinforced concrete is a composite material made up of steel rods and concrete. Where the steel rods support strong tension (pulling) forces, and the concrete supports strong compression (pushing) forces. -Other composites include fibreglass and plastic molded into boat hulls, and plastic with nylon mesh used in garden hoses.

43. Materials-cont. 2. Layered Materials: -Layers of different materials, pressed and glued together, often produce useful combinations of properties. -Lamination: a process in which a layer of material is pressed or glued onto other layers. Examples include car windshields, drywall, linoleum, and plywood.

44. Materials-cont. 3. Woven and Knit Materials: -Hair-like fibres are spun (twisted together) into long, thin strings called yarn are then looped and knotted together to make knit materials, or woven together in a crisscross pattern using a loom. -Knit materials stretch in all directions so they fit well over complex shapes.

45. Materials-cont. 3. Woven and Knit Materials-cont.: -Weaving and knitting are not the only ways to make flexible materials.  Paper and felt are pressed and matted together.  Aluminum foil and plastic wrap are melted and dissolved. -No matter how they are made, materials that can be folded or rolled are extremely useful, especially for lightweight structures that must be moved and / or stored.

46. Choosing Materials Choosing one material over another means balancing the advantages and disadvantages of each possible choice. Stronger materials are often more expensive.

47. Choosing Materials-cont. To pick the most suitable materials for a structure, architects, engineers, and designers will usually consider the following 4 factors: Cost: Appearance Environmental Impact Energy Efficiency

48. Choosing Materials-cont. Cost: The lowest cost materials may not always be best. They can be poor quality, or hard to work with They could wear out quicker or require more maintenance. There are times; however, where the least expensive material could do an acceptable job.

49. Choosing Materials-cont. 2.Appearance: The lifespan of a structure is often considered when deciding on which materials to use. Structures like bridges and buildings last a long time and so the materials they are made from need to remain attractive and strong over time. Structures with short lifespans, such as cardboard boxes, need not be made of materials that weather well over time.

50. Choosing Materials-cont. 3.Environmental Impact: Where the material comes from - is it recycled, a renewable resource, or a non-renewable resource? How it is made - does it require processing that is damaging to the environment? How it is put together – does it require harmful chemicals or materials to assemble?

51. Choosing Materials-cont. 4.Energy Efficiency: The cost of many structures includes more than just the cost of materials and construction. Once a structure is completed the energy it requires to operate is a function of the materials used to build it. Insulating materials in homes, refrigerators, and freezers all impact the cost of operation. The materials used in a furnace impact its ability to transfer heat.

52. Joints Decisions about how to fasten structures together are critical because structures are often weakest where their parts are joined together. There are two main types of joints: Mobile joints Rigid joints

53. Joints-cont. Mobile Joints: Mobile Joint: a joint that is designed to allow movement; examples of a mobile joint include a door hinge and an elbow. Their complicated parts are tricky to make, and they must be coated with a lubricant so that they move smoothly.

54. Joints-cont. 2.Rigid Joints: Rigid Joint: a device designed to fix an object into place; a joint that allows no movement; examples of a rigid joint include a nail and a screw. Most rigid joints fit into 5 categories: 1.Fasteners 2.Ties 3.Interlocking shapes 4.Adhesives 5.Melted joints

55. Joints-cont. 2.Rigid Joints-cont.: 1.Fasteners: Examples of fasteners include: nails, staples, bolts, screws, rivets, and dowels. A major problem with fasteners is that the holes they make weaken the materials they fasten. Nails and staples are usually forced in the materials, which can also cause cracking and separating of the material. Bolt, screw, and dowel holes are often predrilled which doesn’t weaken the materials as much but is more time- consuming

56. Joints-cont. 2.Rigid Joints-cont.: 2.Interlocking Shapes: Are carefully shaped parts, in rigid and flexible materials, that can hold themselves together. Examples of interlocking shapes include: Lego , dovetail joints, folded seams in sheet metal, and hems in clothing.

57. Joints-cont. Rigid Joints-cont.: 3.Ties: Thread, string, and rope are all examples of materials used to tie material together. Tying material together may, simply, be done by hand or it may take special equipment like a sewing machine where the needle and bobbin thread are intertwined.

58. Joints-cont. 2.Rigid joints-cont.: 4.Adhesives: Adhesive: a sticky substance, such as glue or epoxy cement, that is used to hold objects or materials together. Adhesives increase the bonding surface area and the strength of the bond by flowing into tiny rough areas on the surface of the pieces it joins.

59. Joints-cont. 2.Rigid Joints-cont.: 4.Adhesives-cont.: When glue hardens it locks the pieces together. Thermosetting glues: like those found in glue guns, harden when cooled. Solvent-based glues: harden as they dry out. The strongest glues create a special kind of force between the smallest particles of the pieces being joined.

60. Joints-cont. 2.Rigid Joints-cont.: 4.Adhesives-cont.: Even the strongest glued joints fail under extreme conditions. If the glue is stronger than the material it is bonding, the material next to the joint may break. Some adhesives can be a health hazard because the bond as soon as they touch moisture. (Crazy gluing body parts together) Some adhesives can be a health hazard because of the powerful chemicals they release as they harden.

61. Joints-cont. 2.Rigid Joints-cont.: 5.Melting: Pieces of metal or plastic can be melted together. Welding: a process in which pieces of metal or plastic are fused together by the application of heat. Soldering: a process in which a melted material is applied to a different type of material; the melted material hardens when it cools, forming a rigid joint that hold the other material in place.

62. Joints-cont. 2.Rigid Joints-cont.: 5.Melting-cont.: To increase the strength of the soldered joint the pieces to be joined may be twisted or folded together. When soldering the pieces must be cleaned before joining, and the melted material must be cooled slowly and carefully to avoid brittle or weak joints. There are many ways to weld joints, including: torches, an electric spark, strong chemicals, and even sound waves.

63. Grade 7 Unit 4 Topic 3 Mass and Forces

64. Mass Mass: the amount of matter in a substance; often measured with a balance. Mass The metric system measures mass by comparing objects to a standard mass.

65. Mass-cont. Primary Standard: The name given to a small cylinder of metal on which the kilogram (kg) is based; equivalent to 1 kg. Kilogram (kg): the primary measurement of mass in SI, equal to 1000 g; 1 kg is the primary standard for mass.

66. Mass-cont. Exact copies of the primary standard kilogram are kept in various countries, including Canada. Smaller masses are usually expressed in grams (g). “Kilo” means thousand; therefore 1 kilogram is a thousand grams.

67. Mass-cont. “Milli” means “one thousandth”; therefore 1 milligram (mg) is a thousandth of a gram (0.001 g) It would take 1000 milligrams to make one gram.

68. Mass-cont. Balance: A device to measure mass; many balances work by using the force of gravity. Many balances compare the pull of gravity on the object being measured with the pull of gravity on standard masses.

69. Mass-cont. An object’s mass stays constant no matter where it is. An object’s mass will only change if additional matter is added, or some matter is taken away.

70. Forces and Weight While mass stays constant, weight will change if an object moves from the Earth to the Moon. To understand why weight changes you need to understand that weight is a force.

71. Forces and Weight-cont. Force: A push or pull. Force Newton: The standard unit of force in the Système international d`unitès (SI). 1 N (Newton) is just enough to lift a D-cell battery.

72. Forces and Weight-cont. Forces need to be quantified, measured, in order to better understand what forces affect structures. Force Meter: A scientific device used to measure force; also called a spring scale. A bathroom scale is a type of spring scale

73. Forces and Weight-cont. Force meters are low cost devices, common in laboratories; however they are not that accurate compared to other electronic sensors. Very large forces, or difficult to measure forces, are often calculated by observing their effect on the motion of an object.

74. Forces and Weight-cont. To completely describe a force, you need to determine both its direction and its size.

75. Weight Weight is a force, and like all forces is properly measured in newtons. Isaac Newton, 1642-1727, theorized that there is a force between any two objects, anywhere in the Universe, that tries to pull them together.

76. Weight-cont. Newton mathematically examined the size of this force, which he called gravity. Newton Gravitational Force: The force exerted by gravity on an object; measured in newtons (N); the preferred scientific term for the everyday term “weight”.

77. Weight-cont. The Gravitational Force between two objects depends on the masses of the two objects and the distance between them. The greater the mass and the closer the distance, the greater the gravitational force.

78. Weight-cont. Weight: The force of gravity exerted on a mass. Weight and gravitational force are the same thing. Weight can be stated as: “A 1 kg mass has a weight of 10 N on Earth” (It’s actually 9.81 N, but 10 is close enough for most purposes).

79. Weight-cont. Since gravitational force depends on distance between objects, your weight changes depending where you are. Gravitational force also depends on the mass of the object; therefore your weight changes depending on what very large mass you are close to. You weigh 1/6 th of your Earth weight on the moon. Gravitational

80. Picturing Forces Force Diagram: A drawing that uses arrows to represent the direction and strength of one or more forces. The circle stands for the object Each force is shown with an arrow The length of the arrow shows the size of the force The direction of the arrow indicates the direction of the force.

81. Picturing Forces-cont. The arrow, in a force diagram, is usually drawn pointing away from the place where the force is acting. Mathematicians are able to use scale drawings and calculations to predict what will happen when many forces are acting on a given object.

82. Picturing Forces- An example As a cannonball is fired the force of the cannon’s firing is the greatest force acting on the cannonball. A. B. C. A.The force of the cannon firing B.Friction of the air (This stays the same) C.Gravity

83. Picturing Forces- An example As the cannonball continues its flight the force of the firing begins to decrease compared to the force of gravity A.The force of the cannon firing B.Friction of the air (This stays the same) C.Gravity A. B. C.

84. Picturing Forces- An example At the top of the cannonball’s flight arc the force of firing and the force of gravity become equal. A.The force of the cannon firing B.Friction of the air (This stays the same) C.Gravity A. B. C.

85. Picturing Forces- An example The cannonball then begins to descend, as the force of gravity becomes the strongest force. A.The force of the cannon firing B.Friction of the air (This stays the same) C.Gravity A. B. C.

86. Picturing Forces- An example The force of firing continues to decrease compared to the force of gravity, and the cannonball picks up speed as it heads towards the ground. A.The force of the cannon firing B.Friction of the air (This stays the same) C.Gravity A. B. C.

87. Picturing Forces- An example When the cannonball comes to rest, on the ground, the only force still acting on the cannonball is the force of gravity. A.The force of the cannon firing B.Friction of the air (This stays the same) C.Gravity C.

88. Picturing Forces- An example A force diagram of the whole flight would look like this. A. B. C. A.The force of the cannon firing B.Friction of the air (This stays the same) C.Gravity

89. Grade 7 Unit 4 Topic 4 Forces, Loads, and Stresses

90. An Overview When a force is applied to any object that is free to move 3 things can happen: The object’s motion can speed up. The object’s motion can slow down. The object’s motion can change direction.

91. An Overview-cont. External Force: Stresses that act on a structure from outside. External forces produce internal forces. Internal Force: A force that acts on an object from the inside.

92. An Overview-cont. Internal forces can cause deformation. Deformation: The change in a structure’s shape or size when a force is acting on it; deformation is an indicator that a structure is stressed.

93. An Overview-cont. Deformation can result in either: Repairable damage The complete failure of a structure. Engineers must understand external and internal forces to prevent structural failure due to these forces.

94. External Forces External forces are divided into 2 groups: Dead Loads Live Loads

95. External Forces-cont. Dead Load: The weight of a structure upon itself. The dead load is a permanent gravitational force that can cause a structure to sag, tilt, or pull apart should the ground beneath it shift, or the materials compress under the load.

96. External Forces-cont. Live Load: The force or forces that act in or on a structure but are not part of the structure; examples include the wind, the weight of people, furniture, snow, and impact forces such as collisions.

97. Internal Forces There are 4 main internal forces: Tension Forces Compression Forces Shear Forces Torsion Forces

98. Internal Forces-cont. Tension Force: A force that pulls on a material and stretches it apart. Tensile Strength: a measure of the largest tension force that a material can withstand before changing shape or breaking apart.

99. Internal Forces-cont. 2.Compression Force: A force that compacts or squeezes a material. Compressive Strength: A measure of the largest compression force that a material can withstand before changing shape or breaking apart.

100. Internal Forces-cont. 3.Shear Force: A force that bends or tears a material by pushing parts of it in opposite directions. Shear Strength: Measures the largest shear force a material can stand before ripping apart.

101. Internal Forces-cont. 4.Torsion Forces: A force that acts on a material by twisting its ends in opposite directions. Torsion Strength: A measure of the largest torsion force that a material can withstand and still be able to return to its original shape.

102. Internal Forces-cont. Bending Force: A combination of push (compression) and pull (tension) forces that results in a temporary curving change in the shape of some structures. Tension Compression

103. Did You Know? Dragline fibre, a kind of spider silk, is the strongest material known. Dragline fibres are 5 times stronger than an equal mass of steel. Scientists are currently using genetic engineering to develop bacteria containing the gene for spider silk, so that large quantities could be produced.

104. Resisting Stress- The Inside View The forces between the tiniest particles within the material determine the strength of a material. Steel has a high tensile strength: It has strong forces pulling its particles together. Each metal particle attracts a few other particles very strongly. The forces are quite directional, so the particles form a regular arrangement in space.

105. Resisting Stress- The Inside View-cont. Graphite: has low shear strength. Its particles are arranged in layers with strong attraction within the layers and weak attraction between layers. Layers of graphite slide over one another easily making for an excellent dry lubricant. Rubber: has high torsion strength, and hold together even when twisted out of shape. Each rubber particle attracts many other particles in all directions

106. Grade 7 Unit 4 Topic 5 How Structures Fail

107. An Overview No structure or material is perfect. Even small forces, acting in a vulnerable place can cause damage. Learning how structures can possibly fail helps engineers design better structures.

108. Levers Create Large Forces Long rigid frame structures can fail due to their tendency to act like levers when a force is applied to them. Lever: A device used to change the amount of force needed to move an object. Fulcrum: the part of a lever that does not move.

109. Levers Create Large Forces-cont. Wind Effor t Forc e Load Forc e Pole tips When an effort is applied as an external force to the lever, a large enough force is created to lift a heavy load.

110. Levers Create Large Forces-cont. As the Wind continues to push Effort Force builds Load Force causes base joint to fail and the structure topples Pole tips more Unintentional lever action can damage a structure, or even collapse it completely.

111. Levers Create Large Forces-cont. Strong winds can produce other forces that can result in structural failure. TornadoWind Damage

112. How Materials Fail Structures can fail in the key ways: Shear Bend or Buckle Torsion Each type of failure is cause by a certain kind of internal force:

113. How Materials Fail-cont. Shear. Microscopic cracks or weaknesses in material can enlarge or break apart when compressed. This internal compression force causes a shear failure in the material.

114. How Materials Fail-cont. 2.Bend or Buckle Thin panels tend to bend and buckle when a compression force is applied. To prevent bending and buckling, thin paneled structures like aircraft and boats are reinforced with stringers and/or ribs.

115. How Materials Fail-cont. Torsion Twisting forces can cause material to fail in two ways: 1. Brittle structures, such as dry spaghetti, shear when they are twisted. 2. Flexible structures, such as hoses and rubber bands, fold up and twist losing shape.

116. Making Use of Stress Bend, buckle, snap, twist, and shear behaviours can also be put to good use. Bending: bending allows the middle, or camber, of a cross-country ski to contact more snow and give the skier a better grip when pushing off. Buckling: buckling structures, such as car bumpers and grass, can absorb energy from impacts in car crashes and in falls on the soccer field.

117. Making Use of Stress-cont. 3.Shearing: shear pins and clutches are both designed to permit shearing, thus preventing more serious damage to other components. 4.Twisting: Twisting fibres, such as cotton, wire, or wool, allows them to lock together creating stronger and more flexible materials.

118. Metal Fatigue Metal Fatigue: A weakening of metal due to stress, resulting in an accumulation of small cracks. Metals weaken when they are stretched or bent because the arrangement of their particles is changed.

119. Metal Fatigue-cont. This movement of the particles weakens the forces holding them together. Over time, this movement of the particles causes small cracks to develop. These small cracks may eventually cause the material to fail under only a small amount of stress.

120. Metal Fatigue-cont. Metal fatigue is still a problem, especially in lightweight, flexible structures such as aircraft.

121. Grade 7 Unit 4 Topic 6 Designing with Forces

122. An Overview An Engineer’s design of a new structure undergoes three steps: The general features of the structure are studied. Then they analyze the types of forces that are likely to be the greatest. Finally they choose details to counteract those forces.

123. An Overview-Cont. 3 methods designers use to help structures withstand forces: Distribute the load throughout the structure so no single part is carrying too much load. Direct the forces along angled components so that the forces hold the pieces together. Shape the parts to withstand a specific type of force.

124. Structural Components All of these structural components are designed to resist forces: Triangle forms: like those found in roof and bridge trusses are much stronger than rectangles.

125. Structural Components-cont. 2.Load Sharing: frame structures use many vertical support posts to share the load of a structure. This means that no single part of the structure carries a large load.

126. Structural Components-cont. 3.Cantilever: a horizontal span supported by very strong column at one end. Cantilever bridges and other large cantilevers often have cantilever arms that project out from the tower and utilize truss construction. The upper cantilever arms pull upwards on the ends of the cantilever. The lower cantilever arms push upwards on the ends of the cantilever

127. Structural Components-cont. 4.Arches: This shape directs the forces along the stones and down to the ground. The central stone is the keystone. Domes and shell structures are based on the arch concept.

128. Structural Components-cont. 5.Braces: can be added to beam structures to direct the forces through the columns and down to the ground.

129. Structural Components-cont. 6.Beams: A solid beam is strong, but also extremely heavy and expensive. By changing the shape the beams strength to weight ratio can be increased.

130. Structural Components-cont. 7.Corrugations: The wavelike shape found in cardboard and other materials gives the structure strength, while using less material than a solid piece would.

131. Structural Components-cont. 8.Hollow Bones: similar to the box beam these naturally occurring structural pieces provide strength at a reduced weight.

132. Structural Components-cont. 9.Flying Buttresses: an early structural method to support the weight of tall walls. Attached at the top of the wall, they worked in much the same way as Arches. Examples of manufactured and natural Flying Buttresses

133. Strengthening Structures Most materials have one kind of strength but not another. It is important for engineers to analyze the forces on a structure carefully so that they can select the appropriate materials with the appropriate kinds of strength.

134. Strengthening Structures-An Example A.The ropes on the swing undergo a lot of tension; therefore a rope or chain with high tensile strength is needed.

135. Strengthening Structures- An Example B. The crossbar forms a triangle, adding stability to the legs. Steel is a good choice of material because of its high tensile strength.

136. Strengthening Structures- An Example C. Concrete is used for the anchors because of its high compressive strength and ability to withstand the moisture in the ground.

137. Strengthening Structures- An Example D. The twisting of this joint, caused by the swinging motion, requires that material used have a high torsion strength.

138. Strengthening Structures- An Example E. The legs, slanted for stability, experience both compression and tension when the swing moves. Using steel provides both a high compressive and tensile strength.

139. Strengthening Structures- Another Example Shear forces were a big problem in early railways. Tiny cracks, caused during the manufacturing process, caused the rails to fail over time. A Canadian metallurgist, J. Cameron Mackie, discovered these small cracks and devised a method to eliminate them

140. Strengthening Structures- Another Example By cooling the red-hot steel slowly, Mackie found that these tiny cracks no longer developed. This process, of slowly cooling newly formed steel, quickly became the common practice around the world.

141. Using Frictional Forces Friction: A force that resists, or works against the movement of two surfaces rubbing together. Friction is important for assembling structures because it can help keep pieces of the structure from moving apart.

142. Using Frictional Forces-cont. A brick wall is an example of a structure that uses frictional forces to prevent deformation. Friction is also important in wood frame structures, where friction between the fasteners (nails and screws) and the material (wood) keeps the joints rigid.

143. Using Frictional Forces-cont. Different types of fasteners provide different amounts of frictional force.

144. Using Frictional Forces-cont. Friction between the ground and the bottom of a structure is an important design consideration, as it helps hold the structure in place. Too little or too much friction can cause problems, and there it is an important design consideration for structures.

145. Grade 7 Unit 4 Topic 7 Stable Structures

146. An Overview There is more than one way to collapse a structure. Almost all structures can lean a bit without falling down; however the force of gravity will pull them down if they lose their balance.

147. An Overview-cont. While athletes intuitively make adjustments to their body positions, architects and engineers must analyze structures to predict how or when a structure will become unbalanced. Stable: Of a structure, tending to maintain its shape and position.

148. Balancing Act Objects are generally more stable if: They rest on a large area. Have most of their mass close to the ground.

149. Balancing Act-cont. These general principles, however, are not precise enough for engineers when building structures. Calculations on foundation size and where to place heavy heating and air conditioning equipment is an essential element to the design process.

150. Balancing Act-cont. Engineers are also called upon to design stable structures, such as aircraft and rockets, that are not resting on the ground.

151. Balancing Act-cont. Centre of Gravity: The point at which all of the gravitational force of an object may be considered to act. All objects have a centre of gravity.

152. Balancing Act-cont. When an object falls, the centre of gravity falls in a straight line, while all other points rotate around it.

153. Balancing Act-cont. To determine whether a structure is balanced or not, locate its centre of gravity and draw a line directly down.

154. Firm Foundation Solid ground is not always stable, especially if it is moist. Water content in the soil can cause expansion and contraction, due to freezing and drying. As the soil shifts spaces between the soil particles can develop.

155. Firm Foundation-cont. Potholes, sinkholes, and landslides can all be caused from moisture changes in the soil. Moist soil can even flow like a thick liquid when it is shaken or vibrated.

156. Firm Foundation-cont. Builders use three key strategies to create a firm foundation: Pilings: A large, cylindrical structure used to carry the weight of a structure to a solid foundation In Canada foundation walls of about 1.5 m deep reach firm layers of soil that give enough support and never freeze.

157. Firm Foundation-cont. 2.Packing: Road builders, and others, pack loose soil surfaces before paving or building. Less stable material can also be excavated, and more stable material put in before packing.

158. Firm Foundation-cont. 3.Footings: A base for a wall in the foundation of a structure; a footing is wider than the wall to spread the weight over a larger area.

159. Rapid Rotation Gyroscopes: A circular device with a heavy outer rim that spins at a very fast rate, stabilizing the axis so that the axis always points in the same direction. Spin stabilization: The tendency of an object that is spinning on its axis to move in a predictable manner; an example of spin stabilization is the motion of a bicycle wheel.

160. Rapid Rotation-cont. Spin stabilization is especially useful for structures that do not rest on the ground, such as satellites and rockets. Toy tops, yo-yos, and Frisbees all use spin stabilization.