Forces

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  • 1. Forces LabForces Lab | Materials Lab | Loads Lab | Shapes LabAbout This LabThis lab simplifies the real-life forces and actions that affect structures, in order to illustrate key concepts.IntroForces act on big structures in many ways. Click on one of the actions at left to explore the forces at work and to see real-life examples.Squeezing (Compression)Compression is a force that squeezes a material together. When a material is in compression, it tends to become shorter.Compression: See It In Real LifeThe lower columns of a skyscraper are squeezed by the heavy weight above them. This squeezing force is called compression.Stretching (Tension)Tension is a force that stretches a material apart. When a material is in tension, it tends to become longer.Tension: See It In Real LifeThe weight of the roadway and all the cars traveling on it pull on the vertical cables in this suspension bridge. The cables arein tension.BendingWhen a straight material becomes curved, one side squeezes together and the other side stretches apart. This action iscalled bending.Bending: See It In Real LifeThe top side of the metal bar is pulled apart in tension, and the bottom side is squeezed together in compression. Thiscombination of opposite forces produces an action called bending.Sliding (Shear)Shear is a force that causes parts of a material to slide past one another in opposite directions. ÝShear: See It In Real LifeDuring an earthquake, parts of this roadway slid in opposite directions. This sliding action is called shear.Twisting (Torsion)Torsion is an action that twists a material.Torsion: See It In Real LifeIn 1940, the Tacoma Narrows Bridge twisted violently in strong winds and collapsed. The twisting force that tore this bridgein half is called torsion. Ý
  • 2. Loads LabForces Lab | Materials Lab | Loads Lab | Shapes LabAbout This LabThis lab simplifies the real-life conditions that affect structures, in order to illustrate key concepts.Intro/InstructionForces that act on structures are called loads. All structures must withstand loads or theyll fall apart. In order to build astructure, you need to know what kinds of external forces will affect it.Dead LoadThe weight of the structure itself is called the dead load. Anything permanently attached to the structure is part of itsdead load -- including the columns, beams, nuts, and bolts.Live Load Failure IntroThe weight of the stuff on the structure is called the live load. Things that move around in or on a structure, like people,furniture, and cars, are all examples of live load.Live Load FailureThe beam failed because it could not support the heavy weight of the live load above it.Live Load Success IntroThick Beam: The thicker a beam, the less likely it is to bend. Thick beams are used in structures that experience live anddynamic loads.Live Load SuccessThick Beam: The thick beam made this structure very strong. Now the beam wont bend from the heavy weight of the liveload on top of it.Dynamic Load Failure IntroLoads that change over time are called dynamic loads. Dynamic loads -- from wind gusts to pounding objects -- createvibrations that can become bigger and more dangerous over time.Dynamic Load FailureThe beam was vibrating too much from the dynamic load. This kind of vibration would be unacceptable to people occupying abuilding or driving across a bridge.Dynamic Load Success IntroThick Beam: The thicker a beam, the less likely it is to bend. Thick beams are used in structures that experience live anddynamic loads.Dynamic Load SuccessThick Beam: The thick beam absorbed the vibrations caused by the dynamic load and prevented the structure from bendingand galloping wildly out of control.Wind Load Failure IntroWhen wind blows on a structure, it is called wind load. Wind loads push horizontally on a structure.Wind Load FailureThe structure collapsed because it couldnt withstand the strong gusts of wind.Wind Load Success IntroCross-Bracing: Diagonal braces, usually made of steel, are used to strengthen and stabilize all kinds of structures.
  • 3. Wind Load SuccessCross-Bracing: Cross-bracing is an excellent way to stiffen a structure experiencing wind load. When the wind blows, thediagonal brace squeezes together and prevents the structure from flopping over.Thermal Failure IntroWhen a structure expands or shrinks with the temperature,it is experiencing thermal load. The temperature causes thebeams and columns to change shape and push and pull on other parts of the structure.Thermal FailureThe intense sun made the beam expand, throwing the entire structure out of whack.Thermal Success IntroRoller Joints: Roller joints are used in structures that get really hot or cold. They give columns and beams the freedom toexpand and contract as the temperature changes.Thermal SuccessRoller Joints: Thanks to this roller joint, the beam can swell in the sun and slide over the column without damaging thestructure.Earthquake Failure IntroWhen the ground beneath a structure jerks back and forth during an earthquake, the structure is experiencing anearthquake, or seismic load. Earthquake loads push and pull horizontally on a structure.Earthquake FailureThat was more rattling and shaking than this poor structure could handle.Earthquake Success IntroShear Walls: Solid walls of reinforced concrete or masonry -- called "shear walls" -- have great stiffness in the horizontaldirection. They resist loads that push or pull horizontally on a structure.Earthquake SuccessShear Walls: Shear walls can handle being pushed, pulled, rattled, and shaken during an earthquake. Theyre a great way tostrengthen a structure prone to earthquake load.Settlement Failure IntroWhen the soil beneath a structure settles unevenly, it is called settlement load. Structures will sink and change shape whenthey experience settlement load.Settlement FailureThis structure is in bad shape -- literally!Settlement Success IntroDeep Piles: Heavy concrete pillars, or piles, are used to support structures on soft soil. The piles rest deep in the earth onstable, solid soil and support the weight of the heavy structure above.Settlement SuccessDeep Piles: The massive concrete piles, sunk deep into the earth on hard, solid soil, keep the structure safe and sound whereit should be -- above ground!
  • 4. Materials LabForces Lab | Materials Lab | Loads Lab | Shapes LabAbout This LabThis lab simplifies the real-life properties of a selection of materials, in order to illustrate key concepts.Intro/InstructionsWhat you build a structure out of is just as important as how you build it! Different materials have vastly differentproperties. Click on a material at left to find out more about it, and put it to the test.Wood PropertiesType: Spruce (softwood)Wood Pros+ConsStrengths: Cheap, lightweight, moderately strong in compression and tensionWeaknesses: Rots, swells and burns easilyWood ApplicationsBridges, houses, two- to three-story buildings, roller coastersExample: Son of Beast -- Cincinnati, OhioWood Compression MessageYou squeezed this block easily, but it took a lot of effort to make it break. Wood is cheap and pretty strong in compression.Thats why people build houses out of wood!Wood Tension MessageIt wasnt easy to break this block of wood because wood is strong when you pull it in the direction of its fibers. It wouldhave been three times easier for you to break this block if youd stretched it from the top and bottom, across the directionof its fibers.Plastic PropertiesType: High-strength plastic fabric Ingredients: Long chains of moleculesPlastic Pros+ConsStrengths: Flexible, lightweight, long-lasting, strong in compression and tensionWeaknesses: ExpensivePlastic ApplicationsUmbrellas, inflatable roofs over sports arenasExample: Georgia Dome -- Atlanta, GeorgiaPlastic Compression MessageCompared to steel, you squeezed this plastic block easily, but it took a lot of effort to make it break. The long chains ofmolecules that make up plastic can be pulled and pushed in many directions without failing.Plastic Tension MessageYou stretched this plastic pretty far before it finally broke. The long chains of molecules that make up plastic can be pulledin many directions without snapping. Thats one of the reasons why circus tents are made of plastic fabric!Aluminum PropertiesType: Aluminum alloy Ingredients: Aluminum with magnesium & copper
  • 5. Aluminum Pros+ConsStrengths: Lightweight, doesnt rust, strong in compression and tensionWeaknesses: ExpensiveAluminum ApplicationsAirplane wings, boats, cars, skyscraper "skin"Example: Petronas Towers -- Kuala Lumpur, MalaysiaAluminum Compression MessageIt was pretty hard for you to break this aluminum block. Thats because the magnesium and copper inside this block makes italmost as strong as steel!Aluminum Tension MessageIt wasnt easy to break this aluminum block. Thats because aluminum, when combined with metals like magnesium andcopper, is almost as strong as steel!Brick PropertiesType: Ordinary brick Ingredients: Burned clayBrick Pros+ConsStrengths: Cheap, strong in compressionWeaknesses: Heavy, weak in tensionBrick ApplicationsWalls of early skyscrapers and tunnels, domesExample: Original Thames Tunnel -- London, EnglandBrick Compression MessageYou had to push this brick very hard to make it crumble. Bricks are very strong in compression. Thats why early houses weremade of brick!Brick Tension MessageYou pulled this brick apart easily! Thats because bricks are very weak in tension.Concrete PropertiesType: Fine-grain concrete Ingredients: Cement, water, small stonesConcrete Pros+ConsStrengths: Cheap, fireproof and weatherproof, molds to any shape, strong in compressionWeaknesses: Cracks with temperature changes, weak in tensionConcrete ApplicationsEarly arch bridges and domesExample: Pantheon - Rome, ItalyConcrete Compression MessageYou had to squeeze this concrete block really hard to make it break. Thats because concrete is very strong in compression.Concrete Tension MessageYou pulled apart the small stones and cement in this concrete block easily. Thats because concrete is weak in tension.Reinforced Concrete PropertiesType: Fine-grain concrete with high-strength steel Ingredients: Steel bars hidden in concrete
  • 6. Reinforced Concrete Pros+ConsStrengths: Low cost, fireproof and weatherproof, molds to any shape, strong in compression and tensionWeaknesses: Can crack as it cools and hardensReinforced Concrete ApplicationsBridges, dams, domes, beams and columns in skyscrapersExample: Hoover Dam - Nevada/Arizona borderReinforced Concrete Compression MessageYou had to squeeze this block really hard to make it break. Thats because concrete and steel are both very strong incompression.Reinforced Concrete Tension MessageIt was hard to pull this concrete block apart because the steel bars inside make it very strong in tension. Thats why someof the tallest skyscrapers in the world are made of reinforced concrete.Iron PropertiesType: Cast iron Ingredients: Iron with lots of carbonIron Pros+ConsStrengths: Molds to any shape, strong in compressionWeaknesses: Weaker than steel in tension, breaks without warningIron ApplicationsArch bridges, cannons, historic domesExample: Iron Bridge - Shropshire, EnglandIron Compression MessageIt wasnt easy for you to squeeze this cast-iron block. Cast iron is strong in compression. Thats why early arch bridges weremade of cast iron.Iron Tension MessageIt was easy for you to pull this cast-iron block apart. Thats because cast iron is brittle -- it snaps without warning.Steel PropertiesType: High-strength steelIngredients: Iron with a touch of carbonSteel Pros+ConsStrengths: One of strongest materials used in construction, strong in compression and tensionWeaknesses: Rusts, loses strength in extremely high temperaturesSteel ApplicationsCables in suspension bridges, trusses, beams and columns in skyscrapers, roller coastersExample: Sears Tower - Chicago, IllinoisSteel Compression MessageYou had to push extra hard on this steel block to make it bend and break. Steel is stronger than any other material incompression. Thats why engineers choose steel beams and columns to support most skyscrapers.Steel Tension MessageYou had to pull this block incredibly hard to make it break because steel is stronger than any other material in tension.Thats why the cables in the Golden Gate Bridge are made of steel.
  • 7. Shapes LabForces Lab | Materials Lab | Loads Lab | Shapes LabAbout This LabThese labs simplify the real-life conditions that affect structures (shapes), in order to illustrate key concepts. In the realworld, many variables affect the strength and stability of any give shape. The choice of materials, joints, load distribution,and size and thickness of a structure all affect its ability to resist loads. For example, a triangle made of paper wouldcollapse much sooner than an arch made of steel -- an effect that was not demonstrated in this lab.The shape comparisons in this lab depend upon the following conditions: each shape is of equivalent thickness, the joints arehinged, and the live load is applied downward to the structure at a single point at its top and center.Intro/InstructionsThe shape of a structure affects how strong it is. Rectangles, arches, and triangles are the most common shapes used tobuild big structures.One Elephant on RectangleThe weight pushes down on the rectangle and causes the top side to bend.One Elephant on ArchThe weight presses down on the arch and is spread outward along the curve to the ground below.One Elephant on TriangleThe weight causes the top two sides to squeeze together and the bottom side to pull apart.Three Elephant on RectangleThe weight caused the top side to bend too much, so it failed!Three Elephant on ArchThe arch likes to be pushed and squeezed. The weight pushes this arch into a stable, tightly squeezed shape.Three Elephant on TriangleUnlike the rectangle, the sides of the triangle did not bend under the tremendous weight. This is why the triangle is stillstanding.Six Elephant on RectangleThe weight caused the top side to bend too much, so it failed!Six Elephant on ArchThe arch likes to be pushed and squeezed, but not this much! When an arch is pushed too hard, the sides spread apart andcollapse.Six Elephant on TriangleThe triangle is still standing because the pulling force in the bottom side is balancing the pushing forces in the upper sides.Nine Elephant on RectangleThe weight caused the top side to bend too much, so it failed!Nine Elephant on ArchThe arch likes to be pushed and squeezed, but not this much! When an arch is pushed too hard, the sides spread apart andcollapse.
  • 8. Nine Elephant on TriangleEven triangles have their limits! All this weight made the third side stretch so much that it snapped in half.Push RectangleWhat happens when you push the side of a rectangle?The rectangle is a wobbly, unstable shape. When you push the side, it flops into a slanted parallelogram. This happens withoutany of the rectangles sides changing length.Push Braced RectangleNow when you push the side, the diagonal brace gets squeezed, preventing the rectangle from flopping over.Push ArchWhat happens when you push down on an arch that is not supported on both sides?The force of the finger pushes the sides of the arch outward.Push Braced ArchAs the arch tries to spread outward, external supports, called buttresses, push back on the sides of the arch and prevent itfrom spreading apart.Push TriangleWhat happens when you push the side of a triangle?The outer edge squeezes together, and the inner edge pulls apart. When one side experiences these two forces at the sametime, it bends. The weakest part of the triangle is its side!Push Braced TriangleWhen you poke the top of the triangle, the two sides squeeze together and the bottom side pulls apart. The triangle doesntbend because each side experiences only one force at a time. When used properly, triangles are the most stable and rigidshapes used in construction today.