Unit d - 2.2 external forces

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Unit d - 2.2 external forces

  1. 1. Mr. Ilko
  2. 2.  Tacoma Narrows Bridge – 2 mins http://www.youtube.com/watch?v=P0Fi1Vc bpAI Building Collapse http://www.youtube.com/watch?v=uKeENd yIluI&feature=related
  3. 3.  Anexternal force is a force that is applied on a structure from something else • Such as walking into the wind or swimming against the waves Theforce we feel everyday is GRAVITY, gravity is the external force acting on everything on the planet
  4. 4.  Thedownward force acting on us due to the Earth’s downward pull MASS is the amount of matter that the Earth is pulling down When we leave Earth the gravity is removed and we are able to float because our mass is not being pulled down
  5. 5.  Gravity acts on a structure everywhere, but there is an imaginary point where balance is maintained • THAT IS THE CENTER OF GRAVITY Thelocation of the center of gravity determines the stability of a structure
  6. 6.  Inorder for us to increase a structures stability we use two main methods; • 1. Increase the Base compared to the height • 2. Place the majority of the mass close to the ground
  7. 7.  Stand up and balance on one foot…. • 1. • 2. • 3. CanI balance a meter stick on one finger focusing only on its center of gravity?
  8. 8.  Take a ruler and balance it on your finger How did you get it to do that? What do you notice about the ruler? Moveyour finger one centimeter to the left. Now another, and another what happens??
  9. 9.  Symmetry is a balanced arrangement of mass that occurs on opposite sides of an object Fora structure to be stable, its mass must be distributed equally around the center of the structures base
  10. 10.  When structures are built, the creators consider the load that structure will have to resist • A book shelf resists its weight and the books • A bridge resists gravity, maybe a car, and the materials its made from Load is split into two different types; • Static • Dynamic
  11. 11.  Is the weight of the structure itself and the non-moving load it supports  Nails, wood, steel, cables are a few examples Even though these parts make up the load, they create a external force because gravity is pulling on these items Static Load is also known as DEAD LOAD
  12. 12.  Is the opposite of static load It is an external force that moves or changes with time These can change very quickly such as a sudden gust of wind or an earthquake An example of a dynamic load is cars driving over a bridge, or students walking up stairs • Dynamic is also known as LIVE LOAD
  13. 13.  Bridges are prime examples of structures designed to withstand loads Different conditions call for different bridges because all bring their own pros and cons
  14. 14.  Is the most common type of bridge used Is flat and is supported at its two ends Longer beam may be supported by addition piers
  15. 15.  Isa light weight but strong bridge, made of trusses (triangle-shaped frames) along its sides
  16. 16.  Hangs between two ends that hold it up Modern ones have very tall towers on either end that support its main cables that hold up the bridge The main cables are anchored into concrete, and the smaller cables support the roadway
  17. 17.  Designed to withstand very heavy weights The dynamic load of people or cars causes each piece of wood or stone to push against the next one This push eventually leads to the end supports embedded into the ground The ground pushes back strengthening the bridge

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