1. Mr. Ilko

2. Tacoma Narrows Bridge 2 mins http://www.youtube.com/watch?v=P0Fi1VcbpAI Building Collapse http://www.youtube.com/watch?v=uKeENdyIluI&feature=related 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. Thedownward force acting on us due to the Earths downward pull MASSis 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. 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. 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. 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. Take a ruler and balance it on your finger Howdid 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. 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. Whenstructures 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. Is the weight of the structure itself and the non-moving load it supportsNails, 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. Is the opposite of static load It is an external force that moves or changeswith time These can change very quickly such as asudden gust of wind or an earthquake An example of a dynamic load is cars drivingover a bridge, or students walking up stairs Dynamic is also known as LIVE LOAD 13. Bridgesare prime examples of structures designed to withstand loads Differentconditions call for different bridges because all bring their own pros and cons 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. Isa light weight but strong bridge, made of trusses (triangle-shaped frames) along its sides 16. Hangs between two ends that hold it up Modernones 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. 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