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Bridge Basics

Teaching bridge basics to high school teachers for implementing more engineering curriculum in their classes. This was followed up with many hands on and digital bridge building exercises. Made for the program.

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Bridge Basics

  1. 1. Design Your Future: Bridges
  2. 2. <ul><li>Simple </li></ul><ul><ul><li>Basic geometric shape. </li></ul></ul><ul><li>Strong </li></ul><ul><ul><li>Can’t be deformed without changing a length of a side or breaking a joint. </li></ul></ul><ul><li>Cool </li></ul><ul><ul><li>Three is a magic number. Yes it is. </li></ul></ul>triangles
  3. 3. truss bridges
  4. 4. <ul><li>Rigid bodies </li></ul><ul><ul><li>Statics </li></ul></ul><ul><ul><li>Dynamics </li></ul></ul><ul><li>Deformable bodies </li></ul><ul><li>Fluids </li></ul>engineering mechanics
  5. 5. <ul><li>Newton’s Laws of Motion </li></ul><ul><ul><li>A body persists its state of rest or of uniform motion unless acted upon by an external unbalanced force. </li></ul></ul><ul><ul><li>Force equals mass times acceleration ( F  =  m a ) </li></ul></ul><ul><ul><li>To every action there is an equal and opposite reaction. </li></ul></ul><ul><li>Statics is the branch of mechanics concerned with forces in equilibrium </li></ul><ul><ul><li>So Statics is F  =  m a and a = 0, so F =0. </li></ul></ul>statics
  6. 6. <ul><li>Forces have a direction and a magnitude, aka a vector such as 5 lb f at 20 o </li></ul><ul><li>For Bridges: Loads, Reactions & Internal Forces </li></ul><ul><li>Loads – Applied Force </li></ul><ul><li>Reactions – Newton’s 1 st Law </li></ul><ul><li>Internal Forces – Developed within members </li></ul><ul><ul><li>Truss bridges are designed so that members are in Tension or Compression. </li></ul></ul>forces 5 lb f
  7. 7. internal forces: compression
  8. 8. internal forces: compression
  9. 9. W=mg internal forces: compression N
  10. 10. internal forces: tension
  11. 11. internal forces: tension
  12. 12. internal forces: tension W=mg T
  13. 13. free body diagram
  14. 14. N 1 free body diagram F L =10 lb f N 2 <ul><li>Sum the forces in Y </li></ul><ul><li>F=ma=m*0, so EF=0 </li></ul><ul><ul><li>N 1 + N 2 – F L = 0 </li></ul></ul><ul><ul><li>N 1 + N 2 = F L </li></ul></ul><ul><ul><li>Assume N 1 = N 2 </li></ul></ul><ul><ul><li>2N=F L </li></ul></ul><ul><ul><li>N = F L /2 = 10lb f / 2 </li></ul></ul><ul><ul><li>N=5lb f </li></ul></ul>y x
  15. 15. Compression and Tension F L =10 lb f F 1 F 2 y x <ul><li>The top members are in T or C? </li></ul><ul><li>See that F 1 is greater than half of F? </li></ul>
  16. 16. Compression and Tension F 1 F 2 y x <ul><li>How are these Fs related to the last Fs? </li></ul>N 2 N 1
  17. 17. Compression and Tension F 1 y x <ul><li>Is the bottom member in T or C? </li></ul>F 3 N 1
  18. 18. free body diagram
  19. 19. <ul><li>Sponge experiment from PBS Building Big : </li></ul><ul><li>Simple truss bridge forces: </li></ul>bridges
  20. 20. <ul><li>Background image adapted under CC-SA </li></ul><ul><li>Tire Swing Barnwood Gallery (CC-SA ala Google) </li></ul><ul><li>Turtle adapted under GNU-FDL </li></ul><ul><li>This Astoria-Megler Bridge , the longest continuous truss bridge in North America. </li></ul><ul><li>Sponges adapted from PBS Building Big </li></ul><ul><li>Bridge from West Point Bridge Builder </li></ul>Credits
  21. 21. Next Steps <ul><li>Physical Bridge Curriculum </li></ul><ul><ul><li>Static misconceptions </li></ul></ul><ul><li>Digital Bridge Curriculum </li></ul><ul><li>Links </li></ul><ul><ul><li>Engineering process bridge curriculum </li></ul></ul><ul><ul><li>PBS Building Big and small exercises </li></ul></ul><ul><ul><li>Amazing manila folder curriculum </li></ul></ul><ul><ul><li>Lots of bridge links </li></ul></ul><ul><ul><li>Interesting simple analysis </li></ul></ul>