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Bridge Basics

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Bridge Basics

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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 http://www.dyfference.org program.

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 http://www.dyfference.org 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>

Editor's Notes

  • Compare a straw in the shape of a diamond with the same straw in the shape of a triangle.
  • What is common between all these common truss bridges? From wikipedia: “A truss bridge is a bridge composed of connected elements (typically straight) which may be stressed from tension , compression , or sometimes both in response to dynamic loads. Truss bridges are one of the oldest types of modern bridges. The basic types of truss bridges shown in this article have simple designs which could be easily analyzed by nineteenth and early twentieth century engineers. A truss bridge is economical to construct owing to its efficient use of materials.” Image based on Based on “Truss Identification: Bridge Types,” Historic American Engineering Record HAER T1-1, National Park Service, 1976. from
  • F  =  m a and a = 0, so F =0 …. How easy is that! Statics is the study of stuff that is not undergoing any acceleration. Often people say “Statics is the study of stuff that isn’t moving”, which isn’t accurate… stuff can be moving, but it must be at a constant velocity. In conclusion: In University we spend an entire semester on one simple equation, F =m a …. And a is zero.
  • Have students push hands against one another. Ask what they feel. Image from http://upload.wikimedia.org/wikipedia/commons/1/12/Forbidden-City-Bronze-Tortoise-4017.jpg
  • Have students push hands against one another. Ask what they feel. Image from http://upload.wikimedia.org/wikipedia/commons/1/12/Forbidden-City-Bronze-Tortoise-4017.jpg
  • Have students push hands against one another. Ask what they feel. Image from http://upload.wikimedia.org/wikipedia/commons/1/12/Forbidden-City-Bronze-Tortoise-4017.jpg
  • Have students hold hands with another and pull slightly. Ask what they feel. Image from Tire Swing
  • Have students hold hands with another and pull slightly. Ask what they feel. Image from Tire Swing
  • Have students hold hands with another and pull slightly. Ask what they feel. Image from Tire Swing
  • You can use moments to prove N1=N2. N1 causes a clockwise rotation about the finger, N2 a counter clockwise. If N1 and N2 are equidistance from the center line, then N1 = N2.
  • Reaction forces have an y and an x component, therefore total reaction force is greater than just the y component. Efy=0 F1*sin( θ ) + F2*sin( θ )-FL=0 Do Efx=0 to see that F1 = F2 as a scalar 2*F1*sin( θ )-FL=0 F1=F2=FL/[2*sin( θ )] So at 90 degrees, F1=F2=5 So at 60 degrees, F1=F2=5.77 So at 90 degrees, F1=F2=7.07 So at 30 degrees, F1=F2=10 So at 15 degrees, F1=F2=19 So at 5 degrees, F1=F2=57
  • Reaction forces have an y and an x component, therefore total reaction force is greater than just the y component. Efy=0 F1*sin( θ ) + F2*sin( θ )-FL=0 Do Efx=0 to see that F1 = F2 as a scalar 2*F1*sin( θ )-FL=0 F1=F2=FL/[2*sin( θ )] So at 90 degrees, F1=F2=5 So at 60 degrees, F1=F2=5.77 So at 90 degrees, F1=F2=7.07 So at 30 degrees, F1=F2=10 So at 15 degrees, F1=F2=19 So at 5 degrees, F1=F2=57
  • This must all be backed up with a straw experiment.
  • An example of a homework problem from a University Engineering Statics class.
  • http://www.pbs.org/wgbh/buildingbig/educator/act_mini_beam.html Show a video clip of stresses in a bridge from the software package. Show textbook of bridge forces. Predict compression and tension in sponge. Predict compression and tension in the truss bridge. Simple bridge photos are screenshots from West Point Bridge Builder. (http://bridgecontest.usma.edu/) blue tension, red compression
  • The Astoria-Megler Bridge , the longest continuous truss bridge in North America. It&apos;s 6.5 km (4.0 miles) long, and the last piece of the Olympia-Los Angeles continuous stretch of Highway 101. http://picasaweb.google.com/lh/photo/HBtUC8pXPDES3sRQFT5KHg
  • Connecting to standards Standards: Grade 8: Physical sciences:Forces:b,c,E Grade 9: Physics:Motion and Forces:B,d Way easier to add more, by doing the math and analysis.

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