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Bridges

Bridges. Trusses. Trusses. Before 1800 almost all bridges were made from stone arches The expansion of the railroad, in particular the USA transcontinental railroad led to the need for the building of cheap, quick bridges made with readily available raw materials.

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Bridges

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  1. Bridges Trusses

  2. Trusses • Before 1800 almost all bridges were made from stone arches • The expansion of the railroad, in particular the USA transcontinental railroad led to the need for the building of cheap, quick bridges made with readily available raw materials. • These were initially made from wood, but steel and iron were eventually used • But they were all of a Truss construction

  3. Trusses • Trusses are an economical method of building structures that are required to carry load over long distances • They use the tensile and compressive strength properties of steel and wood

  4. Trusses • Early Truss bridge builders learned through a process of trial and error leading to a set of Rational Design rules • One of the early successful truss designs was the Pratt Truss • Here you can see the load applied though W, with the reaction of the abutments on either side being R • Note ∑W must always=∑R and the job of the truss is to transfer the load efficiently from the bridge deck to the ground

  5. Trusses • There the truss member, the chords take the load and depending on the nature of the truss these may be in either tension or compression • This is an important concept as materials behave differently under compression or tension

  6. http://andrew.triumf.ca/andrew/popsicle-bridge/

  7. Trusses • Compression failure or buckling is a common mode of failure as when buckling begins, alignment becomes skewed and rapid failure may then occur • Buckling may be prevented by making the stick more rigid. Traditionally this is done with e use of box girders or I beams • In out case we need to make stiff laminates http://andrew.triumf.ca/andrew/popsicle-bridge

  8. Trusses • There is a near infinite number of truss variations here are the most common • The Howe • The Pratt • The Warren • Design your own • http://www.jhu.edu/virtlab/bridge/truss.htm

  9. Bridge Stress Scenarios

  10. Bridge BuildThe Challenge Your goal is to build a bridge using 120 popsicle sticks and hot glue – all supplied by Rooney. The Rules are: • Maximum of a total of 120 popsicle sticks. • You will be given a length of balsa wood that can be used for the bridge deck • I must be able to drive a 3 inch wide car across the bridge • The bridge must fit on the Rooney test rig • The bridge will be 24 inches wide. • I must be able to place a 2 inch dia. plunger through to the center of the deck to apply load. • Glue is used only for gluing purposes and not to artificially strengthen the bridge.

  11. Bridge BuildThe Challenge I would recommend you take the following steps: • Fully understand how the JHU bridge applet works • Test different designs using the model • When you are happy with your design draw a full scale plan • Use this plan as a construction guide for your build • Sort through your popsicle sticks and pick the best • Carefully build your side members • Glue can only be used for gluing – not for artificial strengthening • Assemble your bridge • THINK ABOUT THE LOAD BEING APPLIED!!!!!!

  12. Bridge BuildThe Report • Introduction • Design – your design and the reasons you made your decisions • Include copies of your work • Build - How well did it go? did you encounter any problems? • Testing - What happened to your bridge on testing • How did it fail? • How did it compare to the best? • Future - What would you do different next time?

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