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Welcome Back. EA 2 January 2011. Common Truss Designs. Bridge Building. Design Requirements. Needs to span 24 meters Needs to be .4 meters wide Needs to be designed for a “traffic load” of 240 kg Your factor of safety needs to be 2.0 Bridge made of standard manila folders

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  1. Welcome Back EA 2 January 2011

  2. Common Truss Designs

  3. Bridge Building

  4. Design Requirements • Needs to span 24 meters • Needs to be .4 meters wide • Needs to be designed for a “traffic load” of 240 kg • Your factor of safety needs to be 2.0 • Bridge made of standard manila folders • 1/40 scale model bridge • Your bridge will have a 60 cm span • Your bridge will be 11 cm wide • Your bridge will be designed for 6 kg (but remember the factor of safety)

  5. The Plan • Decide on a truss configuration. • Use either a Warren/Pratt/Howe Truss • Pratt Through Truss (Span=6L, Height=0.75L) • Pratt Through Truss (Span=6L, Height=1.25L) • Howe Through Truss (Span=6L, Height=1.25L) • Warren Through Truss (Span=6L, Height=1.25L) • Pratt Deck Truss (Span=6L, Height=L) • Create the structural model (on paper) • Check static determinacy and stability (remember your equation) • Calculate reactions. • Calculate internal member forces. • Determine member sizes. • Check member sizes for constructability. • Design in Solid Edge (with gusset plates – create half of the bridge) • Create a schedule of truss members and a schedule of Build the bridge.

  6. Create a model in Solid Edge

  7. Loading Methods

  8. Factor of Safety / Strength Factor of Safety Strength = (Factor of Safety)(Internal Member Force) For Design Purposes it makes sense to use this equation: Strength > (Factor of Safety)(Internal Member Force)

  9. Example Problem Using a Warren Deck Truss to span 60 cm and carry 6 kg With a span of 6L and a Height of 1.375L. Calculations: L = 10 cm / H = 13.75 cm / W = 58.86 N

  10. Static Determinacy / Reactions • Check Static Determinacy and Stability • 2j = m + 3 • j = 13 and m = 23 • Works • Calculate Reactions RA = RG = (3 * 9.81N / 2) = 14.7 N

  11. Calculate Internal Forces FAB = 9.83 N Compression FCD = 23.3 N Compression FAH = 17.7 N Tension

  12. Determine Member Size • FAB = 9.83 N Compression. Use a tube • FCD = 23.3 N Compression. Use a tube • FAH = 17.7 N Tension. Use a bar • Selecting the required tube size is a four-step process: • Determine the member length. Member AB is 10 cm long (remember our truss) • Calculate the required strength, using the equation • Required strength = (Factor of Safety)(Internal Member Force) • Required strength = (2)(FAB) = (2)(9.83N) = 19.7 N Compression • This tells us that Member AB must be a tube with a compressive strength • of at least 19.7 N • 3) Remember your plots, use the plots and find the point that corresponds to length = 10 cm and strength = 19.7 N • 4) Determine the smallest available tube size that had a strength larger than 19.7 for the same length.

  13. Compression Data of Tubes We can safely use a 6 mm x 10 mm tube for member AB. We could have also used a 10 mm x 10 mm tube with a compressive strength of about 50 N but it does not need to be that strong (or that expensive – more material drives up the costs) We could have also used a smaller tube but we do not have the data available to make that determination.

  14. Homework • In your EA notebook calculate the member size needed for member CD. FAB = 9.83 N Compression. Use a tube FCD = 23.3 N Compression. Use a tube FAH = 17.7 N Tension. Use a bar

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