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DESIGN THE BEAMS for a cheap bridge

  1. Jul 9, 2011 #1
    Need to build a bridge 20 foot wide gully. The bridge is 4 ft wide and will be constructed of four by eight ft plywood sheet which is placed on top of 2 composite beams that are 4 ft apart (see link for picture).The 2 composite beams need to take all the load which is 500 lb/ft distributed over the length of each composite beam, (see link for the picture). The plywood are not attached. DESIGN AND BUILD THE BEAMS using any of the following materials:

    - 3 Plywood, 4 feet by 8 feet by 1/2 in thick, properties unknown.

    - 16 pine wood planks, 1 inch by 10 inch ACTUAL cross section, 25 feet length
    - 16 " " " , 2 inch by 4 inch, actual cross section, 25 feet length
    Pin has Young's Modulus of 2 MSI, and a desgin tens/compression strength of 4000 psi and a desgin shear strength of 500 psi ( dont use safety factors)
    -2000 steel nails, 0.125 in diameter, 4 inch long
    Steel has Young's modulus of elastic of 30 MSI and shear strength of 20,000 psi
    -Liquid adhesive/glue, allowable shear stress 50 psi
    - crosscut-wood-saw
    -Paper, pencil and calculator for the the design

    PICTURE: http://i51.tinypic.com/io3td4.jpg

    I have NO CLUE where to start or what I should do. If someone would explain it so that I will be able to do it.
  2. jcsd
  3. Jul 9, 2011 #2


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    You don't think you could design a simply supported beam 20 feet long which can take the specified load?
  4. Jul 10, 2011 #3
    I dont know how to start it.
  5. Jul 10, 2011 #4
    The self weight of a bridge is (usually) the largest load it carries so you start by making an assumption (guess) as to the self weight. Add the imposed loads and use structural theory to design to carry the total.
    This will give a working model that can be refined until the strength matches the loads.

    go well
  6. Jul 10, 2011 #5


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    Start your design like this:

    1. Determine the maximum bending moment for a simply supported beam with L = 20 feet and with the specified load of 500 lbs/ft.

    2. Draw the shear force and moment diagrams.
  7. Jul 10, 2011 #6


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  8. Jul 10, 2011 #7
    Studiot: I dont think you have to make an assumption for the weight bridge.

    SteamKing: I have found max moment of 75,000 and the max shear of 5000. Do you use the max shear force and use in "q = VQ/I " ?? In order to use that I will have to get the I, right?
    You would find out what q and using q and the shear strength for the nail we can find the shear stress??? Or do we use T (tau) = q/t ??

    turbo-1: Sadly that link does not help me.
  9. Jul 10, 2011 #8


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    Use the max. bending moment and the strength properties of pine to determine the section modulus necessary to support the load so that the beam is not overstressed in bending.
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