You must build a bridge across a 20 foot wide gully. The bridge is 4 foot wide a

Question

You must build a bridge across a 20 foot wide gully. The bridge is 4 foot wide and will be constructed of 4 times 8 foot plywood sheets placed on top of two composite beams that are about 4 feet apart. Design the bridge so that the two composite beams take all the load, which is equivalent to 500 lbs/ft distributed over the length of each composite beam. The plywood lays on the composite beams, but is not attached (cheap bridge). Your task is to design and build the beams. You have just arrived on one side of the gully with a truck that has the following materials on board. Plywood, 4 times 8 ft thick 3 places, properties unknown Pine wood planks, 1 times 10 actual cross section, 25 ft length, 16 pieces Pine wood planks 2 Time 4 actual cross section, 25 ft length, 16 pieces Pine has a modulus of 2 Msi and a design tens/compression strength of 4000 psi and a design shear strength of 500 psi (do not use safely factors) Nails, steel, 0.125 inch diameter, 4 long, 2000 jails Steel has an elastic modulus of 30 Msi and shear strength of 20000 psi Liquid adhesive/glue, allowable shear stress 50 psi Cross cut wood saw Hammer Brush (for spreading the liquid adhesive) Paper tablet, calculator and sharp pencil (for doing the design). Paper tablet, calculator and Design the wood beams and provide a sketch showing me how to build it (i.e. show me the composite beam cross section, and where the nails/adhesive are placed. Prove to me it will work.)

Explanation / Answer

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.

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.

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