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.