You must build a bridge across a 20 foot wide gully. The bridge is 4 foot wide and will be constructed of 4x8 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).
1. Concrete Bridge: Design a series of closely spaced parallel box beams that will support an asphalt road surface over a 10 foot wide, 40 foot long span. The hollow box beams are made of concrete and No 9 steel rods (1.125 inch dia). Maximum beam height is 30 inches. Minimum concrete thickness is 2 inches and in regions where steel rods are used, concrete cover over the steel rods must be at least 2 inches thick. The external bridge loading is 500 lbs per square foot. Assume the asphalt surface is 2 inches thick and weighs 30 lbs per square foot. Also assume that the asphalt surface is free to slide (not attached) over the box beams.
