Design a steel box structure (Figure P12-7) 4 ft wide 8 ft long made of plates

Design a steel box structure (Figure P12-7) 4 ft wide × 8 ft long made of plates to be used to protect construction workers while working in a trench. That is, determine a recommended thickness of each plate. The depth of the structure must be 8 ft. Assume the loading is from a side load acting along the long sides due to a wet soil (density of 62.4 lb/ft3) and varies linearly with the depth. The allowable deflection of the plate type structure is 1 in. and the allowable stress is 20 ksi.

Figure P12-7