I am installing two wine racks in a storage closet, and want them 15-18 inches off the floor so I can store boxes under them. The racks are 23.25 inches deep, 77.125 inches tall, and the total span will be 104 inches.
What would be the best material for the support shelf under the racks, how thick should it be, and what is the support spacing I will need under the shelf to minimize sag and hold the necessary 4,000 lbs?