Layouts of Unit-Load Warehouses with Multiple Slot Heights

We describe a method to generate layouts for unit-load warehouses that use multiple slot heights as a way to maximize warehouse space utilization. The problem has two parts: slots must be arranged into rack-bays, and rack-bays must be arranged into a layout. We describe two methods for the first subproblem, depending on whether the warehouse has directed picking and put-away. For the second, we describe a simulation model and a greedy procedure based on the duration-of-stay storage policy. We observe significant benefits of using multiple slot heights in unit-load warehouses with respect to footprint, expected travel time, and cost of racking. For a typical warehouse, we expect space savings between 25% and 35%, depending on the number of slot types, and savings of between 15% and 25% in annual operating costs.