Travel Models for Warehouses with Task Interleaving

Unit-load warehouses receive and ship material or products in single discrete units, usually as pallet loads. They are commonly found in industry; examples include third-party transshipment warehouses, grocery and beverage distributors, appliance manufacturers and import distribution centers. Many break-bulk facilities, which break down a pallet into smaller volume packages for shipment, also have a portion of their activity dedicated to unit-loads, such as the reserve area that is used to receive and store pallets until they are needed to replenish a forward, or fast-pick area [1]. Travel from a common pickup/deposit (P/D) point to a single pallet location and back again to execute a storage or retrieval request is referred to as a single-command cycle. Since interleaving storage and retrieval requests (referred to as task interleaving or a dual-command cycle when the cycle includes one storage and one retrieval request) makes more efficient use of time and resources, and contemporary unit-load warehouses utilize warehouse management systems to schedule activities, the use of dual-command operations is more common than in the past. Even with the more efficient dual-command cycles, storage and retrieval operations are labor-intensive and account for a significant portion of warehouse operating costs, particularly in the very large unit-load warehouses of today. For this reason, we focus on optimizing the design of unit-load warehouses by minimizing expected travel distances. Figure 1 shows three conventional warehouse designs, with parallel picking aisles and orthogonal cross aisles. Layout A has two cross aisles, while Layouts B and C have three cross aisles.