Within-aisle or across-aisle? Optimisation and comparison of two class-based storage policies in multi-dock unit-load warehouses

As E-Commerce develop, customers' tight delivery requirements are driving warehouses to operate more efficiently. This paper studies two types of class-based storage policies for a multi-dock unit-load warehouse: the within-aisle policy and the across-aisle policy. The aim is to determine the optimal class boundary and explore the dominance situation (i.e. which policy performs better in which situation). We first develop an expected response distance model for each policy based on a general layout of multiple docks. Then, closed-form optimal first zone boundary and warehouse dimension are given for the across-aisle policy. For within-aisle policy, an efficient algorithm is developed (based on the proved unimodality property) for obtaining its optimal first zone boundary. Besides, each policy's dominance situation is analysed for two typical situations. Numerical results show that both policies can bring up to 60% performance improvement compared with the random storage policy. Impacts of docks layout and warehouse dimensions on each policy's dominance situations are illustrated. It shows that the across-aisle policy performs better in the situation where the docks' and wall's centrelines are adjacent, and in the situation where the warehouse is narrow and deep. Managerial insights on policy selection and dock layout design are presented for warehouse managers.