We assume storages and retrievals are performed in single command cycles and all products are stored and
transported on identical storage media (e.g., pallets or totes). Class-based storage policy is used. An open location
for an incoming load is selected randomly within its class, i.e., each location is uniformly utilized and the
assigned products are distributed homogeneously in the space allocated for the class. This assumption implies
that the geometric centre of the class is the same as the load center. We assume that inventory decision has
been made independently to the storage decision and all times required in the storage/retrieval process, except
travel times, are considered independent of storage allocation. We also assume that, there is no congestion
between vehicles/cranes and loads are not relocated. All the parameters of the models are deterministically
known and the travel time between any two positions is readily computed. Once the storage locations are
assigned they cannot be reshuffled during the planning horizon and we assume there is a single input/output
point.
Given the layout of storage area, period-wise planned inventory levels for given products and product
demand, the problem is to establish classes of products and allocate them to storage locations so that total
cost of order picking/handling and storage space is minimized in a single command warehouse exploiting
the saving in required storage space because of clubbing products into classes. In this section, we establish
notation and then formulate a model for the problem defined.