In practice, transshipment has various layouts. In this research the layout is being restricted to one inbound and one outbound door. This restriction is not realistic (in a real transshipment platform) but can be used as a baseline for other layouts. In this model an incoming semi trailer arrives at inbound door and unloads products for various destinations. If the outgoing semi trailer is going to the fine destination, the products are moved directly to outbound semi trailer (direct transit of products), in the other hand, the products are moved to a temporary storage (products in temporary storage). In studied model, the following assumptions are considered:
• Each trailer leaves the inbound door when it is fully unloaded. On the other side, each trailer leaves the outbound door when it is fully loaded.
• The internal operations of cross docking such as sorting and merging are not considered. • The storage capacity is assumed unlimited.
• Each outbound semi trailers leaves only for one destination.
• All incoming and outgoing semi trailers are available at time zero.
• The total numbers of arriving and departing products are equal.
• The products differ by their destination.
• Loading, unloading and transfer time are constant and are not considered.
• Inside incoming semi trailers there are products for different destinations while for outgoing semi trailers there are products for just one destination.
In addition, the information about type of products and the quantity of arriving products for each destination are known. For this problem, the following decision variables are considered: 1. Incoming sequence 2. Outgoing sequence 3. Unloading sequence of semi trailers 4. Unloading policy
The first two variables are obvious, but the third variable corresponds to the fact that unloading order of an incoming semi trailer contains items to be shipped in different destinations, which can influence the efficiency. Obviously, items that can be shipped in the active destination (current outgoing semi trailer) have to be unloaded first. This variable can be free or fixed (due to technical constraints for the unloading operations). For the last case, the optimal decision is usually evident. The fourth variable (unloading policy) corresponds to the following situation: an outgoing semi trailer is positioned at the output door, and items are already waiting on the ground for the same destination. The manager can choose to ship those items or wait till an incoming semi trailer arrives with items that can be shipped directly to this destination. Obviously, the loading and unloading sequences (variables 1 and 2) and products movement policies (variables 3 and 4) are two important factors which affect transshipment performance. For the fourth variable, there are two different policies. At the first policy, products already on the ground are systematically used to complete a semi trailer (fewer inventories). At the second policy, items already on the ground remain for the last semi trailer for their destination. The optimal policy is a combination of the two extreme policies. In general some times, it is better to use inventory, compare to waiting for direct transshipment. Three cases are proposed. The definition of each case is as follows: