The development of the IP model proposed is based in some
main assumptions, which are listed below:
– (A1) The warehouse is rectangular and has docks on two sides
of the warehouse, located in opposite sides (Fig. 1);
– (A2) The reception operation is carried out only on one side of
the warehouse, while the shipping operation can be performed
at the docks located on both sides of the warehouse. This
assumes that the receiving operation is completed prior to the
picking operation, which frees the receiving docks to fulfil the
needs of the shipping operation (Fig. 1);
– (A3) It is defined a reception point from where all the picking
operations start and two shipping points, located on opposite
sides of the warehouse, where the orders are shipped (Fig. 1).
These points are located in the mid distance of the side of the
warehouse where the docks are located. The definition of reception
and shipping midpoints aims to reduce the model complexity
that would result from the computation of the distance from
each warehouse dock to each warehouse floor location. In addition,
the computation of these distances would require knowing
the assignment of the trucks to the shipping docks. For cross
docking centres, this problem is addressed in the literature by
various authors (Bartholdi & Gue, 2000; Miao, Lim, & Ma,
2009; Tsui & Chang, 1992);
– (A4) Each warehouse location is divided into two areas, one
space for products in carton boxes and another area for products
in plastic boxes, which are defined by their capacity;
– (A5) Each warehouse location is defined by one overall capacity
(which is subdivided in capacity per area dedicated to one type
of logistic unit), the minimum operational distance to one
reception point and the minimum distance to one of the shipping
points;
– (A6) The overall layout of floor locations vs. access areas is
known and may have any configuration (i.e., it is not required
to have a specific orientation as in Fig. 1);
– (A7) During the picking operation, each employee can visit, to
distribute the contents of one pallet of one product, several
warehouse locations within the same picking route, fulfilling
the demand of more than one store.