Freight terminals are places where shipments are classified, consolidated, possibly stored for a short time, and moved from incoming to outgoing vehicles. These include LTL terminals, crossdocks, package-handling terminals (such as those of UPS and FedEx) as well as rail, port and airport terminals. These facilities share several features and consequently their design and management can be approached using common methodologies. However, there are also significant differences among them. In this section the analysis is restricted to terminals where material handling is labour inten- sive (e.g. loads are moved by forklifts). This is the case, e.g. of LTL terminals and crossdocks, where, as packages often have different sizes, it is difficult to make use of automatic equipment.
6.8.1 Design issues
The first design decision that has to be made is how manydoors(orgates) a terminal should have. Crossdocks have two kinds of doors:receivingdoors andshippingdoors (see Chapter 1). In both cases, a door is assigned a share of floor space (see Fig- ure 6.18). The numbers of receiving and shipping doors are customarily set equal to the number of destinations that have to be served, although high-volume destinations may be assigned more than one door in order to accommodate multiple trucks at the same time. In LTL terminals, the number of doors may be estimated by means of simple formulae like those used in Chapter 5 for calculating the number of docks of a warehouse. A better evaluation can be done through simulation models once other design variables have been set.
Another key design decision is related to the choice of the terminal shape. Terminals may have hundreds of doors so that distances travelled by workers in a trip may exceed several hundred metres. It is then crucial to choose a shape that minimizes the expected overall workload for a given number of doors. In practice, docks in the shape of anI,L,TandHare quite common. In order to compare the different shapes, two performance measures (named thediameterand thecentralityof a terminal) have been introduced. The diameter of a terminal is the largest distance between any pair
LONG-HAUL FREIGHT TRANSPORTATION 237 Table 6.15 Characteristics of the most common terminal shapes.
Number of corners Shape Centrality (Inside−Outside)
I 4/2=2 0−4
L 4/2=2 1−5
T 6/2=3 2−6
H 8/2=4 4−8
of doors. The centrality is the rate at which the diameter grows as the number of door increases. For anI-shaped terminal the diameter is 2. In fact, if two doors are added at each end, the diameter increases by two doors so that the centrality is 4/2 = 2.
The centrality of the most common shapes are reported in Table 6.15. Larger values of centrality are better for two reasons: the workload is smaller and the forklift traffic congestion is lighter. For example, for anI-shape, forklift traffic may be very heavy in the middle of the terminal (in fact, it varies with the square of the number of the doors) while for anH-shape it is much lighterceteris paribus. On the other hand, designs different from theI-shape show a deterioration in efficiency due to the higher number ofcorners.Cornersreduce the potential number of doors along the perimeter for two reasons (see Figures 6.18 and 6.19). First, each door needs a suitable amount of floor space, otherwise there is an interference between adjacent doors. Second, where orthogonal segments of a terminal join, doors are unusable because vehicles would overlap otherwise. As a result, for a given number of doors, the terminal has to be larger if the number of corners increases.
The previous considerations suggest discardingL-shape designs because they have the same value of centrality as theI-shape but more corners. Simulation-based studies have shown that theI-shape is best for small to mid-sized terminals. TheT-shape is best for terminals with about 150 to 250 doors, while for larger terminals theH-shape should be selected. Figure 6.20 illustrates qualitatively the expected workloads for theI,TandH-shapes. The exact breakpoints depend on the material flow pattern and on how vehicles are assigned to doors.
6.8.2 Tactical and operational issues
At a tactical or operational level, one must decide how vehicles should be assigned to gates. Such a decision is influenced mostly by the type of terminal. In rail terminals and airports, vehicles are dynamically assigned to doors, while in crossdocks doors are permanently labelled as receiving or shipping doors. Moreover, in a crossdock, doors do not usually change designations because this allows workers to be more efficient when handling freight.
For a crossdock, a good quality assignment can be obtained through the following constructive heuristic (alternating heuristic), possibly followed by a local search procedure.
238 LONG-HAUL FREIGHT TRANSPORTATION
Share of floor space
Share of floor space Congestion
Congestion
Figure 6.18 I-shaped terminal.
No doors
Congestion
Congestion
Figure 6.19 L-shaped terminal (unusable doors are shadowed).
Step 1. Sort doors by nondecreasing average distance to all other doors.
Step 2. Sort outbound trailers by nonincreasing freight flows.
Step 3. Assign alternatively the busiest inbound vehicle (trailer) and the busiest out- bound trailer to the best locations still available. If there exist some more trailers to allocate, repeat Step 3, otherwiseSTOP.
Blue Freight has aT-shaped crossdock with 200 doors in Denver (Colorado, USA).
By applying the alternating heuristic the solution shown in Figure 6.21 is obtained.
LONG-HAUL FREIGHT TRANSPORTATION 239
Workload
Doors
100 200 300 400
I T H
Figure 6.20 Expected workloads for theI,TandH-shapes.
Incoming trailers
Incoming trailers
Incoming trailers
Figure 6.21 Trailer allocation provided by the alternating heuristic in the Blue Freight crossdock (receiving doors are bold).