What is a crossdock?

A crossdock serves several purposes. Most immediately it is a consolidation point that helps reduce transportation costs. For example, consider The Home Depot, which experienced phenomenal growth through the 1990’s, at one point opening a new store every 40 hours. When they had only a few stores, it made sense to ship from every vendor directly to every store by less-than-truckload (LTL). However, such a strategy made less sense as they grew, for the transportation costs increased as the product of the number of vendors and the number of stores. With 1,500 vendors, each new store requires another 1,500 direct shipments.

The Home Depot realized that it could significantly reduce transportation costs by opening crossdocks. Now vendors ship to the crossdock, where freight is reorganized and shipped to the stores. When a new store is opened, only one new shipping lane needs to be opened, rather than 1,500. Furthermore, each vendor is now shipping material for multiple stores and so may be able to ship full truck loads, which are much cheaper than LTL. Similarly, the crossdock may be able to ship full truck loads to each store, which is not only cheaper, but also simplifies receiving at each Home Depot retail store.

A crossdock may be thought of as a high-speed warehouse. What enables this speed is the fact that when product arrives at a crossdock it is already bespoke; that is, each item already has a known destination on arrival. Consequently, each item can flow directly from receiving to shipping, bypassing the intervening processes typical of most warehouses (put-away and retrieval).

Product is not stored in a crossdock but it is handled. The opportunities for improvement within the walls are primarily in material-handling, to reduce the labor required to move freight in and out.

How to design a crossdock

Size

The first decision is “how many doors?”.

Generally doors are devoted to one of two types of trailers:

• Incoming, from which freight must be removed; and
• Outgoing, in which freight must be loaded

It is easier to unload than to load. A loader must try to get a tight pack and so may have to dock freight and this double-handling slows him down. A good rule of thumb is that it takes twice as much work to load a trailer as to unload one.

To achieve frictionless flow, you must match the flow into the dock with the flow out of the dock. You can have twice as many outgoing doors as incoming doors; or you can assign pairs of workers to load each trailer and so have equal numbers of incoming and outgoing doors.

Note, however, that crossdocks with many doors are generally less efficient than crossdocks with fewer doors. The reasons are as follows. A door can only have a few near neighbors on a dock and so a dock with more doors means that each door is likely to have few more near neighbors but many more distant neighbors. This means that in general freight must move farther across a large dock. Consequently, labor costs are generally higher at larger docks.

An additional factor is that on larger docks more freight flows past the central doors, which, are the most important because they tend to be close to many doors. In fact, the total flow of freight past a centrally-located door tends to be proportional to the square of the total number of doors. Therefore a dock with twice the doors tends to have 4 times the congestion in front of its central doors, which diminishes their value.

Do not forget to allow enough parking space in the yard for two trailers for every door. This means that for each origin or destination you can have a trailer at the door plus one full and one empty in the yard. This helps you handle surges in freight flow.

The less-than-truckload (LTL) transportation segment runs the most intensive crossdocking operations. Companies such as American Freightways, Yellow Transport, and Old Dominion Freight Lines operate hundreds of crossdocks each in North America. Large retailers such as Wal-Mart and The Home Depot also operate systems of crossdocks.

A small LTL crossdock may have ten doors; the largest that we have seen, a terminal in Dallas, TX operated by Central Freightways (a regional LTL carrier), has 553 doors. (See Figure 5.). The manager must ride a motorcycle on the dock.

Shape

Most crossdocks are long and skinny rectangles, shaped like the letter I. The reason for this is so that freight can be unloaded from an arriving trailer and carried the short distance across the dock to be loaded into an outbound trailer (across the dock). An I-shaped dock seems natural, but there are crossdocks in other shapes, some rather unusual:

Which shape is best? That depends on many issues such as the shape of the plot of land on which the crossdock sits, the patterns of freight flows through the terminal, the material-handling systems within, and so on. Nevertheless some general observations can be made.

The first observations is that very large docks should probably not be I-shaped. Imagine growing a dock as business increases. It would be mistaken to grow the dock wider because then carrying freight across the dock would become more expensive. Instead, it makes sense to grow the dock at the ends (that is, grow a taller I-shape). However, the distances between the farthest doors increases linearly with the number of doors; and the traffic in front of the central doors increases as the square of the number of doors. Consequently, an I-shaped dock will become less efficiant as it grows.

This suggests that large docks should be some shape other than an I, so that there continue to be enough central doors. Shapes such as an X offer more central doors; but it is important to note that each corner has an operational cost. Each interior corner forfeits about six doors and increases the chances of trailer moves interfering with each other in the yard. Because doors are lost the dock must be longer to accomodate a given number of trailers, which means that freight has farther to travel on the dock.

Similarly, every exterior corner loses floor space that might have been used for staging freight. This increases the congestion on the dock, which interferes with the flow of freight.

So: if a dock has many doors, an I-shape will be inefficient and a more complicated shape, such as a T or H or X will have more central doors. However this is offset somewhat by the inefficiencies of the additional corners. For example, an X-shaped dock will lose many door positions because it has four interior corners. This means that, for a given number of doors, a dock in the shape of an X must be larger, which increases travel time for freight.

You can experiment with these issues on the layout below, which is live: Click on it and drag the control points to change the design and the statistics will be updated automatically. The maximum and average door-to-door distances give information about how far freight must travel across the dock.

In the figure below, the trailers are standard: 53 feet long and 9 feet wide. The dock doors are 12 feet apart.

Can you find the best dock with 80 doors? 140 doors? 200 doors? Is one shape always the best?

<applet codebase=”./tools/java/” archive=”crossdock.jar” code= “CrossdockDesigner.class” width=”100%” height=”400”> <param name=”UNIT_OF_DISTANCE” value=”feet” /> <param name=”TRAILER-LENGTH” value=”53.0” /> <param name=”TRAILER-WIDTH” value=”9.0” /> <param name=”DISTANCE_BETWEEN_DOORS” value=”12.0” /> <param name=”d1” value=”20” /> <param name=”d2” value=”0” /> <param name=”d3” value=”8” /> <param name=”d4” value=”0” /> <param name=”d5” value=”30” /> <param name=”d6” value=”8” /> <param name=”d7” value=”30” /> <param name=”d8” value=”15” /> <param name=”d9” value=”8” /> <param name=”d10” value=”15” /> <param name=”d11” value=”20” /> <param name=”d12” value=”8” /> <param name=”x” value=”8” /> <param name=”y” value=”10” /> Your browser does not support Java; consider upgrading… </applet>

Assignment of trailers to doors

It matters where you park a trailer. If the trailer is filled with freight bound for Miami then it is good to park the trailer near a another that is bound for Miami, because then freight can be more quickly moved transferred.

This matters because there may be hundreds of items on the incoming trailer and each of them must be transported to its outgoing trailer. On a large I-shaped crossdock, say over 220 doors, the distance from one end to another is more than 0.3 mile (0.5 km). Most crossdocks in North America have little automation and so freight is likely to be moved manually.

So where should a trailer be parked for loading? Generally, certain doors are reserved for specific outgoing trailers, so that workers can always be sure to which door Miami-bound freight should be sent. On the other hand, unassigned doors are used to park any incoming trailer.

The layout shown in Figure 7 is typical. The dock is I-shaped, with each small square representing a door. The unshaded squares represent doors assigned to specific outgoing doors and the bar on each is proportional to the amount of freight bound for that destination. For example, the longest bar, on a door at the lower mid-left, corresponds to the Miami door, which received the most freight from this Atlanta terminal. The solid squares represent doors at which incoming trailers were parked.

There are at least three things wrong with this layout.

• Travel is excessive. Because the outgoing doors are concentrated at the ends, there is little opportunity to unload freight and carry it the short distance across the dock to an outoging trailer. Instead, most of the travel is left-to-right, along the longest dimension. This is not a crossdock!
• There is certain to be congestion in the corners, because all the outgoing trailers must compete for floor space to dock freight; but the corners have the least space available.
• The dragline will be congested because of the long banks of incoming doors: Any door to the downstream side of a bank will have a hard time finding an available cart or space on the dragline. The effect is to reduce the effective capacity of the dragline.

You may read more about this in the text book.