Crossdocks are high speed warehouses. If an arriving item has already been requested by a customer there is no need to store it as anticipation inventory; instead, the item can move directly from receiving to shipping, without intermediate storage and retrieval. Thus the item can move much more quickly through the facility and the most costly part of warehouse labor can be avoided.
In a high-volume crossdock the turnover times may be measured in hours. To support this velocity of movement, a crossdock may be nothing more than a slab of concrete with a roof and walls punctuated with doors for trailers. Freight is pulled off arriving trailers, sorted and loaded onto departing trailers without intermediate storage.
There is little or no storage provided in a crossdock because items do not stay long enough; but there is generally a lot of material-handling equipment, such as forklifts and pallet jacks, to move freight. Labor is frequently the main cost and it is devoted to unloading incoming trailers, moving the freight to the appropriate outgoing trailers, and loading. Consequently, the issues within a crossdock are those of material-handling and product flow rather than location and retrieval.
The biggest reason to have a crossdock is to reduce transportation costs.
This can be achieved by consolidating multiple shipments so that full truck loads can be sent. The Home Depot is a major retailer and the largest user of Less-than-Truck-Load (LTL) shipping in North America. (LTL means sending shipments that do not fill a trailer and so are not economical to send by themselves. Instead, an LTL freight company consolidates many such shipments and so achieves efficiencies.) At the present writing, LTL costs about twice the cost of Truck Load (TL) shipping, so there is a strong incentive to fill trailers. The Home Depot has begun doing this by having vendors ship full trailers to its crossdock. (The trailers are full because they hold product for many stores.) At the crossdock the product is sorted out for individual stores and consolidated with product from other vendors bound for the same store. The result is that each store has enough freight that it or it and a few close neighbors generate a full truck load from the crossdock. The result can be considerable savings.
Additional benefits include less inventory (because all product flows right through) and less labor (because product does not have to be put away and later retrieved).
Most crossdocking freight terminals are laid out as long, narrow warehouses with doors around the perimeter.
Figure : View from above of a typical high-volume crossdock, which receives freight, sorts, and disgorges it. Each door is devoted to either arriving trailers, which are unloaded, or to departing trailers, which are loaded. Ideally, freight should flow directly across the dock rather than along its length.
A typical terminal, where the small shaded rectangles represent incoming trailers with freight to be unloaded, and small clear rectangles represent (empty) outgoing trailers. Terminals range in size from fewer than 10 doors to more than 500 doors.
Inside a terminal, a variety of material handling methods is used to transport freight. Forklifts and palletjacks carry heavy or bulky items, and carts transport smaller items. In addition, large terminals may have draglines, which circulate carts around the inside perimeter of the dock.
There are two types of doors in a terminal: receiving, or strip, doors, where full trailers are parked to be unloaded, and shipping, or stack, doors, where empty trailers are put to collect freight for specific destinations. Once established, the designations of these doors do not change, although the trailers parked at them will. A shipping door always receives freight for the same destination. A receiving door may be occupied by any incoming trailer, regardless of its origin or contents.
Arriving trucks may deliver their trailers directly to an unoccupied receiving door; or, if none is available, they may place them in a queue. After the trailer is backed into a receiving door, a worker unloads the freight. After unloading items of a shipment onto a cart, the worker walks to the destination trailer and loads the items into that trailer; or he places the cart on the dragline, if the terminal is so equipped. To handle pallet loads, the worker uses a pallet jack, or hails a forklift driver, or finds a forklift and delivers the load himself.
After a trailer has been completely stripped, a driver replaces it with another incoming trailer from the queue of trailers waiting to be stripped. After an outgoing trailer has been filled, a driver replaces it with an empty trailer to be filled with freight for the same destination.
The patterns of freight flow within a terminal—and therefore the work—are determined by:
by which we mean the specification of doors as either receiving or shipping doors and the assignment of destinations to the shipping doors.
The shape of a terminal determines the travel distances between doors and the susceptibility to congestion. (For example, narrow docks tend to be more congested because workers have less room to manoeuver.)
Material handling systems
For example, pallet jacks are slower than forklifts, but they may be more available; draglines reduce walking time, but can impede forklift travel.
For example, terminals having a higher mix of pallet freight require more forklift travel than those receiving a majority of carton freight.
Scheduling In real time, the dock supervisor determines freight flow patterns by assigning incoming trailers to receiving doors.
Changing the geometry or material handling systems of a terminal is expensive; changing the freight mix is a marketing decision with implications outside the terminal. The two remaining ways to take work out of the system—change the layout or change the scheduling—are inexpensive. In particular, the layout can be changed simply by changing the labels on the doors of the crossdock.
There are two kinds of doors on a typical crossdock: Those reserved for outgoing trailers (for example, the “Miami trailer”) and those reserved for incoming trailers. The outbound doors are reserved for specific destinations but the incoming doors are not so specific and may be used by any incoming trailer (because, while departures are scheduled to specific destinations, the terminal does not have full control over arrivals).
As more freight flows across a dock, congestion increases, which interferes with the flow.
There are several distinct types of congestion on a crossdock:
Competition for floor space: Freight may be docked outside a receiving door if, for example, it consists of many unpalletized cartons going to the same shipping door. Then there is an incentive to accumulate it all so that fewer carts must travel to the destination door. On the other hand, freight is very likely to be docked outside a shipping door while the loader figures out how to pack the trailer tightly. When several nearby doors compete for space to dock freight, some invariably interferes with other traffic. At the very least, it takes longer for a worker to manoeveur through the docked freight.
The effects of docked freight are most severe near the inside corners of the dock, where there is less space per door, as shown in this figure.
Figure: There is less floor space per door at outside corners and therefore more likely to be congestion that retards movement of freight.
The need to dock freight suggests that busy outgoing trailers be parked away from the corners of the dock.
Interference among fork lifts: Despite the intention of moving freight simply “across the dock”, most doors will be to the left or right of a door with an incoming trailer and so a significant amount of freight must travel along the length of the dock. Most crossdocks set up two forklift “highways”, one along each long side of the dock. (It is a good idea to set up two so that, when one is blocked, some freight can still flow.) However, the flow of forklifts back and forth along the length of the dock may be interrupted by forklifts making left hand turns into doors with outgoing trailers. This effect can be reduced by parking busy outgoing trailers away from the very middle of the dock (which is also the most convenient location). Note that this works opposite to convenience, which tends to push busy outgoing doors towards the middle of the dock.
Competition for drag line capacity: Each door receiving arriving trailers will need empty carts from the dragline and, after loading a cart, will need empty cart positions on the dragline. This means that there will be diminished dragline capacity downstream of this door. If the door is far from a busy outgoing door then the region of diminished capacity can be large. This creates an incentive to intersperse incoming doors with outgoing doors. In particular, this suggests that current practice, which is to create large banks of incoming doors, reduces the capacity of the dragline.
The first decision in designing a crossdock 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, the capacity for flowing freight into the dock must be balanced with the capacity to flow freight out of the dock. Accordingly, one should plan to have twice as many outgoing doors as incoming doors. Alternatively, one can balance the rates of flow by assignment or workers. For example, if there are equal numbers of incoming and outgoing doors, balance can be achieved by assigning twice as many workers to load. 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.
This follows from the following simple model: Imagine a rectilinear dock as a line with 2n doors (numbered from left to right), and assume that equal amounts of freight move between every pair of doors. Then the flow into any door is of intensity O(n). But the total flow passing the area between door i and i + 1 is i(2n – i), which means that the greatest total flow passes by the middle of the dock, door n, past which flows O(n2) units. But these central doors are exactly those that are nearest to most other doors and therefore are the best locations! Thus, as a dock design grows in length, the lengthwise traffic past the central doors increases rapidly while traffic directly across the dock remains unchanged. Increased traffic means congestion, which helps explain why docks can lose their efficiency as they grow. There are few docks larger than about 200 doors. Most are 80–120 doors long.
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.
What is a good shape for a crossdock? In general, one wants to enable efficient flow of freight from incoming trailers to outgoing trailers.
Typically, a crossdock is a long rectangle, with doors for trailers around it. The capacity of a dock is increased if it has many doors, but without being too close together so that trailers (outside) or freight (inside) interfere with one another.
Figure : A typical crossdock is built in the shape of the letter I (actually, an elongated rectangle), so that freight can flow across from incoming trailers to outgoing trailers.
A typical dock, such as illustrated in the picture, is generally around 120 feet wide (36.6 meters). This is to allow freight to be staged on the floor. A standard (large) trailer is 48 or 53 feet long (14.6 or 16.2 meters) and a “pup” is 28 feet long (8.5 meters); all are 9 feet wide (2.7 meters). The width of the dock should include enough space for the trailer on each side of the dock to stage its freight (about 100 feet total, or 30.8 meters) plus allow space for travel along the length of the dock (for example, two aisles, each 10 feet wide, or about 3.0 meters). We have seen docks as narrow as 80 feet (24.4 meters), but this is practical only when it is possible to avoid staging most freight, such as when the material is palletized and also easily stackable and may be loaded in any order. If a dock is much wider than this, it just adds to the travel time to move the product from incoming trailer to outgoing trailer.
A dock does not have to be shaped like the letter I. For example, shapes of an L, U, T, and H. But every corner in a dock reduces effective capacity:
Crossdocks have been built in a variety of shapes. Clockwise from upper left: An L-shaped terminal of Yellow Transport; a U-shaped terminal of Consolidated Freightways; a T-shaped terminal of American Freightways; an H-shaped terminal of Central Freight
On the outside of a corner you lose floor space per door on which to dock freight.
- This increases congestion on the dock, which interferes with the flow of freight.
- On the inside of a corner, you lose door positions because trailers will interfere with each other in the yard. Because doors are lost, the dock must be longer to accommodate a given number of doors, which means that on average freight will have to travel farther to cross the dock. Thus, for example, freight has to travel farther to cross an H-shaped dock, with four inside corners, than to cross an I shaped dock. (Because the door positions will be lost anyway, inside corners are a good place to locate administrative spaces or hazardous materials storage.)
It is hard to make generalizations independent of specific bills of lading; but in general an L-shaped crossdock is inferior: It incurs the costs of one inside and one outside corner but without getting anything in return. The result is that freight must travel farther because the dock must be longer from end to end to make up for lost doors at the inside corner. Furthermore, there is congestion at the outside corner. The same observations hold even more strongly for a U-shaped dock.
An X-shaped or a T-shaped dock also incur corner costs but they have a compensating benefit: The longest distance from door-to-door is less than that for an I-shaped or L-shaped dock with the same number of doors.
One can reduce labor costs in a crossdocking freight terminal by parking incoming and outgoing trailers so that freight can be efficiently moved across the dock. For example, if much of the freight flowing through the terminal is bound for Miami, the Miami trailers should probably be parked in a convenient location. The challenge is to formalize the notion of “convenient”; then labor-reducing door assignments can be made with optimization models based on the geometry of the terminal, the material handling systems within, and the mix of freight passing through.
References : Copyright 1998–2011 John J. BARTHOLDI, III and Steven T. HACKMAN. All rights reserved. This material may be freely copied for educational purposes—but not for resale—as long as the authors’ names and the copyright notice appear clearly on the copies. Corresponding author: firstname.lastname@example.org