string(98) ‘ trucks are possibly directly designated to a deprive door and have absolutely to wait within a queue until assignment. ‘
Omega watches 40 (2012) 827–846 Contents lists sold at SciVerse ScienceDirect Omega journal homepage: www. elsevier. com/locate/omega Review Cross-docking: State of the art January Van Belle n, Paul Valckenaers, Dirk Cattrysse KU Leuven, Office of Mechanical Engineering, Celestijnenlaan 300B, B-3001 Heverlee (Leuven), Belgium a r to i c l e i and f o Article history: Received 23 June 2011 Accepted 17 January 2012 Processed by simply Pesch Available on the web 25 January 2012 Keywords: Cross-docking Logistics Classi? cation abstract
Cross-docking is a logistics strategy through which freight is usually unloaded by inbound vehicles and (almost) directly packed into outbound vehicles, with little or no storage space in between.
This paper gives an overview with the cross-docking principle. Guidelines for the successful use and implementation of crossdocking are discussed and lots of characteristics are described which you can use to distinguish between different cross-dock types. In addition , this daily news presents an extensive review of the current literature about cross-docking. The discussed documents are classi? d based on the problem type that is tackled (ranging by more ideal or trickery to even more operational problems). Based on this kind of review, several opportunities to increase and prolong the current analysis are mentioned. , 2012 Elsevier Ltd. All legal rights reserved. Material 1 . installment payments on your 3. Intro………………………………………………………………………………………… 827 When and how to employ cross-docking?…………………………………………………………………….. 829 Cross-dock features………………………………………………………………………………. 830 three or more. 1 . Physical characteristics…………………………………………………………………………… 831 3. 2 . Operational characteristics……………………………………………………………………….. 831 a few. 3. Movement characteristics……………………………………………………………………………… 831 Literature review…………………………………………………………………………………….. 832 4. 1 . Position of cross-docks………………………………………………………………………….. 832 4. 2 . Structure design………………………………………………………………………………….. 833 4. several. Cross-docking networks…………………………………………………………………………… 833 four. 4. Motor vehicle routing………………………………………………………………………………… 834 4. a few. Dock door assignment……………………………………………………………………………. 834 4. six. Truck booking……………………………………………………………………………….. 837 4. 6. 1 ) Single tape and bunch door…………………………………………………………………. 837 4. six. 2 . Organizing of incoming trucks………………………………………………………………… 839 4. six. 3. Scheduling of inbound and outbound trucks……………………………………………………… 840 4. six. Temporary storage space……………………………………………………………………………… 841 4. 8. Various other issues…………………………………………………………………………………… 842 Conclusion and research opportunities……………………………………………………………………… 843 Sources………………………………………………………………………………………… 844 4. 5. 1 . Introduction Cross-docking is a strategies strategy today used by many firms in different industrial sectors (e. g. retail? rms and less-thantruckload (LTL) strategies providers). The essential idea behind crossdocking is usually to transfer inbound shipments directly to outgoing Related author. Tel.:? 32 16322534, fax:? thirty-two 16322986. Email addresses: january. [email, protected] kuleuven. always be (J. Van Belle), paul. [email, protected] kuleuven. always be (P. Valckenaers), dirk. [email, protected] kuleuven. be (D. Cattrysse). 0305-0483/$ , observe front matter , 2012 Elsevier Limited. All legal rights reserved. doi: 10. 1016/j. mega. 2012. 01. 005 n automobiles without saving them in between. This practice can serve different desired goals: the consolidation of deliveries, a shorter delivery business lead time, the reduction of costs, and so forth The function of cross-docking in industry even appears to increase [1–4]. In a traditional distribution center, merchandise are? rst received after which stored, as an example in pallet racks. When a customer demands an item, personnel pick this from the safe-keeping and dispatch it to the destination. Via these four major functions of warehousing (receiving, storage, order selecting and shipping), storage and order picking are usually the most costly.
Storage can be expensive due to inventory holding costs, order picking since it is labor 828 J. Truck Belle ain al. as well as Omega 40 (2012) 827–846 intensive. One approach to reduce costs could be to boost one or more of such functions or improve how they interact. Crossdocking however is usually an approach that eliminates the 2 most expensive controlling operations: storage space and purchase picking [5–8]. A de? nition of cross-docking provided by Kinnear  is usually: “receiving merchandise from a supplier or perhaps manufacturer for a number of end destinations and combining this product to suppliers’ product for common? al delivery destinations”. In this de? nition, the focus is usually on the consolidation of deliveries to achieve financial systems in transportation costs. The Material Handling Market of America (MHIA) sobre? nes cross-docking as “the process of shifting merchandise through the receiving dock to delivery [dock] intended for shipping devoid of placing that? rst into storage locations” . The focus is actually on transshipping, not keeping stock. This involves a correct sync of inbound (inbound) and outgoing (outbound) vehicles. Yet , a perfect sync is dif? cult to accomplish.
Also, in practice, staging is essential because many inbound shipments need to be fixed, consolidated and stored until the outbound transport is complete. So , this strict restriction is calm by many authors. Cross-docking then can be described as the process of combining freight together with the same vacation spot (but coming from several origins), with nominal handling and with little if any storage among unloading and loading in the goods. If the goods will be temporally kept, this should be only for a short period of a period. An exact limit is dif? cult to de? at the, but many writers talk about twenty-four h (e. g. [5, several, 11, 12]). If the goods are placed in a storage place or about order picking shelves or if the staging takes many days or even weeks, it is far from considered as crossdocking but as (traditional) warehousing. Yet , even if the products are staged for a longer time, a lot of companies consider it cross-docking, as long as the products move via supplier to storage to customer almost untouched apart from truck launching [3, 13]. Various organizations make use of a mixture of warehousing and cross-docking to combine the bene? ts of the two approaches .
A terminal committed for cross-docking is called a cross-dock. Used, most cross-docks are extended, narrow rectangles (I-shape), but other forms are also used (L, T, Back button,… ) . A crossdock features multiple packing docks (or dock doors) where trucks can dock to be loaded or unloaded. Incoming vehicles are designated to a ‘strip door’ where freight is unloaded. Then the goods are moved to its appropriate ‘stack door’ and loaded with an outbound vehicle. Mostly, there is absolutely no special facilities to level freight. In the event goods must be stored in the short term, they are put on the? oor of the cross-dock (e.. before the dock door where the leaving truck is or will be docked). However , it is possible which the cross-dock contains for instance a pallet storage, undoubtedly if cross-docking is along with warehousing. Fig. 1 presents a schematic representation of the material managing operations at an I-shaped cross-dock with 15 dock gates. Incoming vans are both directly designated to a deprive door and have absolutely to wait in a queue right up until assignment.
You read ‘Cross-Docking: State of the Art’ in category ‘Essay examples’ When docked, the freight (e. g. pallets, packages or perhaps boxes) in the inbound vehicle is not loaded and the vacation spot is identi? ed (e. g. con scanning the barcodes mounted on the goods). Then, items are transferred to the specified stack door by a few material handling device, for instance a worker functioning a forklift or a conveyor belt system. There, items are packed onto an outbound vehicle that serves the committed destination. Once an inbound truck is very unloaded or an telephone truck is completely loaded, the truck can be replaced simply by another vehicle. Cross-docking matches with the desired goals of trim supply string management: smaller sized volumes of more obvious inventories which have been delivered more quickly and more often .
In the books, several other (possibly intertwined) benefits of cross-docking in contrast to employing classic distribution centers and point-to-point deliveries happen to be mentioned (e. g. [2, a few, 6, 12-15, 16]). Some positive aspects compared with classic distribution centers are: cost reduction (warehousing costs, inventory-holding costs, controlling costs, labor costs), shorter delivery business lead time (from supplier to customer), better customer service, decrease of space for storage, faster products on hand turnover, fewer overstocks, reduced risk for loss and harm.
Some features of cross-docking in comparison with point-to-point deliveries are: cost reduction (transportation costs, labor costs), consolidation of deliveries, improved source utilization (e. g. full truckloads), better match among shipment amounts and actual demand. Fig. 1 . Materials handling in a typical cross-dock. These positive aspects make cross-docking an interesting logistic strategy that could give businesses considerable competitive advantages. Wal Mart is known as a well-known case in point , but as well several other businesses have reported the powerful implementation of cross-docking (e.. Eastman Kodak Co. , Goodyear GB Ltd. , Dots, LLC  and Toyota ). Although cross-docking has already been utilized in the eighties (e. g. by Wal Mart), it has only captivated attention from academia much later and generally during the recent years. For instance, much more than 85% from the academic papers found by the authors happen to be published by 2004 upon. During these years, a considerable number of documents have been published and because from the growing fascination from industry [1–4], the creators expect that still even more research about this topic will be performed the coming years.
The objective of this paper is to present an overview in the cross-docking strategy. First, recommendations for the successful employ and setup of cross-docking will be discussed. Further, a number of characteristics will be described to distinguish between different types of cross-docks. Following, the conventional paper will provide an assessment the existing materials about cross-docking. The mentioned papers are classi? ed based on the situation type. These kinds of problems range between more proper or technical to even more operational concerns. This review can help (future) cross-docking M. Van Superbe et ‘s. Omega forty (2012) 827–846 829 professionals to? nd the correct literary works to start or improve their cross-docking operations. With no proper rendering, it is not possible to l?be? t in the above-mentioned positive aspects. Based on the provided review, the writers try to determine gaps of knowledge and interesting areas intended for future exploration. The term cross-docking usually refers to the situation in which trucks or trailers1 will be loaded and unloaded in a cross-docking terminal. However , the operations to handle shipping at a harbor or airport are sometimes very similar.
At a possess for instance, containers are unloaded from a ship and temporarily put onto the quay until they are loaded onto an additional ship or onto a truck. An international airport can also be seen as an kind of cross-dock for copying passengers and their baggage. In the literature, a number of papers is found that deal with similar problems because encountered in crossdocking, nevertheless speci? c for harbors or airfields (e. g. how to identify the layout associated with an airport terminal [19, 20], how to designate airplanes to gates , and so forth ). These kinds of papers are not taken into account to get the books review provided here.
The paper concentrates on the typical cross-docking in which merchandise are transported between vehicles at a cross-dock. The speci? c application or perhaps industry (e. g. less-than-truckload (LTL) or perhaps courier, share and package (CEP) industry) is not important, given that the applied material controlling can be considered while cross-docking. To the best of the knowledge, only two paperwork present an assessment cross-docking documents. Boysen and Fliedner  discuss papers about the truck organizing problem and offer a classi? cation with the considered concerns. The procedure taken ere is even so more general and several issue types associated with crossdocking are discussed, including the truck booking problem (see Section 4. 6). Agustina et ing.  provide a general picture of the statistical models employed in cross-docking papers. These versions are classi? ed depending on their decision level (operational, tactical or strategic) after which subdivided by simply problem type. However , another classi? cation is shown here as the creators do not completely agree with the proposed classi? cation (the considered problem types and the assignment of papers to problem types).
For instance, Agustina et ing.  do not consider motor vehicle routing and temporary safe-keeping and the documents about cross-dock networks are discussed in two several sections (transshipment problems and cross-docking network design). Also, some papers about pier door assignment are mentioned in the section about cross-docking layout design. In addition , the review shown here is even more extensive, even more papers happen to be included as well as the papers are discussed much more detail. This kind of paper also includes a general summary of cross-docking and describes a lot of cross-dock qualities. The daily news is prepared as follows.
The next section covers in which scenarios cross-docking is actually a suitable approach and works with the requirements for the successful implementation. In Section 3, you will are discussed that can be used to differentiate among alternative cross-docking systems. The literature assessment is shown in Section 4. The discussed papers are classi? ed based upon the problem type they manage. The results with in order to improve and extend the latest research will be summarized in Section a few. Fig. installment payments on your Suitability of cross-docking (adapted from Susceptible and Viswanathan ). 2 .
When and how to use cross-docking? Although cross-docking is nowadays used by many firms, it is most likely not the best strategy in every case and in almost all circumstances. It brie? sumado a describes the existing 1 In the following webpages, the terms truck, trailers and car will be used substituted. literature that gives some guidelines intended for the effective use and implementation of cross-docking. Apte and Viswanathan  go over some factors that in? uence the suitability of cross-docking in contrast to traditional syndication. 2 A? rst important factor is the item demand charge.
If there is a great imbalance between your incoming insert and the out bound load, cross-docking will not work well. Hence, items that are far better for cross-docking are the ones that possess demand rates that are approximately stable (e. g. grocery and regularly consumed perishable food items). For these items, the warehousing and transport requirements are much more predictable, and consequently the planning and execution of cross-docking becomes much easier. The unit stock-out cost is an additional important factor. Since cross-docking decreases the level of inventory at the warehouse, the possibility of stock-out situations is definitely higher.
Nevertheless , if the product stock-out cost is low, the bene? ts of cross-docking can outweigh the elevated stock-out price, and so cross-docking can still become the preferred approach. As shown in Fig. 2, cross-docking is as a result preferred to get products with a stable require rate and low product stock-out price. The traditional storage is still better for the opposite situation with an unstable demand and excessive unit stock-out costs. Intended for the two various other cases, cross-docking can still be applied when appropriate systems and planning tools are in position to keep the amount of stock-outs to a reasonable level. Some other factors that can in? ence the suitability of crossdocking are definitely the distance to suppliers and customers (higher distances boost the bene? ts of consolidation), the product worth and existence cycle (a larger reduction in inventory costs for items with a bigger value and shorter existence cycle), the demand quantity (a larger reduction in inventory space and costs for items with a bigger demand), the timeliness of supplier deliveries (to assure a correct harmonisation of inbound and outbound trucks), and so forth [1, 23, 24]. Some writers use a more quantitative way of study the suitability of cross-docking. For instance, Galbreth et al. 6] review the transport and controlling costs among a situation where a supplier needs to ship goods to several clients with just direct deliveries and a scenario in which likewise indirect deliveries via a cross-dock are possible. For the second situation, a mixed integer programming (MIP) model is definitely proposed to ascertain which items should go straight from supplier to customer and which items should be sent via a cross-dock to meet the (known) requirements. The transportation costs are modeled in a realistic approach:? xed for truckload shipping and delivery, while the less-thantruckload shipping costs are modeled using a modi? d all-unit discount (MAUD) cost function. The keeping costs in the customers happen to be proportional towards the quantity and the holding time between arrival time and due date. The expenses for both the situations are compared below varying working conditions. The authors determine that cross-docking is more beneficial when demands are less a couple of It is assumed which the demand quantities are small , and otherwise point-to-point deliveries are usually more suited. 830 J. Truck Belle ainsi que al. / Omega 40 (2012) 827–846 variable then when unit keeping costs in customer locations are bigger.
On the other hand, it can be less important when the average demands will be close to pickup truck load ability. Other quantitative approaches generate a comparison among a situation with a cross-dock and a situation having a traditional division center. As an example, Kreng and Chen  compare the operational costs. Besides the vehicles and having costs, the production costs (more speci? c the set up costs) of the goods with the supplier happen to be taken into account. Every time a cross-dock is utilized, more regular deliveries towards the cross-dock are required and the batch size must be smaller, that causes higher create costs.
Waller et al.  turn to both conditions from a listing reduction point of view. Schaffer  discusses the successful implementation of crossdocking. When a company wants to expose cross-docking, the introduction must be prepared perfectly. If the necessary equipment is currently available and because cross-docking seems simple, a single easily assumes that cross-docking can be implemented without much work. However , cross-docking itself is fairly complex and requires a high degree of coordination between your supply cycle members (e. g. the timing of arrival and departure).
Therefore , the requirements for successful cross-docking should be comprehended thoroughly as well as the implementation ought to be planned cautiously. In , Schaffer elaborates upon six categories of requirements for any successful setup. According to Witt  and to Yu and Egbelu , software to plan and control the cross-docking procedures (e. g. a factory management system or perhaps WMS) takes on an important part in the good implementation of cross-docking. The mandatory (automated) hardware for a cross-docking system (material handling devices, sorting systems, etc . ) might arrive off the corner and is easily obtainable today.
But the software needs to be tailored to the speci? c requirements and is also in general relatively less produced, although it can be as important as hardware to cross-docking success. This is also con? rmed by a survey among professionals who take part in cross-docking and who denote IT program support as being a key barrier to successful cross-docking [3, 4]. Hence, the program requirements need to be carefully sobre? ned and studied to be able to prevent putting in the physical system to find out afterwards there is not any information and communication system in place for successful procedure.
This software system can only function correctly if it is fed with accurate and timely information. Compared with standard distribution, the data? ow to support cross-docking is usually signi? cantly more important . As an example, to coordinate the incoming and telephone trucks for the appropriate docks, the coming time and the destination of the freight should be known prior to physical appearance of the items (e. g. via advance shipping notice (ASN)). Many information technology tools are available to appreciate this information? ow, e. g. lectronic info interchange (EDI), shipping box marking (SCM), bar-coding and scanning of goods using universal product code (UPC) . Irrespective of which technology is chosen, the supply string partners should be able focused enough to deliver the required information by way of this technology. A good cooperation across the source chain will make or break the cross-docking implementation [8, 13, 24]. docking [1, 29]. In a two-touch or perhaps single-stage cross-dock, products happen to be received and staged for the dock right up until they are loaded for outbound transportation. Generally, the goods are put into areas corresponding for their strip or perhaps stack door (see Fig. 3).
In the matter of a multiple-touch or two-stage cross-dock, goods are received and taking place on the boat dock, then they happen to be recon? gured for delivery and are loaded in outbound trucks. Within a typical que incluye? guration, the incoming gets is? rst put in zones corresponding for the strip doors. The goods will be then sorted to the areas and specific zones corresponding to the stack doorways (see Fig. 4). An additional distinction can be made in accordance to if the customer is definitely assigned to the individual products . In predistribution cross-docking, the customer is designated before the delivery leaves the supplier who also takes care of planning (e. g. labeling and pricing) and sorting.
This permits faster handling at the cross-dock. On the other hand, in post-distribution crossdocking, the allowance of goods to customers is performed at the cross-dock. Still some other distinctions are possible. The German supermarket retailer Metro-AG for instance differentiates sourceoriented and target-oriented cross-docking based on the location Fig. 3. A single-stage cross-dock in which the products happen to be staged in zones matching to the stack doors (adapted from Gue and Kang ). a few. Cross-dock attributes Several attributes can be considered to distinguish between different kinds of cross-docks (and cross-docking).
A common difference made in the literature is founded on the number of touches  or perhaps stages . In one-touch cross-docking, products are touched only one time, as they are received and filled directly in an outbound pickup truck. This is also referred to as pure cross- Fig. some. A two-stage cross-dock when the products will be staged in zones corresponding to the deprive and bunch doors and therefore are sorted in the middle (adapted via Gue and Kang ). J. Vehicle Belle et al. / Omega forty (2012) 827–846 831 of the cross-docking terminals relative to suppliers and clients .
Napolitano  distinguishes various kinds cross-docking based upon the meant use and , eight several crossdocking techniques are detailed. In this section, several attributes are referred to that can be used to distinguish between diverse cross-dock types. 3 Remember that real world attributes of the cross-dock are considered, rather than the properties from a speci? c decision problem related to cross-docking. For the papers as part of the literature assessment (Section 4), the characteristics with the considered cross-docks will be listed in tables according to the characteristics defined here. Nevertheless , the structure of Section 4 is not based upon these attributes, but within the considered issue type. You will can be broken into three organizations: physical qualities, operational qualities and characteristics about the? ow of goods. 5 Within the next sections, these types of groups will probably be described much more detail. a few. 1 . Physical characteristics The physical attributes are attributes of the crossdock that are supposed to be? xed (for a rather very long time). The following physical features are considered. Condition: Cross-docks may have a large variety of designs.
The shape can be described by the letter corresponding to the condition: I, D, U, To, H, Electronic,… Number of pier doors: A cross-dock is also characterized by the number of dock gates it has. In practice, cross-docks selection in size via 6 to 8 doors to more than 200 gates, and even a cross-dock exceeding 500 gates exists . In the literature, sometimes the number of ipod dock doors is limited to only 1 or 2. In these cases, the theory is never to model an authentic cross-dock, but for gain a few insight simply by studying a simpli? impotence model. Internal transportation: The transportation within the crossdock could be executed physically (e.. by workers using forklifts) or there can be an automatic system in place (e. g. a network of conveyor belts). The available system will of course be determined by the type of gets that is dealt with in the cross-dock. For instance, LTL carriers deal with mostly palletized freight so make use of forklifts. Conveyor devices on the other hand will be among others used by parcel companies, as they deal with many (small) packages. A mixture of both vehicles modes is also possible. three or more. 2 . Detailed characteristics Several operational decisions can in? uence the functioning with the cross-dock.
These operational constraints lead to the next characteristics. Support mode: According to Boysen and Fliedner , the support mode of any cross-dock can determine the examples of freedom in assigning incoming and telephone trucks to dock doorways. In an unique mode of service, every single dock door is either entirely 3 A few of the characteristics defined here are just like the characteristics employed by Boysen and Fliedner  to make a classi? cation of truck booking problems. However , they  consider not only real world attributes, but likewise characteristics from the (mathematical) models. At least for the papers through which these features are referred to, i. at the., in which real-world details of the cross-dock are believed (Sections four. 5–4. 8). 5 This kind of classi? cation is rather hazy. For some attributes, it is not clear in which group they? t best or they can be given to multiple groups. As an example, temporary storage area is considered as a? ow characteristic. However , short-term storage can even be seen as a physical characteristic (storage is not possible because of space constraints) or perhaps operational characteristic (it is definitely an operational decision that storage area is prohibited, e.. to prevent congestion inside cross-dock). dedicated to inbound or perhaps outbound vans. If this kind of service function is used, generally one aspect of the cross-docking terminal is usually assigned to inbound vehicles and the various other side to outbound vehicles. A second function is blended mode. From this mode, incoming and telephone trucks could be processed at all doors. These two modes can be combined. In this combination method, a subsection, subdivision, subgroup, subcategory, subclass of doors is operated in exclusive mode while the rest of the doorways is operated in mixed mode. Pre-emption: In the event that pre-emption can be allowed, the loading or perhaps unloading of your truck could be interrupted.
This kind of truck can now be removed from the dock and another pick up truck takes its place. The el? nished pickup truck has to be docked later on to? nish the loading or unloading. several. 3. Stream characteristics You will of the? ow of goods that contain to be prepared by a cross-dock can be very distinct. The following qualities are recognized. Arrival style: The arrival times of the goods are dependant on the introduction times of the inbound vehicles. The introduction pattern can be concentrated for one or more durations if the inbound trucks get there together in (more or less) a similar times.
For instance, a cross-dock in the LTL industry offering a certain physical area usually receives shipping at two periods. Merchandise that have to be transported from the inside that location to another region are found during the day and everything pickup trucks get to the evening in the cross-dock. Items are then sorted at night time and the telephone trucks keep in the morning. To simplify the problem, several papers assume that the inbound vehicles arrive with each other (at first the time horizon). On the other hand, shipment from outside of the region although destined for that area comes in the early morning and is after that istributed throughout the day. Another opportunity is that the introduction pattern is scattered as well as the inbound trucks arrive at different times in the daytime. The introduction pattern posseses an in? uence on the blockage of the cross-dock and on the scheduling of workers and resources. Leaving time: The departure times during the the trucks can be limited or not really. In many cases you will find no constraints and the vans leave the cross-dock in the end freight is definitely loaded or unloaded. Nevertheless , it is also which the vans have to go away before some point in time, for instance in order to be on time for a next transportation task.
In this case, there can be restrictions made on the starting times of the inbound vehicles only, so that these pickup trucks have to be not loaded on time. In a similar fashion, it is possible that only the telephone trucks need to leave the cross-dock prior to a certain second. 6 For example, in the package delivery sector, the telephone trucks generally leave by a? xed point in time. Parcels arriving past due have to possible until another pickup truck departs for the same destination. Additionally it is possible that both inbound and outbound vehicles have limited departure instances.
Product interchangeability: The gets handled at a cross-dock is in standard not compatible. In this case, all products are dedicated to a speci? c destination7 or a speci? c outbound truck (pre-distribution). Information about the destination or the committed truck is normally known prior to products arrive at the cross-dock. It is however also possible that interchangeability of products can be allowed (post-distribution). In this scenario, only the type of products to be loaded around the outbound vans and the related quantity is famous (see footnote 7).
If the products are interchangeable, usually some value-added activities (e. g. labeling) need to be performed. 6 This era can be influenced by the (due dates of the) real load with the truck. several The job of the items to a speci? c outbound truck is then an operational decision. 832 J. Van Belle et al. as well as Omega forty five (2012) 827–846 Temporary storage space: In real cross-docking, the arriving shipment is immediately transported to outbound trucks, so no storage should be used. In practice nevertheless , this is hardly ever the case. Generally speaking, the goods will be temporarily stored on the? oor of the cross-docking terminal (e.. in front of the collection doors) or even in a (small) warehouse. Nevertheless , it is possible that goods are certainly not allowed to be stored. As an example, if chilled products need to be cross-docked in a non-cooled airport terminal, these products have to be directly relocated from a cooled inbound to a cooled outbound pick up truck. 4. Literature review Cross-docking practitioners have to deal with many decisions during the design and operational phase of cross-docks. These types of decisions may have a serious impact on the ef? ciency, thus they have to end up being carefully used. In the literary works, several decision problems are analyzed.
Some of these trouble is more concerned regarding decisions with effects over a longer term (strategic or tactical), while others cope with short-term decisions (operational). This section gives a report on the existing books about crossdocking problems. The literature review is methodized according to the fundamental planning procedure a manager, wanting to start with cross-docking, can be confronted with. The? rst decisions that have to be taken during the planning process happen to be strategic decisions: where can a cross-dock (or crossdocks) be located and what is the best design of a cross-dock.
Once the cross-dock is available, it will be part of a supply network (with a number of cross-docks). A tactical decision that has to be produced then is definitely how the products will? ow through the network to minimize the costs, while producing supply encounter desire. Next, the manager is faced with the operational decision (although it includes also trickery aspects) of vehicle routing: before coming to the cross-dock, freight has to be picked up in various locations, and the goods have to be sent to multiple locations after debt consolidation at the cross-docking terminal.
Other operational decisions deal with the assignment of trucks to dock doorways or the organizing of the vans, and with the position where merchandise will be briefly stored. Naturally , the administrator will also be confronted with problems that aren’t speci? c for cross-docking: the booking of the inner resources for the loading and unloading with the freight (e. g. the workforce), finding the right staging strategy and identifying an optimal truck packaging sequence. Another sections illustrate the cross-docking problems dealt with in the literature.
Only the issues that are speci? c for cross-docking are believed. First, the strategic decisions are reviewed: the location of cross-docks and layout design and style. The technical problem of cross-docking systems is defined next. Further more, the functional decisions are handled: vehicle routing, boat dock door project, truck scheduling and non permanent storage. Finally, some paperwork that examine other concerns related to crossdocking are mentioned. 4. 1 . Location of cross-docks The location of one or maybe more cross-docks is part of the style of a syndication network or perhaps supply sequence.
An important tactical decision that should be made problems the position of the crossdocks. This challenge cannot be taken care of isolated through the decisions that determine how items? ow through this network. The dedication of the? ow of goods is definitely discussed in Section some. 3, yet problems that likewise involve a decision about the location are considered right here. The problem where you should locate establishments (e. g. distribution centers or plants) has attracted a considerable amount of interest. 8 The papers talked about in this section determine and also the optimal? ow of goods throughout the network.
Furthermore, they consider the features to be cross-docks because they will explicitly take individual automobiles into account or perhaps because momentary storage is definitely not allowed. A? rst examine about the place of cross-docks is performed by simply Sung and Song . Inside the considered issue, goods need to be transported via supply to demand nodes via a cross-dock (direct shipments are not allowed). The cross-dock can be selected from some possible cross-dock locations, every with a great associated? xed cost. The requirements are assumed to be known and there are two styles of automobiles with a diverse capacity and cost. The goal is to? m which cross-docks should be applied and how many vehicles happen to be needed on each link in order to minimize the entire cost. This total expense consists of the? xed costs of the employed cross-docks as well as the transportation costs. The creators present a great integer programming model of the situation. This model is incredibly similar to the model presented simply by Donaldson ainsi que al.  and Letras et ing.  (discussed in Section 4. 3) and comparable simplifying presumptions are applied. Compared with those two papers however , the way of Sung and Song  will not consider direct shipments although does include the location decision.
Because the is actually NP-hard, a tabu search-based algorithm is usually proposed to fix the problem. The solutions figure out how the goods? ow through the network. Based on this? ow, the quantity of vehicles can be derived simply by solving a subproblem. Several computational tests are performed on generated test situations and indicate that the suggested algorithm? nds good possible solutions in a reasonable time. Sung and Yang  extend this kind of work and propose a little improvement for the tabu search algorithm.
The authors also present a set-partitioning-based ingredients of the problem and offer a branch-and-price algorithm based on this formulation to obtain exact solutions. The computational benefits show that the algorithm gives better results with regards to the number of (smallscale) problem occasions solved plus the required computation time compared to the benefits obtained by simply solving the integer coding model with all the optimization software package CPLEX.? Gumus and Bookbinder  study a similar issue, but now direct shipments will be allowed and multiple merchandise types are viewed as (multicommodity).
The facility price for each crossdock consists of a? xed cost and a throughput cost incurred per device load. The transportation price also has two components: a? xed price for each pick up truck and a variable cost per device load every unit distance. A last cost that is considered is the cost for intransit inventory. In this approach, the synchronization of inbound and outbound vans is not really taken into account. The authors provide a mixed integer programming type of the problem. Simply by solving a number of smaller issue instances suitably (with the optimization software packages LINGO and CPLEX), the in? ence of several cost variables is analyzed. The experts conclude the optimal range of cross-docks can be an increasing function of the ratio between the (? xed) pick up truck cost as well as the (? xed) facility expense. A different way is taken by Jayaraman and Ross . That they study a multi-echelon injury in which merchandise (from multiple product families) have to be transported from a central manufacturing plant to one or more distribution centers. From there, the goods are relocated via cross-docks to the customers. The problem is discussed in two stages. In the? st stage, a strategic unit is used to select the best pair of locations for the circulation centers and cross-docks. The authors provide an integer development formulation that aims to minimize the? xed costs associated with functioning open syndication centers and cross-docks as well as the 8 A number of references can be obtained from the paperwork discussed with this section. L. Van Superbe et ‘s. / Tissot 40 (2012) 827–846 833 various transportation costs. Demand splitting is not allowed: clients have to be designated to single cross-docks while crossdocks have to be assigned to single syndication centers only.
In the second stage, a great operational style decides upon the quantities of each merchandise type that need to be transported through distribution centers and cross-docks. The style tries to reduce the transport costs whilst satisfying buyer demand. The[desktop] is less restricted than the? rst model (it relaxes as an example the demand dividing assumption) and can be executed as soon as the open circulation centers and cross-docks are determined with the aid of the? rst model. Equally models will be more simpli? education compared with the prior approaches.
As an example, individual automobiles are not considered and the vehicles cost is proportionate to the volume to deliver. The authors propose a simulated annealing approach to fix larger difficulty instances. The computational tests on generated problem circumstances indicate that the heuristic provides results having a deviation of around 4% from the optimal answer (obtained with LINGO), but 300–400 moments faster. In , the same creators present two other heuristics to tackle the problem. Both equally heuristics are based on simulated annealing but use an extra system to avoid regionally optimal alternatives.
The? rst heuristic makes use of a tabu list, the other heuristic permits a sudden re-scaling of the ‘system temperature’. Pertaining to both heuristics, the solution top quality and computational performance are tested for different ‘cooling schemes’. The fresh results reveal that the simulated annealing heuristic combined with tabu search gives better solutions in slightly more time. Bachlaus et approach.  contemplate a multi-echelon supply string network, which includes suppliers, crops, distribution centers, crossdocks and customers. The goal is to optimize the material? w through the entire supply string and to determine the optimal quantity and location of suppliers, plant life, distribution centers and crossdocks. The problem is formulated as a multi-objective optimization style that attempts to minimize the whole cost and maximize the plant and amount? exibility. Because of the computational difficulty of the trouble, the experts propose a variant of particle swarm optimization (PSO) to design the supply chain. A few computational tests are carried out and the outcomes show the fact that proposed remedy approach gives better results when compared to a genetic criteria and two other PSO variants. his at the cost of additional 4 corners which decrease the labor ef? ciency (two inside and two outside corners pertaining to T, several inside and four outside corners for X). An inside part renders several doors useless, while doorways around an outside corner have less? oor space open to stage freight. So , these kinds of additional corners are a? xed cost, which will begins to pay up for larger docks. It is however not always easy to predict which will shape is much better, because this as well depends on electronic. g. the freight? ow pattern. Various other papers deal with the design of the storage area where freight may be temporarily taking place (on the? or or perhaps in racks). In many cases, the freight is put in several parallel rows and the workers can move among these rows. Vis and Roodbergen  deal with the operational decision where to temporarily store incoming freight (see Section 4. 7). The proposed protocol can also be used through the design phase to determine the optimal number of seite an seite storage rows and their extent. The (single-stage or two-stage) storage area can be organized in parallel lanes directly next to each other which could only be accessed at the two ends.
Gue and Kang  make use of simulation to examine the behavior of the so-called workplace set ups queues. The results suggest that, for a single-stage storage area, it is best to have even more short lanes than fewer long types, at least when the employees follow a rational approach. The results as well indicate that two-stage cross-docking has a signi? cantly decrease throughput than single-stage cross-docking. 4. a few. Cross-docking systems Some creators do not examine problems concerning a single cross-dock, but think about a network that contains one or more cross-docks.
The aim is to determine the? ow of products through this sort of a network in order to reduce costs, while producing supply encounter desire. The research of Lim ainsi que al.  extends the standard transshipment issue. The transshipment problem consists of a number of source, transshipment and demand nodes. The couronne between these nodes will vary capacity limits and costs. The objective should be to? nd the very least cost? ow that satisfies all demands and the potential constraints. In the extended transshipment problem, safe-keeping is allowed at the transshipment centers.
These kinds of centers can be viewed as as cross-docks because the purpose of the style is to minimize or eliminate holdover products on hand. Moreover, this problem takes distributor and customer time home windows into account and considers the capability and holding costs with the crossdocks. Almost all shipments have to pass using a cross-dock, therefore no direct shipments are thought. Similar to the unique problem, the aim is to decrease the total expense (transportation costs and holding costs) when meeting require and respecting the time windows and capacity constraints.
In the event that multiple departures and deliveries within a period window are allowed (multiple shipping– multiple delivery), the authors demonstrate that a time-expanded network can be used to formulate the challenge as a minimum cost? ow issue (MCFP) which can be solved in polynomial time. For various other cases, the authors provide evidence that the problem is NP-hard. For the special case when only 1 delivery or perhaps departure is usually allowed in a time window and the departure and appearance times will be? xed (single shipping–single delivery with? xed schedules), a genetic criteria is produced by Miao ou al. .
This heuristic offers better results (in terms of solution quality and calculation time) than solving the integer coding formulation of the problem with CPLEX (with a period limit). Chen et ‘s.  study a similar issue which they phone the multiple cross-dock trouble. The major distinctions are that supplies and demands happen to be not-splittable and that different numerous be considered (multicommodity? ow problem). Also, vehicles time is this approach not taken into account. 5. 2 . Design design As soon as the location of the cross-dock is determined, another tactical decision that has to be made should be to choose the design of the cross-dock.
The layout can be interpreted while the dimensions and shape of the cross-dock, as well as the dimension and form of the internal cross-dock areas and their arrangement. Bartholdi and Gue  give attention to the shape of your cross-dock. Many existing cross-docks are extended, narrow rectangles (I-shape), although there are also cross-docks shaped as an L, U, T, L or E. The crossdock shape is sometimes determined by straightforward constraints (e. g. style of the great deal on which it will eventually stand), however in this conventional paper the focus is definitely on how the design affects cross-dock performance.
Many experiments are performed when the labor costs (estimated by total travelling distance)9 are measured several shapes. The experiments suggest that an I-shape is the most ef? cient to get smaller cross-docks (fewer than about 150 doors). For docks of intermediate size, a T-shape is best and then for more than 2 hundred doors (approximately) an X-shape is best. Cross-docks with a To or X-shape have a larger ‘centrality’. Yet , they achieve 9 Here and in the subsequent pages, the travel distance is the distance traveled (by workers, forklifts,.. ) in order to transfer the goods internally in the inbound to the outbound pick up truck. 834 M. Van Superbe et al. / Omega watches 40 (2012) 827–846 An integer coding formulation from the problem is presented, together with a proof of its NP-completeness. The authors suggest three heuristics (simulated annealing, tabu search and a mix of both) to fix the problem. These heuristics provide better alternatives than those acquired by fixing the integer programming ingredients with CPLEX, within only less than 10% the time employed by CPLEX.
Among the three heuristics, tabu search seems to supply the best benefits. The previous studies represent the shipment of products as? ows. Individual transport units are not considered as well as the transportation expense is proportional towards the quantity to ship. Yet , to take advantage of loan consolidation, the vehicle travel cost needs to be taken into account. A? rst strategy that does consider the transportation cars explicitly (and this is why the authors view it as cross-docking) is definitely taken by Donaldson et al. .
In the regarded as problem, the goal is always to determine if to route freight straight from suppliers to customers or perhaps via a cross-dock and how a large number of vehicles should be scheduled on each transportation link in order to reduce the transport costs. In contrast to the previous strategies however , this problem is more simpli? ed, elizabeth. g. safe-keeping at the cross-docks is certainly not considered plus the synchronization of inbound and outbound pickup trucks is left out of the trouble. The authors eliminate links with a large transport time in an attempt to consider time home windows.
However , when the due dates at the vacation spot nodes may vary for the various goods, it will be easy that the motor vehicle allocation of an obtained solution violates the due times in practice. The authors present an integer programming type of the problem. Because the problem is dif? cult to resolve with branch-and-bound algorithms, another solution approach is proposed. From this approach, a great iterative process is used through which either the integrality limitations on the backlinks from beginning nodes towards the cross-docks or perhaps on the backlinks from the cross-docks to the destination nodes are relaxed.
This kind of relaxation heuristic provides near optimal solutions in an suitable time. The authors used this approach to compare a lot of scenarios (with a different volume of cross-docks for different places) for the network design of a nota service company. The same problem is likewise studied by simply Musa ou al. . They propose an ant nest optimization (ACO) heuristic to solve the problem and possess that this heuristic gives in a short time slightly greater results than a branch-and-bound approach (with the search engine optimization software package LINDO) that requires a far longer period.
The procedure of Ma et ing.  requires most of the above-mentioned concerns into consideration. The apparent shipment consolidation problem (SCP) considers distributor and client time glass windows and also the transport times between network nodes. Moreover, storage at the transshipment centers (cross-docks) is considered, shipments can be transported right to their destination or using a cross-dock and the transportation price accounts for the quantity of trucks. Yet , only one sort of products is regarded as (single commodity).
Again, the objective is to lessen the total cost (transportation and inventory cost) while satisfying the constraints imposed by the time windows. The authors present an integer programming model of the problem and have absolutely that it is NP-complete in the solid sense. Therefore , the experts propose a (two-stage) heuristic algorithm to fix the problem. The standard idea of the algorithm should be to consider? rst trucks which can be fully crammed and then to? nd alternatives that combine several more compact loads which are not considered but. In the? street stage, an entire truckload prepare (TL plan) and an initial less-than-truckload plan (LTL plan) are made. In the second stage, this initial LTL plan is improved iteratively through a metaheuristic (squeaky wheel search engine optimization or genetic algorithm). The computational experiments indicate the fact that proposed heuristic gives competitive results when compared to CPLEX (with a time limit) within a much shorter period. 4. 5. Vehicle redirecting Freight destined for a cross-dock needs most of the time to be found at numerous locations, and must be brought to multiple places after debt consolidation at the cross-dock.
Both the pick-up and the delivery process is visible as a vehicle routing difficulty and some studies consider cross-docking and motor vehicle routing together. A? rst approach is usually taken by Shelter et al. . The aim is always to? nd a great optimal course-plotting schedule pertaining to pickup and delivery (within the planning horizon) that minimizes the amount of transport cost and? xed costs of the vehicles. It is assumed that split transport are not allowed and all pick-up vehicles should arrive at the crossdock at the same time to prevent holding out times to get the telephone trucks. Although this can be a valid constraint for some cases (see Section several. ), this may not be generally accurate. The experts present a great integer encoding model of the situation, which nevertheless seems ineffective to solve the described problem. A tabu search protocol is recommended to? nd solutions. This method corresponds to the solving of two car routing concerns (one pertaining to pickup and one to get delivery). The 2nd routing issue will only commence when the? rst one is? nished and the complete process needs to be? nished within a certain preparing horizon. Liao et al.  recommend another tabu search criteria to solve similar problem. Wen et ing. 12] study the so-called car routing issue with cross-docking (VRPCD). In this trouble, orders from suppliers need to be picked up with a homogeneous? eet of cars. These purchases are in that case consolidated by a cross-dock and quickly delivered to consumers by the same set of automobiles, without intermediate storage with the cross-dock. Throughout the consolidation, items are not loaded from the inbound vehicles and reloaded upon outbound cars. The unloading must be accomplished before reloading starts. The authors imagine the duration of the unloading consists of a? male impotence time for preparation and a duration proportional to the weight size. Additionally it is assumed that if the delivery will be executed by the same vehicle while used for pick up, the unloading is not required (independent from the sequence in which the vehicle is loaded through the pickup tour). A time window is de? ned for all suppliers and customers and orders aren’t splittable. In the case without debt consolidation, the solution with this problem can be found by fixing two motor vehicle routing concerns (one to get pickup and one for delivery). Due to consolidation nevertheless , the pickup and delivery routes are certainly not independent.
Only trying to reduce the distance with the pickup and delivery routes is not suf? cient, the exchanges of orders at the cross-dock also have to be studied into account. The two of these aspects usually con? ict with each other. The authors present a combined integer programming formulation from the problem in that this objective is usually to minimize the whole travel time of all cars. This formulation contains various variables and constraints, therefore the authors offer to use tabu search embedded within an adaptable memory process. This method is tested upon realistic info involving approximately 200 supplier–customer pairs.
Experimental results present that the criteria can produce alternatives less than 1% away from the ideal within brief computing occasions (less than 5 s) for small problem occasions. For bigger instances, the gap with a lower certain is less than five per cent while the computation time stays below five min. four. 5. Ipod dock door assignment When an incoming or telephone truck gets to the cross-dock, it has to be made a decision to which boat dock door the truck should be assigned. A great assignment may increase the efficiency of the cross-dock and can cure the (handling) costs. So , the dock door assignment issue tries to? g the ‘optimal’ assignment of inbound and outbound trucks to boat dock doors. It is assumed that there are for least all the dock entry doors as vans, so each truck can J. Vehicle Belle ain al. as well as Omega forty (2012) 827–846 835 Desk 1 Qualities of the paperwork discussed in Section 4. 5. A great ‘n’ signifies that not just one value with the characteristic can be valid, yet that all beliefs can be used, ‘ns’ indicates that the characteristic is definitely not speci? ed. Paper(s) Shape Number of doors d n and n n n in n in n d n Internal transport By hand Manually Manually Manually By hand n Support mode Unique Exclusive Distinctive Exclusive Distinctive Exclusive
Special Exclusive Blended Exclusive Merged Exclusive Interchangeability Temporary storage space Yes Not any ns natursekt ns Certainly ns Certainly Yes ns ns ns Peck  Tsui and Chang [49, 50]#@@#@!? Bermudez and Cole  Cohen and Keren  Wow et ing.  Bartholdi and Gue  Gue  Brownish  (semi-permanent) Brown  (dynamic) Bozer and Carlo  (semi-permanent) Bozer and Carlo  (dynamic) Yu et approach.  I I and I We I I n n n in n Physically Manually Physically Manually Personally Manually Pick up truck Destination Destination Destination Vacation spot Destination Destination Destination Pickup truck Destination Truck Destination at the assigned to another door and time elements are not taken into account. If this condition is not really ful? lled, the ipod dock doors can be seen as (scarce) resources which have to be planned over time. This can be the so-called truck scheduling issue. Both challenges can be quite complex due to the range of doors as well as the dynamic character of the problem. This section handles the pier door job problem, whilst truck booking problems are reviewed in Section 4. 6. The task of boat dock doors can be executed on a mid-term or short-term intervalle .
Several documents solve the assignment issue on a mid-term horizon. Then, each pier door will serve a speci? c incoming or telephone destination for a for a longer time period of time (e. g. 6th months). 10 All trucks coming from the same origin or perhaps having the same destination will be assigned for the same boat dock. Such a? xed task is easier to get workers since they find out exactly where dock door they need to dispatch each fill, but it provides the expense of your reduced? exibility. Even if a? xed assignment is used, it is vital that the dock doors will be reassigned once there is a signi? cant change in the shipping pattern.
When ever data about the inbound trucks happen to be known significantly enough beforehand, the project of the vehicles can be resolved on a shortterm horizon. The trucks alone are assigned to the pier doors based upon the actual shipping? ow. This ‘? oating dock’ strategy is submit by Peck  who have studied the material handling businesses in an LTL terminal. This assignment means that the workers are every day confronted with a different door for the same vacation spot and have to consider care that the freight can be loaded into the correct truck. The use of contemporary information technology (e. g. areal code or RFID checking together with a WMS) can be useful for this end. A combination of equally is also possible. Several papers consider a cross-dock in which places are given to collection doors (so the telephone trucks will be assigned on a mid-term horizon), while the project of the inbound trucks is performed on a short-term horizon. The characteristics of the cross-docks considered inside the following documents are summarized in Stand 1 . Because time aspects are neglected and there are enough available pier doors, the preemption, introduction pattern and departure time characteristic are certainly not relevant here and are not really shown.
In his dissertation, Peck  builds up a detailed ruse model of a great LTL port and tries to assign the trucks to dock gates in order to reduce the travelling time11 in the shipments. The assumption is that the travel around time to transportation the products among This includes which the cross-dock operates in exclusive service mode. Here and in the next pages, the travel time is the time required to copy the goods inside from the inbound to the outbound truck. eleven 10 two trucks may be expressed as a function of the distance, depending on the actual items of the vans and the required means of transportation (2-wheeler, 4-wheeler or forklift).
The naming of doors while either strip or collection doors is? xed in advance. The problem is developed as a great integer programming model and because of the computational complexity, a heuristic (greedy balance algorithm) is offered to solve it. Simulation shows that his heuristic improves a great assignment based upon experience and intuition. One other early research about the assignment of trucks to dock entry doors is performed by simply Tsui and Chang . From this paper, a crossdock is regarded as in which not any storage is provided, every shipments go directly from inbound to telephone trucks.
The problem is solved on a mid-term distance, so the origins and spots have to be given to ipod dock doors, certainly not the trucks itself. The designation door as strip or stack doors is usually? xed. The assignment is actually formulated like a bilinear programming problem that tries to decrease the travel and leisure distance with the forklifts (the number of forklift trips needed to carry a specific load is usually assumed to become known). To resolve it, the authors propose a simple heuristic method to? nd a local the best possible.
The creators do not present test effects, but conclude that the discovered solution is a good starting place for the cross-dock administrator. There are present exact methods to solve bilinear optimization complications, but these are not very suited for this problem since the same writers mention in Tsui and Chang . From this paper, a branch-and-bound protocol is suggested to solve the dock door assignment difficulty exactly. The numerical testing show that algorithm is usually however computational expensive.? Bermudez and Cole  cope with a very similar trouble, but now you cannot find any? ed status for opportunities. All doorways can have got assigned either an origins or a vacation spot. The statistical model of Tsui and Chang  is usually adapted to consider this into account. The objective function minimizes the overall weighted travel distance instead of the real travel around distance. A genetic criteria (GA) can be propose
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