Crossdocking

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Crossdocking
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Literature and interesting Web sites

• Lecture material
• Bartholdi Hackman, Chpt. 11
• Kevin Gue, Crossdocking Just-In-Time for
  Distribution, Tech. Report, Graduate School of
  Business Public Policy, Naval Postgraduate
  School, Monterey, CA, May 2001
• J. Bartholdi and K. Gue, The Best Shape for a
  Crossdock, working paper
• K. Gue, The Effects of Trailer Scheduling on the
  Layout of Freight Terminals, Transportation
  Science, 334, pg. 419-428, November, 1999.
• An interesting site
• http//web.nps.navy.mil/krgue/Crossdocking/crossd
  ocking.html

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The driving idea behind crossdocking

• Crossdocking seeks to eliminate the expensive
  functions of inventory holding and order picking
  from modern distribution centers by taking
  advantage of the information system
  infrastructure in modern supply chains.
• Hence, at a crossdock, incoming material is
  already assigned to a destination, and therefore,
  the only required functions are consolidation and
  shipping.
• In this way, material is staged at the facility
  for less than 24 hours.
• gt Just-In-Time for distribution

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Major requirements for justifying and
effectively deploying a crossdock operation

• Significant and steady product flow
• easy to handle material / unit-loads
• Good and reliable information flow across the
  entire supply chain
• pre-distribution crossdocking the customer is
  assigned before the shipment leaves the vendor,
  so it arrives to the crossdock bagged and tagged
  for transfer.
• post-distribution crossdocking the crossdock
  itself allocates material to its stores.

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Examples

• Home Depot operates a pre-distribution crossdock
  in Philadelphia serving more than 100 stores in
  the Northeast area.
• Wal-Mart uses
• traditional warehousing for staple stock - i.e.,
  items that customers are expected to find in the
  same place in every Wal-Mart (e.g., toothpaste,
  shampoo, etc.)
• crossdocking for direct ship - i.e., items that
  Wal-Mart buyers have gotten a great deal on and
  are pushing out to the stores
• Costco uses pallet-based post-distribution
  crossdocking
• Computer firms like Dell consolidate the major
  computer components in merge in transit
  centers.
• JIT manufacturers consolidate inbound supplies in
  a nearby warehouse
• LTL and package carriers (UPS, FedEx) crossdock
  to consolidate freight

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Crossdock Operations
Strip doors doors where full trailers are parked
and unloaded. Any incoming trailer can be
unloaded to any strip door.
Stack doors doors where empty trailers are put
to collect freight for specific destinations.
Each stack door is permanently assigned to a
distinct destination.

• Typical material handling modes
• manual carts for smaller items
• pallet jacks and forklifts for pallet loads
• cart draglines (reduce walking time but impede
  forklift travel)

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A queueing model for the crossdock operations

• Customers Inbound trailers
• Servers Strip doors
• Buffering Queue Parking lot
• Processing times
• customer-dependent freight mix
• server-dependent distance of the strip door
  under consideration from the stack doors serving
  the destinations of the trailer freight

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Optimizing the crossdock performance

• The major operational cost for crossdock is the
  labor cost.
• Hence, the system performance is optimized by
  seeking to maximize the throughput of the
  crossdock operations by establishing an efficient
  freight flow.
• Factors affecting the freight flow
• Long term decisions
• Number of doors and shape of the building
• Employed material handling systems
• parking facilities
• Medium term decisions
• Crossdock layout, i.e., the characterization of
  the various doors as strip or stack doors, and
  the assignment of specific destinations to the
  stack doors
• Short term decisions
• Inbound Trailer Scheduling

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The number of doors and the parking lot size

• Number of stack doors determined by the volume
  of freight moved to each customer, and any
  potential delivery schedules
• Number of strip doors since trailer unloading is
  a faster job than trailer loading, a common rule
  of thumb is to have twice as many stack doors as
  strip doors, so that you balance the incoming
  with the outgoing flow.
• In general the larger the number of doors in the
  crossdock, the larger the distances that must be
  traveled.
• The parking lot should provide parking space for
  two trailers per door, so any flow surges can be
  accommodated without considerable problems.

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The shape of the crossdock building

• Corners are bad! Specifically
• Internal corners take away door locations (about
  8 doors per corner)
• External corners take away storage space in
  front of the door (w/2 doors worth of
  floor space)
• On the other hand, a building shape that
  minimizes its corners increases
• the travel distances
• the traffic congestion in front of the most
  centrally located (and therefore,
• the best) doors
• Some characterizations of the crossdock building
  shapes
• diameter max door-to-door distance
• centrality the ratio of the obtained number of
  extra doors over the resulting diameter
  increase for a symmetric expansion of the
  building by two doors at each end of it.
• Suggested building shapes
• I for small crossdocks (up to 150 doors)
• T for medium size crossdocks (between 150-250
  doors)
• X for the largest crossdocks (above 250 doors)
• Frequently, the building shape is determined by
  other constraints, e.g.,
• available land, an existing building, etc.

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Crossdock layout

• In general, centrally located doors should be
  reserved for the uloading activity and for
  destination with large outgoing flows.
• On the other hand, if the freight on each inbound
  trailer is destined to a small and stable set of
  customers, then the facility can be decongested
  by establishing distinct hubs serving clusters of
  destinations that tend to have their freight on
  the same incoming trailers.
• Two extensively used heuristics are
• the block heuristic Assign first the unloading
  activity to the best doors (i.e. the doors having
  the smallest average distances to all other
  doors). Subsequently, assign the remaining doors
  to outbound destinations, prioritizing them in
  decreasing order of their flow intensities
• the alternating heuristic The door assignment
  alternates between a strip door and a stack door
  to the destination with the next highest flow
• gt The alternating heuristic produces solutions
  that are typically 10 better than the solutions
  produced by the block heuristic.

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Trailer Scheduling

• How should we pick the next inbound trailer to be
  processed at a free strip door?
• If the freight mix tends to be uniform across all
  inbound trailers, then a simple rule like FIFO
  will perform well.
• Otherwise, the selected trailer should be the one
  that will have the smallest processing time
  w.r.t. the considered strip door, among those
  currently waiting in the parking lot.