Transcript Crossdocking - Georgia Institute of Technology

Crossdocking
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, 33:4, pg. 419-428, November, 1999.
• An interesting site:
– http://web.nps.navy.mil/~krgue/Crossdocking/crossdocking.html
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.
• => Just-In-Time for distribution
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.
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
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)
A queueing model for
the crossdock operations
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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
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
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.
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.
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
– => The alternating heuristic produces solutions that are typically 10%
better than the solutions produced by the block heuristic.
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.