Warehouse Zoning & Bucket Brigades (Chpt. 9 in Bartholdi & Hackman; also see http://www.isye.gatech.edu/~jjb/bucket-brigades.html)

The two main concepts of zoning in contemporary warehousing Warehouse zoning: The physical and/or logical segmentation of the warehouse / picking area, through the employment of different storage modes and practices due to the product differentiation w.r.t. dimensions, physical characteristics storage and material handling requirements throughput, etc. the parallelization of the order-picking activity.

Zone-based order picking Progressive Zoning / Order assembly Order To packing and shipping Z1 Z2 Z3 Z4 Z5 Parallel/Simultaneous Zoning (typically organized in pick-waves with downstream sortation) To sorting and consolidation Z1 Z2 Z3 Z4 Z5 Order (Batch)

The problem and the problems of establishing effective logical zones in Warehouse operations The problem: Try to achieve maximum utilization of the picking resources, by distributing “equally” the total (picking) workload among the defined zones. The problems: The warehouse picking environment usually is a very dynamic environment; workload profiles are constantly changing. Existing zoning systems seek to balance the average workloads across zones, based on some hypothetical order work-content and worker behavioral models. Furthermore, constant zone redefinition requires a lot of effort from, both, the warehouse management (who must keep track of all the workload changes and re-establish the zones) and the warehouse pickers (who must adjust to the new policies). The results: Very limited scientific literature.

Bucket Brigades ( c.f. Bartholdi & Hackman, Chpt. 10) A dynamic self-balancing scheme for progressive zoning. The three main requirements of bucket brigades: Carry work forward, from station to station, until someone takes over your work. If a worker catches up to his successor, he remains idle until the station is available (i.e., no overpassing is allowed) Workers are sequenced from slowest to fastest.

The self-balancing property of bucket brigades Theorem: Letting v_i denote the working rate of the i-th worker, the line operation under the three main requirements of bucket brigades, converges to a balanced partition of the effort, wherein the fraction of work performed by the i-th worker is equal to v_i / _{j=1}^n v_j and the line production rate is equal to _{j=1}^n v_j items per unit time.