Aisle Configurations for Unit-Load Warehouses

Naval Research Logistics, 2009

Unit-load warehouses store and retrieve unit-loads, typically pallets. When storage and retrieval operations are not coordinated, travel is from a pickup and deposit (P&D) point to a pallet location and back again. In some facilities, workers interleave storage and retrieval operations to form a dual-command cycle. Two new aisle designs proposed by Gue and Meller ("Improving the unit-load warehouse." In Progress in Material Handling Research: 2006. Material Handling Industry of America, Charlotte, NC, 2006) use diagonal aisles to reduce the travel distance to a single pallet location by approximately 10 and 20% for the two designs, respectively. We develop analytical expressions for travel between pallet locations for one of these-the fishbone design. We then compare fishbone warehouses that have been optimized for dual-command to traditional warehouses that have been optimized in the same manner, and show that an optimal fishbone design reduces dual-command travel by 10-15%.

Facility Logistics: Approaches and Solutions to Next Generation Challenges, 2007

Order picking is typically the most costly operation in a warehouse and traveling is typically the most time consuming task within order picking. In this study we focus on the layout design for a rectangular warehouse, a warehouse with parallel storage blocks with main aisles separating them. We specifically analyze the impact of adding cross aisles that cut storage blocks perpendicularly, which can reduce travel times during order picking by introducing flexibility in going from one main aisle to the next. We consider two types of cross aisles, those that are equally spaced (Case 1) and those that are unequally spaced, which respectively have equal and unequal distances among them. For Case 2, we extend an earlier model and present a heuristic algorithm for finding the best distances among cross aisles. We carry out extensive computational experiments for a variety of warehouse designs. Our findings suggest that warehouse planners can obtain great travel time savings through establishing equally spaced cross aisles, but little additional savings in unequally-spaced cross isles. We present a look-up table that provides the best number of equally spaced cross aisles when the number of cross aisles (N) and the length of the warehouse (T) are given. Finally, when the values of N and T are not known, we suggest establishing three cross aisles in a warehouse.

IIE Transactions, 2011

We investigate the effect of assigning the most-active items to the best locations in unitload warehouses with non-traditional aisles. Specifically, we report the performance of flying-V and fishbone designs when products exhibit different velocity profiles. We consider both single-and dual-command operations in a warehouse where receiving and shipping are located at the midpoint of one side of the warehouse. For dualcommand operations, a fishbone design shows similar reductions in travel distances for both random and turnover-based storage policies. The fishbone designs that provide the best performance have a diagonal cross aisle that extends to the upper corners of the picking space, and are approximately half as tall as they are wide. In general, warehouse design parameters that perform best under random storage also perform well under turnover-based storage.

Engineering and Management Innovation, 2007

Order picking is typically the most costly operation in a warehouse, and traveling is typically the most time-consuming task within order picking. In this study, we focus on the layout design for a rectangular warehouse, a warehouse with parallel storage blocks with main aisles separating them. We specifically analyze the impact of adding cross aisles that cut storage blocks perpendicularly, which can reduce travel times during order picking by introducing flexibility in going from one main aisle to the next. We consider two types of cross aisles, those that are equally spaced (Case 1) and those that are unequally spaced (Case 2), which respectively have equal and unequal distances among them. For Case 2, we extend an earlier model and present a heuristic algorithm for finding the best distances among cross aisles. We carry out extensive computational experiments for a variety of warehouse designs. Our findings suggest that warehouse planners can obtain great traveltime savings by establishing equally spaced cross aisles, but little additional savings in unequally spaced cross aisles. We present a look-up table that provides the best number of equally spaced cross aisles when the number of cross aisles (N) and the length of the warehouse (T) are given. Finally, when the values of N and T are not known, we suggest establishing three cross aisles in a warehouse.

The International Journal of Advanced Manufacturing Technology, 2016

This paper considers the layout design problem of a single block order-picking rack-based warehouse that employs turnover-based storage assignment in both vertical and horizontal dimensions. An analytical expression for vertical travel distance is derived which is incorporated in the pick distance model. The effect of inventory staggering on storage space requirement is considered in arriving at warehouse dimensions. A model that incorporates area cost along with handling cost in optimizing warehouse design is developed and a solution algorithm is presented. The analytical model for vertical travel and the optimization model are applied to data from a real life case. It was found that the model would offer considerable operational cost savings, especially when space costs are high. Computational experiments show that the effect of inventory staggering is quite significant in the estimated storage space. Experiments also demonstrated the importance of segregating products based on turnover in the vertical dimension.

2017

Order picking is the most costly operation in a warehouse. However, current warehouse design practices have been using the same design principles for more than sixty years: straight rows with parallel pick aisles and perpendicular cross aisles that reduce the travel distance between pick locations. Gue and Meller (2009) altered this design mentality by proposing a fishbone layout that offered reductions in travel distance of up to 20% in unit-load warehouses. However, research in finding alternative layouts for order picking warehouses is lacking. The main result of this dissertation is to show that there are non-traditional designs that reduce the cost of order picking operation by changing the aisle orientation, aspect ratio, and placement of depot simultaneously. We present designs that achieve reductions in travel distance of up to 5.3%. Order picking warehouse layout optimization is computationally complex. Three problems need to be solved: layout design, product allocation, an...

European Journal of Operational Research, 2014

Space required for the order picking area and labor required to perform the picking activity are two significant costs for a distribution center (DC). Traditionally, DCs employ either entirely wide or entirely narrow aisles in their picking systems. Wide aisles allow pickers to pass each other, which reduces blocking, and requires fewer pickers than their narrow-aisle counterpart for the same throughput. However, the amount of space required for wide-aisle configurations is high. Narrow aisles utilize less space than wide aisles, but are less efficient because of the increased likelihood of congestion experienced by pickers. We propose a variation to the traditional orthogonal aisle designs where both wide and narrow aisles are mixed within the configuration, with a view that mixed-width aisles may provide a compromise between space and labor. To analyze these new mixed-width aisle configurations, we develop analytical models for space and travel time considering randomized storage and traversal routing policies. Through a cost-based optimization model, we identify system parameters for which mixed-width aisle configurations are optimal. Experimental results indicate that annual cost savings of up to $48,000 can be realized over systems with pure wide or narrow aisle configurations.

International Journal of Production Research, 2001

This paper considers routing and layout issues for parallel aisle warehouses. In such warehouses order pickers walk or drive along the aisles to pick products from storage. They can change aisles at a number of cross aisles. These cross aisles are usually located at the front and back of the warehouse, but there can also be one or more cross aisles at positions in between. We describe a number of heuristics to determine order picking routes in a warehouse with two or more cross aisles. To analyse the performance of the heuristics, a branch-and-bound algorithm is used that generates shortest order picking routes. Performance comparisons between heuristics and the branch-and-bound algorithm are given for various warehouse layouts and order sizes. For the majority of the instances with more than two cross aisles, a newly developed heuristic appears to perform better than the existing heuristics. Furthermore, some consequences for layout are discussed. From the results it appears that the addition of cross aisles to the warehouse layout can decrease handling time of the orders by lowering average travel times. However, adding a large number of cross aisles may increase average travel times because the space occupied by the cross aisles has to be traversed as well.

Order picking is the warehousing process by which products are retrieved from their storage locations in response to customers' orders. Its efficiency can be influenced through the layout of the area and the operating policies. We present a model that minimizes travel distances in the picking area by identifying an appropriate layout structure consisting of one or more blocks of parallel aisles. The model has been developed for one commonly used routing policy, but it is shown to be fairly accurate for some other routing policies as well.

Citeseer, 2002

Operations in unit-load warehouses include single-command cycles and dual-command cycles, where storage and retrieval operations are interleaved. We model dual-command travel in two warehouse layouts that occur commonly in industry, and compare the efficiency of the two designs. General guidelines for optimizing the two aisle layouts are provided.