In a traditional warehouse, storage racks are arranged to create parallel picking aisles, perhaps with one or more cross aisles to allow workers to move quickly between picking aisles. This structure forces workers to travel rectilinear distances (north-south and east-west) to picking locations.

The traditional design is based on a number of unspoken, and unnecessary, assumptions. Why, for example, must cross aisles meet picking aisles at right angles? Or why do picking aisles have to be parallel? The answer, of course, is that they do not, and our research shows that adhering to these assumptions could result in a significant penalty in labor costs.

We consider the problem of arranging picking aisles and cross aisles in new ways to reduce the cost of travel within warehouses. Our models produce alternative designs that promise to reduce travel distances in a reasonably-sized warehouse by more than 20 percent (for some operations). Below is an example that maintains parallel picking aisles, but allows the cross aisle to take on a different shape. We call this the "Flying V". If travel begins and ends at the bottom of the V, the expected distance to retrieve a single pallet is 10% less in this warehouse than in an equivalent traditional design.  See a simulation of the Flying V.

Travel in a Flying-V is along 3 possible paths. Workers can--and should--travel rectilinear paths to locations near the bottom of warehouse; they use the cross aisle to access both locations above the cross aisle and locations slightly below it. Notice that travel to items below the cross aisle seems less efficient than to items above it. To address this, we relax a second unspoken assumption--that picking aisles must be parallel. We call this the fishbone design.

The expected distance to a pick in a fishbone design is approximately 20% less than in a traditional warehouse. To see why there is such a large improvement, consider the illustration below.

Suppose we wish to travel from the lower dot to the upper dot in each figure. Rectilinear distance (as we are forced to travel in a traditional warehouse) is indicated with the dashed line; straight-line, "as the crow flies" distance with the gray line; and fishbone travel with the black line. For travel directly to the right or directly up from the bottom dot, neither rectilinear travel nor fishbone travel are far from the straightline distance. But for travel near a 45 degree angle, fishbone travel is much shorter than rectilinear travel. This corresponds to picks near the cross aisle in the fishbone warehouse.

A third design is a variation we call chevron aisles.

Chevron aisles are nearly as efficient as fishbone aisles, and have the added advantage that entry to the picking space need not be from a single point.

We are currently working on several extensions of this work. For example, in most warehouses travel is not to and from a single point, but to and from a region of many dock doors. We are working on models for such operations. We are also working on designs for order picking warehouses, in which workers travel to many possible locations during a tour. Aisle designs for these operations facilitate direct travel between dock doors and picking locations, and between different picking locations within the space.

1. ‣To see a simulation of the Flying V design, go here.  A simulation of the Fishbone design is here.
   
   

2. ‣For a more detailed overview, try this document.
   
   

3. ‣Letitia M. Pohl, Russell D. Meller, and Kevin R. Gue, Optimizing the Fishbone Aisle Design for Dual-Command Operations in a Warehouse, Naval Research Logistics 56:5, 389-403, 2009.
   
   

4. ‣Letitia M. Pohl, Russell D. Meller, and Kevin R. Gue, An Analysis of Dual Command Operations in Common Warehouse Designs, Transportation Research Part E 45:3, 367-379, 2009.
   
   

5. ‣Kevin R. Gue and Russell D. Meller, Aisle Configurations for Unit-Load Warehouses, IIE Transactions 41:3, 171-182, 2009.
   

Sponsored by the National Science Foundation