Nowhere in today’s modern distribution centers is the concept of space utilization improvement given a more ‘fertile field’, than in the oldest and largest consumer of warehouse space; pallet-load storage.
The footprint devoted to such storage includes both the physical area that houses the palletized inventory and all access/perimeter aisles. The combination can typically reach 50% to 80% of the warehouse’s total available floor space. In the facilities where pallet storage racking is installed as the primary means of storage, the opportunities to use available space more effectively expands significantly into the realm of true ‘cube’ space utilization of a building…length, width and height!
The first step in designing an effective and superior pallet storage system is to optimally combine the critical (but often opposing) functions of storage density and product selectivity. The factors to be considered for a balanced storage system include: the total number of SKU’s which are palletized, the number which have threshold inventory quantities of more than one pallet load and up to or more than 20 pallet loads, rotation-of-stock (F.I.F.O.) requirements, and the methods by which product selection and replenishment will occur. High density storage enhances space utilization, but minimizes individual SKU pallet-load access. Direct ‘face’ access (selectivity) to all individual pallets typically requires more aisle floor space than high density storage arrays.
The importance of forklifts to rack system design
For many operations, a combination of highly dense pallet storage (floor bulk, drive-in & pallet-flow racks) and single-deep ‘selective’ rack, provides the best solution for accommodating large and diverse SKU bases that have a wide range of ‘in-stock’ pallet loads per SKU. For other operations, the storage systems design is driven by space factors; such as using the maximum available storage height of the building, combined with the least amount of aisle square footage. This alternative requires specialized forklifts that extend higher and require smaller aisle widths than conventional forklifts. In this application it is also common to use less floor bulk storage due to the height limitations of conventional counter-balanced forklifts or the need for individual pallet load access (selectivity), regardless of any individual SKU’s pallet count.
The typical rack layout will fall into one of these three configurations: Standard Aisle (10’+ wide), Narrow Aisle (8’-9’ wide) and Very Narrow Aisle (VNA 7’ or less). These aisle-width classifications are more about the type, features and capabilities of the forklifts used to access pallet locations. Standard Aisle rack layouts typically utilize right-angle stacking, counter-balanced forklifts, such as 3 or 4-wheel ‘sit-down’ forklifts and 3-wheel ‘stand-up’ trucks. Narrow Aisle forklifts are also right-angle stacking forklifts, but use ‘front-end’ outriggers for stability, which makes the overall combined length of forklift and pallet load less than for the standard aisle forklifts. The VNA rack layout employs ‘swing’ (or pivoting) forklift mast technology, which negates having to turn the entire forklift at a right angle to the rack during pallet put-away & retrieval.
Pallet-load forklifts can be either ‘man-up’ (where the operator compartment elevates with the pallet-fork assembly) or ‘man-down’ (operator at ground level). These VNA trucks also may be equipped with ‘guidance’ systems that uses a floor-embedded wire system which allows for increased travel speeds, reduced steering demands and enhanced safety. In warehouse forklift technology, a decrease in aisle size is accompanied by an increase in forklift investment. The added advantage in going ‘narrow’ is to also go higher; the combined result yielding a significant increase in space utilization (smaller storage footprint or added storage in a similar size footprint, as compared to wider aisle applications).
Current challenges & challenging old conventions
The challenge of providing the best available rack layout solution is to maximize the number of usable storage locations given the parameters of pallet load dimensions, required minimum forklift aisle widths for the particular forklift/pallet load, roof-support column locations, height restrictions (building or forklift), physical obstructions (building stability bracing, overhead lighting/piping, floor grating/access plates, etc.), building/fire code regulations, and cost.
If the building for which the storage system is to be designed has not yet been selected or built, the rack layout may impact the selection or design of a future facility, since certain roof-support column placements can compromise even the most creative pallet rack layout design. However, this is more the exception than the rule, and usually the task is to find the best available layout for the ‘current condition’ parameters. Indeed, several alternative layouts may be required in order to ascertain the optimal cost-benefit ratio of the various forklift/storage location combinations.
The key factor in designing rack layouts is in knowing that pallet load dimensions, along with forklift aisle requirements, are often far more important considerations than the roof-support column locations. Conventional wisdom has long sought to first isolate these key building structures by locating them in the ‘flu’ space between two connected rows of racks and then adjust aisle widths, and intermediate pallet rack rows, accordingly. The intent suggests to protect the columns with the rack and to not block any usable pallet storage locations.
Unfortunately, this method can be the greatest deterrent to increased space utilization in rack system design! The first problem is that it disregards the ‘best practice’ to always maintain minimal, uniform aisle widths throughout the warehouse. The benefits of doing so are: better space efficiency, improved forklift operator productivity and decreased rack damage (as operators learn to perform consistent, repetitive forklift maneuvers in aisles of standard widths). At issue, is that this method supports a popular myth in rack design…that to purposely obstruct pallet storage locations is a bad thing. Contrarily, by maintaining minimum aisle widths and forgoing old conventions about locating roof-support columns located only in connected rack row ‘flu’ spaces, it is possible to exact additional rows of rack from the available warehouse space, providing a net gain in pallet locations that can be many times more than the number of those pallet locations that were obstructed in the more creative design alternative.
Rack layout design, even in the largest of ‘big box’ warehouses, is truly a ‘game of inches’. While it remains important to not expose roof-support columns to the potential for forklift impact damage, many creative rack system designs use building column lines as ‘borders’. The detailed design work must consider any interference with rack and columns, but the primary goal remains to optimize storage locations without having roof-support columns in forklift aisles that maintain standard (minimal) widths. That done, the only variable that remains, is the pallet rack itself. We' will examine that in the next installment.
Patrick J. Thibault is General Manager, Chino Division for Wynright Corporation’s Engineering and Integration Group. Pat has nearly two dozen years in sales and management, and has focused on the material handling industry. He holds a B.S. degree in business administration and specializes in design, sales and implementation of rack systems.