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Store: Creative Solutions in Storage Rack Design

Aug. 14, 2013
The combination of rack and aisle can take up to 80% of a warehouse's total available floor space. That's why improving storage requires as much attention to what happens in the aisles as in the rack.

The first step in designing an effective pallet storage system is to 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 SKUs 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 can minimize 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 Forklift Effect

For many operations, a combination of  highly dense pallet storage (floor bulk, drive-in and 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 system 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 also right-angle-stack, 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 and 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 use a floor-embedded wire system allowing 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 can yield a significant increase in space utilization (smaller storage footprint or added storage in a similar size footprint, as compared to wider aisle applications).

Challenge Old Conventions 

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 flue space between two connected rows of racks and then adjusting aisle widths, and intermediate pallet rack rows, accordingly. The intent is to protect the columns and to not block any usable pallet storage locations. 

This pallet rack system rendering uses 11-foot-wide aisles with roof support columns on 50-foot centers. These columns are located near the aisle side face of the rack, causing a loss of pallet positions that is equivalent to 26 full bays of rack. The amount of additional rack made by this design is 162 bays, or an increase of 136 bays. The six rows of 2-deep push-back rack represents 9% of all bays and 16% of all stored pallets, including floor bulk. (This analysis does not consider the conventional rack layout which would have shown all the roof-support columns in the flu space of two connected rows of rack, resulting in 17-foot-wide aisles and considerably fewer rows of rack, only the additions of the 2-deep push-back rack).

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). 

By maintaining minimum aisle widths and forgoing old conventions about locating roof-support columns only in connected rack row flu spaces, it is possible to get additional rows of rack from the available warehouse space. 

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 remaining variable is the pallet rack itself.

The Optimal Rack Layout

Application demands are sometimes incompatible with the building specs. Finding a solution begins with verifying all pertinent details of the building, pallet loads, potential forklifts and expected-use of any bulk storage. The focus here is not isolated specifications alone, but any ranges, deviations or revisions that should also be incorporated. 

A good example is that of forklift aisles. Although they are to be uniform in width, they typically are made fixed within a range of + 5% to 7% of their prescribed norm. Battery box sizes and attachments, as well as front/rear pallet load overhang, can influence the aisle width requirement. 

The next step is to apply a preferred pattern of rack/aisles to a small section of the warehouse (two column bays, at minimum, starting one or more building bays from a parallel-running wall). The information gathered here is crucial.  If this test fails, in that one of the three column lines is exposed (ending up in an aisle), then the same test should be re-run using different starting positions (at least four feet from the original test's start point). Results from this test will determine if more advanced methods are required.

Often, it becomes necessary to add small amounts of expanded-depth rack for a rack design to work as intended. This can be as insignificant as changing the row spacer connector length between two back-to-back rows of single-deep selective rack, or as important as substituting a deeper storage rack, like deep-reach selective rack or push-back rack. The most popular rack width modifier is to reduce a back-to-back row of rack to a single row. On the rare occasion, consideration for rotating the entire rack system 90 degrees may be required. There is no denying that a certain amount of creativity goes into the search for an optimal design, but it is a creativity born from experience.

The accomplishment of the ultimate goal is often based on determining repeatable patterns within the overall rack layout and mirroring those patterns from one or more focal points within the building.  (This is likened to a butterfly or inkblot pattern … as created when a partial rack design is "folded" out from a center "focal" location).  The design skill here is in determining the locations and frequency of the pattern centers throughout a given building. Of course, it is important to provide cost-justifications, in terms of a benefit analysis, for any proposed solution. 

Other Design Considerations 

Beam level positioning should account for more than just pallet-load heights and an industry-standard lift-off space (4").  Often, additional lift-off space is required, as in reach forklift applications where the outrigger's inside dimension is narrower than the width of the pallet load. This requires the operator to perform an "up and over" pallet maneuver to clear the 5-1/2" tall outrigger height when placing or extracting a ground level pallet load.  

Also, at beam levels above 20' it is usually necessary to increase the "lift-off" space for any forklift application due to load tilting, mast deflection and reduced visual acuity of the operator at such vertical distances.

Additional considerations in pallet storage rack system design include several items related to building and fire codes (seismic stability, floor slab strength, longitudinal and transverse flue spaces, maximum storage heights, egress path code compliance, in-rack and overhead fire sprinkler protection).  In addition, pallet-storage rack systems must also consider lighting, rack load capacity and directional signage, storage location labeling/bar code identification, forklift impact protection devices, in-floor wire/rail guidance systems and a host of accessory features of the rack itself.  Furthermore, it must be code compliant and provide protection from impact damage that might otherwise render the system structurally noncompliant.  Finally, it should promote both operator effectiveness and personal safety. 

Patrick J. Thibault is general manager, Chino Division for Wynright Corporation's Engineering and Integration Group. He has nearly two dozen years in sales and management, and has focused on the material handling industry.    

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