DAILY NEWS Jan 6, 2013 12:57 PM - 0 comments

Designing Optimization Part II: Finding the optimal rack layout

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By: Patrick Thibault
2013-01-06

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 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. In Part I of this series we discussed the importance of forklifts to rack system design and we challenged old conventions on the path to creating new solutions.

In Part II we focus on the pallet rack itself.

Finding the optimal rack layout; a creative process

While it is true that many rack system layouts will follow the roof-support column pattern in a precise and workable manner, it is also true that many times the application demands are seemingly incompatible with the building specs. In these situations, an experienced professional needs to apply appropriate design methods, in an effort to devise a reasonable solution.

 The process begins by 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 impact the aisle width requirement. The next step in the process is to apply a preferred pattern of rack/aisles to a small section of the warehouse (2 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 3 column lines is exposed (ending up in an aisle), then the same test should be re-run using different starting positions (at least 4’ from the original test’s start point). Results from this test will determine if more advanced methods are required.

The advanced methods that follow, necessary to find the optimal rack layout solution, include various ways of modifying the width footprint of designated rack segments within the overall pallet rack layout.  Often, it becomes necessary to add small amounts of expanded-depth rack in order 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.

By way of example, the attached depicts a rack system solution that includes strategically placed 2-deep ‘push-back’ rack connected (back-to-back) with a single-deep selective rack. A 2-deep selective rack could have also worked in lieu of the ‘push-back rack.  However, 2-deep selective rack also requires that a ‘deep-reach forklift’ be part of the fleet.  While the ‘push-back’ solution is more costly for the rack purchase, it has the added benefits of:  1) not requiring that one or more forklifts have a special deep-reach feature; 2) it is usable in a VNA application (where the first alternative would not); and 3) the ‘push-back’ rack is self- replenishing once the front location is depleted, thereby promoting more efficient picking.  Keep in mind that this particular use of a deeper storage is intended to be sporadic, but at the same time is justified in its nominal use by Pareto’s Principal of distribution (also called the 80/20 rule).  When applied here, it suggests that most inventories have within them multiple pallet SKU’s; in sufficient numbers (20%), to allow for modest amounts rack having ‘2-deep’ storage locations.

Other factors of the design

The details of an effective rack design also extend to the vertical dimension.  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 then its’ outrigger inside dimension is narrower than the width of the pallet load; thus requiring 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 the ‘tilting’ the load, mast deflection and reduced visual acuity of the operator at such vertical distances between him/her and the higher level pallet locations.

Additional considerations in pallet storage rack system design include several items related to building and fire codes (seismic stability, floor slab strength, longitudinal & transverse flu 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 & 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.

In the current LEAN thinking business environment, where purpose, process and people are the building blocks which transform ideals from maximization to optimization, the approach of creating a customized, ‘best solution’ pallet-load storage systems for each client is well suited to that LEAN agenda  This can be especially effective when planning relocations or expansions to new facilities, where racking expertise can make all the difference  in determining the storage system application which brings the best solution for the client’s operational challenges.

Note: The attached exhibit depicts a ‘real world’ application drawing of a pallet rack system that uses 11ft wide aisles with roof support columns on 50ft 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 that was 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 2 connected rows of rack, resulting in 17’ wide aisles and considerably fewer rows of rack, only the additions of the 2-deep ‘push-back’ rack).                                                                                                       

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.

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