Logistics News

By Cliff Holste

October 12, 2011

Optimizing Cubic Storage Space In The DC Is A Cost Effective Strategy

Companies Can Avoid Off-Site Storage Expenses By Adopting High Bay Technologies

While the primary purpose of a DC is to provide speedy flow-through order fulfillment, maintaining a limited amount of inventory is unavoidable. However, many DCs find it necessary to maintain a warehouse capable of holding large quantities of SKUs for several weeks or months. In either of these situations, efficient storage, and quick access to products are key requirements for a competitive operation.

Whether you’re planning a new Distribution Center or upgrading an existing one, optimizing your building’s pallet storage capacity is an option proven to be far less expensive than leasing additional off-site warehousing space. Temporarily storing pallet loads in trailers staged in the yard may provide convenient and relatively inexpensive storage, but requires considerable extra handling, which will prove neither productive nor cost effective.

For most DCs, the factors that determine the optimum storage types and configurations are both numerous and interwoven. The type and variety of products handled, throughput requirements, building design, and staffing levels will all influence the choice of storage systems, as will budget and ROI.

Holste Says: For most DCs, the factors that determine the optimum storage types and configurations are both numerous and interwoven. What Do You Say? Click Here to Send Us Your Comments Click Here to See Reader Feedback

Common Pallet Storage Rack Considerations

Other than simply stacking pallet loads on the floor (bulk storage), which may be an option for selected high velocity SKUs, most DC storage schemes utilize one or more of the following pallet rack configurations:

Single deep selective racks, - are the most widely used type of pallet rack. There biggest advantage is accessibility - they offer 100% access to every pallet position, thereby providing maximum pallet slot utilization including the option of picking individual cases. However, storage density is somewhat compromised due to the number of lift truck aisles required to access every single pallet position.

Double deep racks, - consists of two back-to-back racks that are two pallets deep, substantially increasing storage density by cutting the number of lift truck aisles required in half when compared to single deep rack. However, the potential for some honeycombing is increased along with reduced load selectivity if/when different SKUs are placed in the same slot.

Drive-in/drive-through racks, - offer a way to eliminate nonproductive aisle space. If your operation involves many pallet loads that contain the same SKU, drive-in/drive-through racks could be a perfect high density solution. However, because they produce a first-in/last-out product flow, they often aren’t the right choice for applications that require stock rotation.

Pallet flow racks, - feature first-in-first-out (FIFO) stock handling, high storage density, and low maintenance. Although they can be get to be a little pricy when compared to the above static racks, they are never the less ideal for applications that have a relativity small number of SKUs along with a high pallet throughput requirement.

Mobile rack systems, - can be considered when there is sever space restrictions and low to medium throughput requirements. A mobile rack installation requires a single aisle and fits into cramped quarters.

Automated Unit Load Storage & Retrieval, - deployed in highly automated, high-rise operations, Automated Storage & Retrieval Systems (AS/RS) can provide both high storage density and high throughput, coupled with nearly flawless inventory control.

Obtaining the Best Storage Cube Utilization

One of the most basic planning decisions relative to pallet rack layout is the width of the lift truck aisles and the number of pallet loads high. Do you want to use a conventional; a narrow-aisle (NA); or a very-narrow-aisle (VNA); rack configuration? The following is a brief overview of what each approach has to offer.

Conventional

Conventional pallet rack layouts typically use aisle widths of 12 feet, and are usually no more than 4 pallets high permitting the use of standard counterbalanced fork lift trucks. To translate this into storage positions, 1,000 pallets will require about 10,500 sq. ft. of floor space.

Narrow Aisle (NA)

By choosing to go with a narrow-aisle (NA) layout, you will be able to store the same 1,000 pallets in about 6,000 sq. ft. of floor space, which equates to a savings of about 40%. This is because the NA layout utilizes a specially designed compact Reach Truck that can stack pallets 6 high while operating in an 8 ft. wide aisle. The Reach Truck is equipped with a pantograph type of reach mechanism that scissors the pallet forward from the mast. In operation, the truck turns a right angle to face the rack, and then extends the pallet load out into its proper slot. Due to the truck’s compact overall length and the fact that it carries its forks in a retracted position, it requires substantially less room to turn than does a conventional counterbalanced fork lift truck.

A variation of this truck design is the double-reach, which uses a double pantograph mechanism to increase the length of its reach. A double-reach truck can store pallets two-deep, permitting you to use double-deep rows of rack. Double-reach trucks require an additional 6” of aisle width for their turns (8’-6” total). With this layout you’ll only need 4,500 sq. ft. to accommodate 1,000 pallets.

Very Narrow Aisle (VNA)

If you need to squeeze even more storage capacity out of your warehouse you should consider a very-narrow-aisle (VNA) rack layout. With VNA storage you can narrow the aisle widths down to 5’-6” or even less, while increasing rack heights to as many as 9 slots (40 feet.). Now you’ll need just 3,200 sq. ft. of floor space to store 1,000 pallets. VNA requires a highly specialized rack-guided or wire-guided Swing-Reach Turret Truck, the price of which reflects their sophisticated designs and performance capabilities.

In addition, the aisle floor must be “super flat”. There are concrete floor specialists that work with contractors to pour a super flat floor for new construction and/or provide corrective concrete floor laser grinding in existing buildings.

Another important consideration is the local fire protection code that limits product stacking height. For general merchandise and most consumer goods, the National Fire Protection Association (NFPA) has approved ESFR systems for use in storage areas where the stacked product height does not exceed 35 feet, and the ceiling height is 40 feet or less (see – Fire Protection Code Limits Product Stacking Height).

One Size Doesn’t Fit All

Some DCs employ multiple methods of storage, such as various types of racking or mezzanines. Combining different types of storage systems is important in optimizing space utilization. However, the storage types and configurations should be selected based on SKU handling characteristics, velocity and inventory turns.

The Pareto principle states that 80% of the activity in a DC comes from 20% of the SKUs, and these SKUs are the fast movers within the warehouse. The next 15% of the activity comes from 30% of the SKUs, which are the medium movers. Finally, 5% of DC activity comes from 50% of the SKUs, which are the slow movers.

By separating the fast, medium, and slow moving SKUs, throughput can be increased by optimizing access to the fastest moving products. Easier access to the products that generate the most DC activity reduces time spent traveling while increasing productivity.

Fast movers typically are stored in bulk storage and selective pallet rack. For companies using reach trucks, fast moving SKUs can be stored in narrow aisles. Medium movers are often best stored in narrow aisles that are equipped with selective pallet rack, carton-flow rack or carousels. Slow moving SKUs generally are stored in very narrow aisles and utilize turret trucks, carousels and shelving.

Final Thoughts

The type of pallet storage system and its configuration, along with the type of lift trucks deployed should all be selected based on storage and throughput requirements while taking facility constraints into considering.