The Drawbacks of High-Density Storage

As we all know, land is a finite resource. Particularly scarce is developable land that increases the supply of real property in the marketplace and keeps the price of real estate from increasing.

Because we can't just create more useful land, real property costs — commercial, industrial or residential — continue their steady upward march.

Warehouses and distribution centers, in particular, take up a lot of space and need lots of surrounding land to support inbound and outbound tractor trailers. Anything that can be done to reduce the size of a distribution center reduces its cost as well as the fixed overhead of the company using it.

Since space is so valuable for its rarity, those planning distribution center layouts often consider very-narrow-aisle (VNA) storage systems. VNA storage can increase the storage density of a facility dramatically without sacrificing selectivity. But, if not applied with some wisdom and upfront analysis, a VNA storage system can also become an unexpected bottleneck, constraining a distribution operation from achieving the throughput customers require.

There are several factors managers must keep in mind when evaluating a VNA storage strategy. Some preparation and knowledge can help ensure a facility benefits from maximum density while avoiding potential throughput limitations.

How Narrow?

Most VNA systems have aisles that range from six-feet to five-feet, six-inches. There are narrower systems, such as those applying shuttle forks where a standard pallet (40 × 48 inches) is carried down the aisle with its 48-inch dimension perpendicular to the direction of travel instead of the usual parallel orientation, but let's focus on the more common VNA applications.

Common VNA layouts use turret or swing reach lift trucks to service the aisles. The trucks are so named because the forks of the vehicle turn 90 degrees to the left or right, allowing a pallet to be placed in a rack storage position without the truck turning.

A fixed rail system or floor-embedded wire guidance system allows the trucks to move rapidly and safely in the aisle without hitting the racks. The truck moves only straight forward and backward in a VNA, which is at least three feet narrower than the narrowest reach truck aisle.

The density increase is clear; three feet times the length of a storage aisle (let's say 300 feet) times the number of aisles in the warehouse (let's say 20) is a minimum footprint reduction of 18,000 square feet. At $50 per square foot to construct, that's a $900,000 capital savings through building footprint reduction. If the VNA system is in a facility with a taller clear height (Large turret trucks can service systems in a facility with nearly 60-foot clear heights.), it is possible to reduce the square footage even more. Those savings are significant, even after allowing for the higher price of turret lift trucks, which can cost three times as much as conventional counterbalance or reach trucks.

The Downside

But those savings come with sacrifices. The density increase requires a solid understanding of storage throughput requirements, coordination of workload planning for pallets moving in and out of the VNA system and an ability to ensure that volumes can be spread across the aisles.