Over my 30-year career, I have seen many building shapes you won’t find in a geometry book. Most of these odd building shapes were a result of taking advantage of site dimensions and years of expansions. While traveling, I often look out the airplane window at the various building shapes and sizes. The majority of buildings are rectangles with various lengths and widths. There are also unique designs, such as, L-shaped buildings that work well for sharing dock resources and finger dock usage that results in buildings shaped like the letter ‘E’. When booking your next air travel, make sure to pick a window seat, and look at the buildings during take-off and landing! So, what is the right building shape for your operation and does it even matter?
Many businesses leverage a specific building shape for dock utilization, cross-docking, and other unique material flow requirements. But, if you are not building a new facility, finding the exact shape can be a challenge. More often than not, you are leasing an available building that meets the majority of your design requirements and has favorable commercial terms. Then, you must adjust the internal processes, material flow, and equipment layout to accommodate the building. Yes, we don’t like this either and prefer to design the optimum operation and build-to-suit.
So, let’s dream big, and explore the most common desirable facility shape and other factors that drive efficient operational designs. You might be able to make a business case for new constuction versus settling on an available building.
Size & Shape
A rectangular building is the most dominate shape used by distributors and developers. The building depth is ideally defined by the dock staging requirements and length of storage aisles. For example, many operators want a dock deep enough for staging and handling pallets for unloading and loading a 52-feet deep trailer. The remaining building depth is defined by the length of the storage aisles. Common storage aisles are from 100 to 125 feet, before requiring a cross-aisle (~12 feet) to satisfy egress dictated by local fire codes. Larger warehouses have the cross-aisle positioned in the middle of the storage area with aisles that can reach 250-300 feet long or more. Assuming docks are only on one side of the building, these assumptions make a common building depth 350 to 400 feet or more.
The length of a building is defined by the number of storage aisles, rack equipment depth, and the number of dock doors required. The length is almost always longer than the depth of the building. In most situations, the storage area is large (wide) enough that the resulting building length provides more than enough dock doors and staging space.
According to the “2021 Warehouse/DC Operations Survey: Automation as a Disruption Response” article in Logistics Management magazine, the average warehouse size is about 200,000 square feet. And, when a company has more than four (4) distribution centers, the average size increases to 530,000 square feet. These buildings sizes (200,000 to 530,000 square feet) account for about 50% of the surveyed companies, with 40% using buildings less than 100,000 square feet, and the remainder with larger than 1 million square feet buildings. You can read more about these other distribution trends in their entirety in the Logistics Management article here.
Bay spacing refers to the distance between the building’s structural support columns. The St. Onge drafting team works on hundreds of building designs per year, so I asked them to tell me the most common bay dimensions. They came back with more than 15 bay spacing examples, ranging from 35 to 60 feet! That is a result of working with clients in buildings dating back 20+ years to newer constructions. From my recent experience, it seems many spec buildings are being designed with 50bay spacings of 50 feet or more.
But, the reality is the ‘best’ bay spacing depends on your storage equipment (single deep, double deep, etc.) and aisle width (wide, narrow, very narrow aisles) requirements. The primary layout planning objective is to not have a column in the middle of a travel aisle by ‘burying’ it within the back-to-back rack bay flue space (longitudinal flue space), or within the rack bay (losing multiple pallet positions).
The bay spacing and resulting number (and size) of columns in a building impacts the construction cost. From an operational standpoint, we want fewer columns in a building. But, fewer columns can mean they are bigger, so they still impact layout planning. From a sampling of many building drawings, most columns range in size from 6 to 12 inches (some are round, others are square and yet others are I-beams).
The column size also impacts the longitudinal flue space dimensions between rows of pallet storage racking when burying columns. When there is no column to work around, a common longitudinal flue space is 12 inches. This provides a 3-inch pallet overhang in back-to-back rack bays and leaves 6 inches between the pallets. This space is dictated by local fire codes to provide required fire protection. When dealing with a common 6-inch building column, this flue space can expand to 18 inches in order to provide these same clearances.
Ultimately, you don’t want to make rack type and aisle width decisions based on your column bay spacings. So, it is recommended to draft your optimum layout and adjust to limit the impact of columns on the design.
The placement of the receiving and shipping docks has a big impact on the material flow of an operation. The primary options are to have docks on one side of the building, or use a cross-dock design with docks on opposite sides of the building. From my experience, most operations are designed with docks on the same side of the building to enable sharing labor resources, enhance management, and reduce the size of the building. The dock area is often designed with nothing overhead, meaning there is a lot of wasted vertical space. However, the clear height in the dock area is often utilized for conveyor, sorters, and mezzanines for packing operations.
For others, having the dock on opposite sides of the building is required to provide the dock door and staging requirements, and to enable a flow-through design. Many manufacturers use a cross-dock (flow through) design to move raw materials from one side of the building to the other to support production areas. Often, the decision on dock placement is a personal preference, but it should be driven by what benefits the operation in terms of material flow, labor efficiency, dock door and staging requirements.
One of the first things that I observe when entering a building is the clear height and whether it is fully utilized. The same survey referenced above shows building clear heights range from 20 to more than 50 feet. And, the survey reports the average building height is around 32 feet. My observations support these findings, while new buildings are being constructed at 36 to 42+ clear heights. After all, the taller a building is the smaller the foot-print requirements.
The ideal clear height depends on your pallet heights, number of storage levels, type of storage and material handling equipment. And, sometimes operators are just not comfortable working within storage areas with more than 5-levels of storage. A lift truck operator does move slower when accessing top storage levels when using a sit-down truck. And, while cameras can be installed on lift trucks to improve visibility, it is still a slower operation. But, again, the footprint (and operating costs) of the building drastically shrinks with taller buildings. Buildings that are more than 42+ feet high typically have automation and can reach 100+ feet.
There are many factors that influence the shape and size of distribution centers. Most are driven by your operational requirements, while others are influenced by your past experiences and comfort levels. Do your best to allow operational requirements drive a building selection, but be ready to make compromises unless you can build new.
While there are good reasons behind a desired building shape and size, what matters most is the operational design inside, so ‘Don’t Judge a DC by its Cover’. There are numerous building shapes and sizes, but the processes within them are the key to achieving operational excellence.
—Norm Saenz, St. Onge Company