Where do you buy paper towels? How about headphones? Office supplies? Maybe not at a traditional brick-and-mortar store like you used to.

It isn’t news that online shopping is transforming retail.  “E-commerce” has taken significant market share from traditional brick-and-mortar retail stores, and why not? With lower prices, better selection, quick delivery, and the ability to get all or most of your shopping done from the comfort of your couch, there might not be a reason to leave the house again.

As online shopping continues its rapid growth, it requires modern distribution and fulfillment centers designed specifically for how merchandise is shipped, stored, and distributed to today’s customer. It’s no surprise then that the market for new distribution and fulfillment center facilities is booming, and the design of these new facilities is changing and adapting to what’s driving the future of distribution:

  • Taller + bigger
  • Multi-story
  • Material handling automation

Taller + Bigger

You might’ve driven past a new distribution center recently and thought, wow, that’s a huge building.  You’re right – they just keep getting bigger.

Twenty to thirty years ago, distribution center clear height (the vertical clear distance between the ground floor slab and the underside of structure) was commonly 24, 28, or 32 ft. However, as land close to cities has become scarce and more expensive, and as fire protection and material handling technology has advanced to allow for higher storage, developers and tenants are optimizing space by building taller. Today 36 and 40 ft clear is the new normal, and the market continues to demand even taller buildings.

Where large tracts of prime developable land are available, large distribution center footprints are attractive because they offer scale advantages. According to Dodge Data & Analytics, the number of 1 million sq ft or larger distribution centers built per year has more than doubled from 23 projects in 2007 to 48 in 2018. These larger buildings are typically “cross-dock” configurations (dock doors on both long walls) with widths of 500 to 600 ft between dock walls, and lengths extending as far as available land will allow – regularly exceeding 1000 ft to over 2500 ft.

Taller buildings allow for taller storage racking systems, which results in heavier loads on the concrete slab on ground. While slab thicknesses generally increase with building height, improvements in slab on ground design and detailing practices have also helped to mitigate heavier loads. Slab reinforcing, both continuous light reinforcing and dowels at sawcut joints, provide load carrying capacity benefits. Popular proprietary construction joint dowel systems transfer storage rack and forktruck loads across joints while also allowing concrete to shrink in all directions without causing cracks. Proprietary “jointless” or “reduced joint” slabs systems are also becoming common as they can eliminate miles of sawcutting and joint filler in a building while also reducing joint maintenance over time. In addition, many proprietary slab systems publish point load capacities that exceed that of conventional concrete slabs.

While demand grows for taller and larger buildings, the fire suppression system most commonly used in distribution centers (ESFR sprinklers) limits the building height to 45 ft to the underside of roof deck. This limitation can make the taller clear heights desirable to owners and tenants difficult to achieve without special structural design accommodations. The 36 and 40 ft clear buildings may require very shallow roof framing members, especially at lower portions of the roof near drains and gutters. In addition, roof slopes as flat as 1/8 inch per ft are often necessary, but require special structural design considerations to mitigate ponded water due to rain or snow melt. In extreme cases where a 40 ft or taller clear height is required, roof drains may be required throughout the interior of the roof footprint, vs. the typical practice of locating drains or gutters only at the building perimeter. However, some relief may be coming in the near future as the industry anticipates an increase in the ESFR roof deck height limitation to 48 ft, making 36 and 40 ft clear heights more achievable.

As distribution center buildings become larger, the magnitude of thermal expansion and contraction in the plane of the roof increases, particularly during construction when the structure is exposed to weather. Thermal movements require careful consideration and detailing in order to prevent damage to architectural or structural components. Building plan dimensions exceeding approximately 400 to 600 ft require a thermal expansion/contraction joint, and many large distribution centers require multiple joints.

Multi-Story

As customers demand faster delivery of online orders, fulfillment centers need to be as close as possible to large population centers. This can be a challenge near large cities due to limited land availability and higher real estate costs. A resulting trend that is just now emerging is to go vertical and build multi-story facilities on smaller sites.

The vertical distribution of product between floors is a unique design challenge for multi-story warehouses. Some facilities provide truck dock access at multiple floors, either by varying grade around the building perimeter or building elevated ramps, bridges, and dock court structures.  Other facilities rely on vertical material handling lifts within the building to move product from floor to floor.

Elevated floors typically consist of concrete slab on metal deck and composite steel beams or bar joists. Warehouse floors must be designed for a minimum storage live load of 125 (“light” storage) or 250 (“heavy” storage) lbs/sq ft, or higher if required by the specific use of the floor.  A portion of this live load must be included in the building’s seismic calculations, putting additional demand on the lateral force resisting system that would not be required for an office building, hospital, or many other uses.

Whereas typical single-story warehouses are clad with load-bearing tilt-up or precast concrete wall panels, taller multi-story facilities may require different solutions. Some options that can be used include stacked tilt-up or precast panels, insulated metal panels (IMP) supported by cold-formed or structural steel backup, or a combination of concrete panels at lower floors and IMP at higher floors.

Material Handling Automation

Welcome to the 21st century – the products you ordered and that were shipped to your house may have been picked from a fulfillment center shelf by a robot. Automation sometimes get a bad rap, but in reality, it eliminates the grueling human job of walking up and down concrete warehouse aisles carrying products all day. However, robots still demand some level of attention. If robots are to function properly, the floors they travel on may need to be designed for special requirements such as restrictions on floor flatness and levelness, deflection of suspended floors, surface roughness and friction, and maximum allowable slab crack width.

Automation also means sophisticated material handling systems that can require miles of conveyors, catwalks, platforms and mezzanines, vertical lifts, and multi-level automated storage and retrieval systems. These items can impact all aspects of the building structure, from roof and suspended floor structures supporting hanging conveyors and catwalks; to slab on ground or foundations supporting platforms, mezzanines, automated storage and retrieval system post loads, and robotic arms; to stability bracing of multi-story vertical product lifts.

Looking Ahead

We expect the distribution and fulfillment center market to continue its strong growth for at least the next several years.  The trends described above will likely continue and even intensify as advancements in fire protection and material handling technology continue to drive up the height of buildings. As demand in urban areas increases, we expect not only more multi-story facilities, but more “one-off” smaller facilities tucked into available lots – a far cry from the multi-site industrial parks built decades ago when land was cheaper and more plentiful.  Whether they’re single story or multi-story, distribution and fulfillment centers are only becoming more challenging and complex, and as the market continues to change, Schaefer is always looking for new innovative solutions to solve these new design challenges.

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