Warehouse Design – Warehouse Racking
Posted on 18th July 2022
When designing a warehouse there are many different racking options available, but which one is the most efficient and cost-effective solution? The aim of this Blog is to present an alternative and pragmatic analysis of each of the options, perhaps offering a different perspective to the information found in the racking companies’ sales literature. This Blog will not consider automated storage options, perhaps this will the subject of a separate Blog at a later date
I personally have specified more Standard Adjustable Pallet Racking (referred to herein as APR) in a wide aisle configuration, than the other options. I do not favour one option over another as each type of rack is suitable to provide a solution to different challenges. This Blog will review at a high level the following rack types:
Standard APR with a wide aisle accessed with a Reach Truck
Standard APR in a narrow aisle accessed using an Articulated Truck (Flexi / Bendi trucks)
Double Deep Racks
Verry Narrow Aisle (see Blog 8 August 2020)
Block Stack (see Blog 25 July 2020)
Shuttle Racking (see Blog 1 August 2020)
Drive in Racking
Pallet Live
Mobile Racking.
Note: Automated and Crane types of solutions will be the subject of separate Blogs at a later date.
Note: Mobile Racking will also be part of a separate Blog at a later date.
Note: Cantilever racks are not covered in this Blog.
Note: Shelving is not covered in the Blog.
Note: Sprinklers are not mentioned in this Blog, at the start of the warehouse design it should be confirmed if sprinklers are required, and to what specification.
When I have delivered training courses the most common mistake people make when designing for a “target number of pallets to store” is that rack utilisation is not understood. There are many factors that will impact on the utilisation factor to applied and these will be discussed for each rack type. The space utilisation for each rack type is also different and consequently some racks offer far greater density of storage than others.
Another major consideration affecting any design will be the cost per pallet stored for each rack type. The racks will be accessed using different equipment and at different productivity rates, this will impact on operational throughput rates and overall costs.
To conclude the Blog, I summarise some work I completed as part of a Case Study. This will consider the space required for different rack types to store a target number of pallets. I will create a part 2 of this Blog where the operational costs will be shared.
Pallet Racking
Overview
In the UK we typically store 2 standard size pallets in warehouses:
UK / Chep pallet 1200 x 1000mm
EU pallet 1200 x 800mm
Non-standard pallet sizes can also be stored in racks, these will require some alternative design ideas.
Plastic Pallets, Box Pallets and metal cages can also be stored in racks.
Wooden pallet (4-way entry)
Pallets bases are either 2 way or 4-way entry. The 4-way entry pallets are the most common and more flexible when designing the storage solution. The final design must consider the type of pallets being stored and the number of pallets by each type. The overall quality of the packed items on the pallet, and if the load overhangs the pallet footprint should be checked. The quality of the pallets in the warehouse should also be considered, pallet support bars may be used, or the load could be put onto a new pallet, or a slave pallet might be required.
1200 x 1000 Closed Box Pallet (Dolav)
Standard APR (also known as Selective) with a Wide Aisle / Narrow Aisle
The most common form of racking found in many warehouses in the UK is APR accessed using either a Reach Trucks or an Articulated Truck. The Reach Truck will generally require a minimum aisle width of 3.0 metres whilst an Articulated truck will require an aisle width of 1.6 metres. Always allow more space if possible, building columns may dictate that a wider aisle is required.
Note: always check with the MHE supplier and analyse the technical specifications to determine what the minimum aisle widths that are required.
Comment: I was advised many years ago that the aisle width for reach trucks should be the length of the truck, plus the load, plus 9 inches, this of course should be validated.
Note: A forklift truck could be used to locate pallets into racks, a 4-metre aisle would be required, and the trucks can only lift to 4 metres in height – subject to the truck specifications and the weight of the load.
Example racking section
See Drawing section below – Bay width 2700mm, pallets stored 1200mm wide and 1000mm deep. The frame depth is 900mm deep. The aisle dimensions are usually specified as “rack to rack”, in the example below this is 2000mm, the other rack dimension typically quoted is “pallet to pallet” in this case 1900mm. As the pallets are design to have 50mm of overhang on the beams and for the weight of the load to be evenly spread on the beams.
Example Section of rack design
In some warehouse some flexibility can be part of the initial design if the storage is for both UK and EU pallet types. Racks can be designed to store the pallets 1200mm deep on a 1100mm deep frame. A common beam width used for these situations is 3600mm wide. This can be used to store either 3 UK pallets per beam or 4 EU pallets per beam.
As mentioned in the introduction the rack utilisation must be considered. The utilisation APR racks, where all of the pallets can be accessed should be planned to a maximum of 95% (and this should only be for short term peaks), above this could result in inefficient operations as replenishment distances and travel could be increased over normal operations.
Note: all rack designs should be in accordance with the required legislation as per the Storage Equipment Manufacturers Association (SEMA) guidelines. All rack suppliers will as part of their design and validation ensure that the guidelines are adhered to.
Wide Aisle APR
Example of wide aisle APR with a Reach Truck
Wide Aisle APR is very popular due to the fact that the racks can be used for both reserve / bulk storage and the low-level locations can used for picking operations. Picking can be performed at the ground level only using a Low-Level Order Picker (LLOP) or a pallet truck, or at 2 levels if ½ pallet positions are created (order picking trucks can be fitted with an access platform). The racks can have shelving designed into the lower levels.
The picking could be from higher levels if a man-up truck is used.
Pallets are usually stored 1200 wide which is ideal for picking from at the lower level, however, they can be stored 1200 deep, but this makes accessing the rear of the pallet more difficult, especially for smaller items.
Other design Considerations
Pick / Replen Aisles
Alternate picking and replenishment aisles can be created with the use of flow racks for either pallets or cartons / totes / trays. This is an effective way of separating pedestrian pickers and picking trucks from replenishment tasks performed suing trucks.
Live Shelving
Tunnels in racks
Tunnels in racks can result in reduced travel distances in larger installations. They can also allow for condensed picking routes.
Pallet Racking and a tunnel
Note: the design of pick layout and alternate strategies will be the subject of a separate Blog at a later date.
Note: when designing racks with ground level picking allow for the first beam level to be at least 2.0 metres clear height, check with Health and Safety representatives on this. Providing the pick operators with “bump caps” is common practice to reduce likelihood of accidents.
Reserve / Bulk pallet storage
The locations above pick faces can be used to store Reserve or Bulk pallets. The reserve pallets can either be located in a dedicated Restock location directly above the pick face, or located by the WMS in a reserve location close to the pick face. The Restock method does allow for more efficient replenishment and can result in increased pick efficiency. The restock method will generates additional warehouse moves and the restock pallet will need to be replenished from bulk. Check the WMS functionality will support this method of replenishment before designing the solution.
When designing the layout consider different ways of hiding the building columns in the rack – see my Blog about Very Narrow Aisles (Section 3 Layout Design) for more details.
Summary of APR:
The cost of the racks per pallet stored is low when compared to other racking solutions
The installation and configuration is relatively straight forward
The beam heights can be adjusted at a later date should pallet heights change (do not change beam heights without checking with the installers first, changing the beam heights could reduce the integrity of the rack structure and this could lead to rack collapse.
The racks can be used to store either reserve / bult pallets and can also provide a picking area.
Each pallet can be accessed and therefore high levels of rack utilisation can be achieved.
Double Deep Racks
Double Deep racks are accessed using a reach truck fitted with a telescopic mast used to locate pallets in the 2nd pallet position. These racks can generate 30% more pallet spaces compared to a wide aisle solution.
Double Deep Racks
The main design consideration for these racks is:
SKU mix, you cannot, or should not mix SKUs in a slot. The 2nd pallet cannot be accessed if the first slot is occupied.
These racks operate at 75% or less utilisation, as may slots will be vacant. Some housekeeping of slow-moving items to a wide aisle location can help improve this but this requires additional resource.
Truck lift heights – the additional weight of the masts fitted to the trucks results in reduced lift heights when compared to a standard reach truck. Check with the truck manufacturers for exact specifications.
Drive in Racks
Drive in racks can be designed in 2 ways – the first is to allow for pallets to be loaded and retrieved from the same side, creating First-In-Last-Out (FILO) inventory retrieval organisation. This would apply for any racks that back onto a wall.
The alternative to this is to load from one side and unload from the other to create First-In-First-Out inventory retrieval (FIFO).
Drive in Racks
The main design consideration for these racks is:
Ideally each bay of racks would contain the same SKU, you cannot access all the pallets when the racks become depleted. You will need to conduct detailed storage profile analysis to establish if drive in racks are suitable to store your product range. First in First Out, batch control, etc will need to be considered.
Either reach trucks or forklift trucks are used to locate and retrieve pallets from racks. The width and lift heights of the trucks should be checked in line with the rack design
Utilisation, typically these racks operate at 65% or less utilisation,
Housekeeping opportunities are limited with Drive in Racks and again these moves require additional resource.
VNA Racking
For a comprehensive description and design guide for VNA Racks and Trucks see Blog 8 August 2020.
VNA Racks plus VNA Combi Truck
The main design consideration for these racks is:
Each pallet is accessible, so high utilisation levels can be achieved
The trucks operate at a low speed, and are expensive when compared to other options considered in this Blog
The time for a truck to change from one aisle to another is up to 5 minutes in practice, and the trucks need 5.5 metres to turn
Drivers will need to be trained to operate a non-standard truck
The trucks can operate at levels up to 17metres (Hyster truck C1.0-1.5 to be verified) I would suggest a height of 14.5 metres but please check with your preferred suppliers also.
Push Back Racks
Push Back Racks
Push back racks allows pallets to be stored on either side of an aisle in a similar way to double deep racks. Pallets are positioned to the front of the rack on a cart, then when a second pallet comes in, it pushes the first pallet back and rests on the middle cart. Similarly, the 3rd pallet pushes back the first 2 pallets and rests on the bottom cart. When unloading the push-back rack, the sequence is reversed.
The main design consideration for these racks is:
High Density Storage
Plan for 75% rack utilisation
Pallet automatically presented at the rack pick face
Limited depth of storage to 3 or possibly 4 pallets
These racks are expensive and may require some maintenance / repairs.
Live / Gravity Racks
Live racks are fitted with rollers and include a braking system, pallets are located to the racks from one side and retrieved from the opposing side. The racks have a 4% incline and gravity causes the pallets to roll towards the retrieval side of the rack. This allows subsequent pallets to move into the forefront once the preceding item is removed. These racks provide natural First in First Out (FIFO) sequencing of pallets.
Live racks can be accessed with Reach trucks or forklift trucks.
Live Racks
The main design consideration for these racks is:
High Density Storage
The slope (typically 4%) can result in the loss of a storage level on a long section of racks
Pallets should be in good condition and well wrapped and stable
Pallets automatically presented at the rack pick face in sequence
Live racks can reduce truck travel distances and cycle times
Live racks can provide a dense storage solution for the marshalling of outbound loads in despatch (see the photo as an example).
Shuttle Racks
See a comprehensive description and design guide for Shuttle Racking in my earlier blog.
Shuttle Racks
The main design consideration for these racks is:
High Density Storage
SKU storage profile needs to be suitable for this storage concept
Plan for 75% rack utilisation
These racks typical operated using a Reach truck.
Mobile Racks
In summary – Mobile racks are opened and closed via a remote control operated by a Reach truck driver – or can be controlled by the WMS if the system is sophisticated.
The racks move on rails recessed in the floor – the floor will need to be of a specific thickness and load tolerances will need to be checked. Ideally this will be designed into a new build facility, as retrospective installation may not always be possible.
Mobile rack solutions are not suitable for fast moving operations as the solution operates with a minimal amount of aisles, is not suitable for a high level of trucks operating in the area at one time and the time to open and close aisles could restrict efficiency.
Mobile Racks do however provide very dense storage and are very popular in high cost facilities such as freezer and chilled applications.
Mobile Racks
The main design consideration for these racks is:
Very High-Density Storage
High utilisation of 95% can be planned for
Accessing pallets can be slow if the truck driver has to wait for aisles to open.
Reach trucks used – typical to a maximum height of 13 metres.
Each pallet accessible
The platform results in approx. 250mm of height
Costs of platform and initial installation of rails is expensive
Check WMS functionality and ability to interface with opening and closing of aisles.
Overall Comparison
Typical Matrix found in Sales literature from the manufacturers
Note: the costs per pallet are from a project I completed in the past. The cost of steel is highly variable, and as such the above is indicative only. Larger scale projects may result in a reduced cost per pallet.
Case Study
Example Project
The image above shows a concept layout used as part of an alternative rack analysis project. The section of rack on the left in the image above is a high bay VNA area used as a bulk storage area for full pallets only. The pallets are either picked as full pallets sent direct to the despatch area, or replenishment pallets sent to the section of rack to the right, where there is a case picking operation.
The VNA area has an overall capacity of 19,200 pallets, and assuming that the maximum occupancy of the area is 95% this equates to 18,240 target to store pallets. To each of the different rack types a utilisation factor is applied and this results in a different target to store number of pallets. In each case the number to store pallets was designed in a new reserve storage area, and the overall space required calculated.
For example, to store 18240 pallets in a double deep rack area, with 75% rack utilisation the target to store pallets is 24,320. The relevant aisle space required for the designated MHE has been applied and the layout drawing was modified, and the overall footprint calculated.
The VNA area was assumed as 8 pallets high, the block stack area 4 pallets high, the live racks 5 pallets high and the other areas 6 pallets high.
Rack Analysis
This analysis and evaluation of options was only performed as part of a hypothetical warehouse design project; however, it demonstrates how the space required to store a high level of pallets varies significantly by storage option type.
Conclusion
There are so many ways to store pallets in a warehouse. In this Blog we have only looked at the non-automated options. It is highly likely that an optimum design for a warehouse is a combination of maybe 2 or more of the options evaluated.
If you are considering undertaking a warehouse design process then the analysis of the current operation and assessment of alternative concepts should be assessed by impartial, independent experts. I have numerous design tools at my disposal to quickly evaluate the options and to allow for an informed decision. Please do not hesitate to ask for any advice. I would be very happy to provide a quotation to provide any assistance required.
Tagged as: Warehouse design, Warehouse racking
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