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Warehouse Design – High Bay Warehouses – Very Narrow Aisle

Updated: Dec 11, 2020




Introduction

One of the many racking storage options when designing a warehouse is Very Narrow Aisle (VNA). As the name suggest the aisle widths are reduced when compared to other options, typically between 1.6 and 2.0 metres. Reduced aisle widths, if coupled with high racks can result in a very efficient pallet per square metre ratio. In VNA racks each pallet is accessible, therefore high levels of rack utilisation can be achieved. VNA areas are usually used for:

  • bulk / reserve storage

  • picking of slow-moving items


Typically, VNA areas are used in high bay warehouses for the storage of slow-moving items. VNA is also used as a bulk / reserve storage area feeding other areas of a warehouse. VNA is also very suitable for warehouses which are temperature controlled, where space utilisation is a key factor in deciding what type of solution is required. VNA racks can also be used for picking of slow-moving items.


VNA solutions are sometimes unpopular with warehouse operators, they report that they are inefficient to operate and that throughput rates are too slow. The VNA trucks are expensive to buy / lease when compared to alternatives. However, the cost of the trucks should not be consideration in isolation. The overall cost of an VNA operation should be compared to the alternatives before making a final decision on a storage concept.

This blog aims to guide you at a high level through the various stages I adopt when designing a High Bay VNA warehouse area.

Section 1 Data Analysis

For any warehouse design, whatever the storage concept being evaluated, the first stage is data analysis. The investment required for a storage system such as VNA is considerable. The analysis stage should not be overlooked, or the complexity underestimated. I will dedicate another blog to data analysis at a later date.

The main data requirements when designing a VNA area are:

  • analysis of the storage requirements by pallet size / height

  • pallet movements into / out of the VNA area

  • picking volumes if applicable

  • the times of activities.

As part of the data analysis process I generate “Day in the Life” models, and support this with my Resource Model. I can also support these with Costs models which are very useful to compare alternative concepts.

Section 2 VNA Trucks

The put away and retrieval tasks in the VNA areas are performed with specially designed VNA Trucks (also referred to as Turret Trucks). The standard trucks operate up to a top pallet height of 14.5 metres, in an aisle up to 2.0 metres wide.

Comment: some VNA trucks can operate above 14.5 metres, as the heights increase the trucks require larger chassis and an increased wheelbase resulting in a wider aisle. For warehouses at this height alternative options such as manual or automated stacker cranes should be considered.

Other design considerations

Floor


Another consideration is that the floor needs to be “Super flat”. You may need to have a floor survey completed by specialist company. If required, the floor may need to be ground to meet the specifications required. The VNA trucks are usually wire guided, so at the time of floor grinding the wire guidance installation is usually completed at the same time.


Wire Guidance


VNA trucks generally operate using wire guidance - Wire Guidance is an electronic and mechanical system that controls VNA enabled forklifts steering by tracking an energised guidewire. Within each wire-guided aisle, this energised wire is buried inside a bevelled cut that is approximately 12mm to 15mm deep.

The alternative is to have a guide rail fitted to the racks. Wire guidance is preferred as it is usually

  • cheaper to install,

  • results in a cleaner operational area,

  • allows for easier housekeeping,

  • there are no rails to get damaged.


Man Up / Man Down


Man-Down or Man-Up VNA trucks are available. The man down truck as the names suggests, the cab stays at ground level, and the forks raise.


Man-Down VNA Truck


The man up trucks are also described as Combi Trucks (combination trucks). The trucks can also be used for picking operations as well as the pallet put away and retrieval tasks. The forks on the truck can be raised by the operator to ensure that when picking the pallet is at the most ergonomic height (see the picture below).

Since the operator is physically closer to the storage location positioning the pallets to the rack is easier when compared to a Man Down truck.

Man-Up VNA Combi Truck


VNA trucks are fitted with a rotating mast, these masts allow for pallets to be accessed from either side of an aisle. The pallets can also be rotated in the aisle if required.


Truck fitted with a Rotating Mast


Semi-Automated / Automated Trucks


The latest technology allows for VNA trucks to operate in semi-automated modes whereby the truck operator can be guided efficiently to locations using RFID and transponder technology. Fully automated VNA trucks are also available.

Note: during an investigation into VNA operations semi and fully automated options should also be evaluated.

Throughput Rates


Although the trucks can operate at speeds of up to 12kph, typically faster than other trucks might operate in a warehouse, the actual throughput rates are generally low when compared to alternative concepts. This is usually a result of tasks not being “dual cycled” (dual cycling is when the WMS allocates a retrieval task following a put away task, thus maximising the efficiency of the truck movements). Typically rates of 20 pallets per hour can be achieved with single cycle operations, and possibly 26 pallets / hour with dual cycles. These rates are however highly variable, and many factors including the frequency of aisle changes can reduce rates.

Alternative Concepts

During the design process it would be amiss not to consider if VNA is the best solution, or if an alternative option should be considered. For a warehouse that is up to 14 metres in height the most popular alternatives are summarised below:

Note: the above costs are indicative only, the prices shown are estimates provided for a recent project in a warehouse that was 13.5 metres high, however it does demonstrate the reason why VNA trucks might be considered expensive.

Note: the pallets overhang the racks by 50mm on each side – therefore aisle widths are typically quoted as rack to rack – but the pallet to pallet aisle width is 100mm less – therefore a 2.00m aisle width rack to rack has a 1.9m pallet to pallet aisle width.

Racking Costs


The costs of the racks in the table above are similar when comparing each alternative, the VNA racks will require additional P&D locations at the rack ends.

As part of my design process I would consider the suitability of other options such as Push-Back, Drive-In, or Shuttle Racks.

Articulated Fork Truck/ Narrow Aisle Concept



Again, like VNA Trucks, the proposition of using Articulated trucks in a narrow aisle can divide opinion with warehouse managers. They have the advantage of being versatile, in that they can also perform other tasks other than put away or retrieval. These trucks can be used in the yard to unload or load vehicles, they can also be used to transfer pallets between locations in the warehouse. Suppliers will advertise that you could operate a warehouse using just this one type of truck. The trucks however can be difficult to operate above 10 metres. The mast of the truck can sway as the pallet raises, and positioning of pallets at height can be difficult for operators.


Reach Truck / Wide Aisle Concept


Reach trucks can operate up to 13.5 metres, usually equipped with driver aids such cameras or pre-set lift height guides. This does require rack levels are consistent through the warehouse. Reach trucks are popular with warehouse operators. Locating pallets using reach trucks above 13 metres requires high levels of skill and dexterity, however, it is slightly easier than using an articulated truck. I usually allow for larger aisles than the technical specification suggest if possible, this will reduce rack damage or accidents.

Section 3 Layout Design

I use AutoCAD to design warehouse layouts. Typically, my clients will have either:

  • an architect drawing of the site

  • racking suppliers’ drawings of the site, (which they are usually happy to share if you ask nicely!)

  • paper copy of site drawing

  • in some cases, there is no scale drawing available, sometimes to varying degrees of accuracy drawings or schematics can be supplied in Excel or Visio format

I will modify an existing drawing or creating a new drawing to create concept layouts.

The warehouse layout will show the building walls and columns, receiving and despatch locations and any other storage or picking areas.


Example Layout


This warehouse has a VNA section to the left in blue, and a wide aisle section to the right in yellow. The VNA section is used for full pallet picking and reserve stock. The yellow section is also used for full pallet picking, with the addition of case picking. The reserve stock for the case pick area is retrieved from either the wide aisle or VNA areas.

Whenever I create a warehouse layout the location of the building columns is always the main consideration. We are looking to maximise storage capacity and if possible, make the columns “disappear” into the racks. To achieve this the racks can be assembled around the columns in a variety of ways:

1. Column located between racks


The racks are assembled either sides of the racks, the flue space is increased, and this will require additional bracing of the racks.




2. Racks positioned either side of the columns


This will require more uprights than continuous racks.




3. Columns hidden in the racks

The final option is to position the racks with the columns inside the racks, and to lose storage capacity of one pallet at each level of rack.

Comment: I have designed warehouse using all the above options, each one is equally viable.

Access Aisles


Another factor to be considered in the layout is the space required at the end of the racks to change aisle. VNA trucks can be up to 4 metres in length and as such will require a turning area of up to 5.5m


Layout showing hatched yellow access aisle.


P&D Locations


The P&D locations can be a single cantilever rack located at the rack ends. Movements onto / off the racks are usually performed using a reach truck (depending on the height of the racks – counterbalance trucks can also be used for low level racks). The bays can be located on one or both sides of the aisles.


P&D Racks - Cantilever


Another method is as above to have staggered bays (see below). These bays are accessed using a reach truck as the removed bay allows for sufficient space for a reach truck to locate the pallets.

Design considerations


P&D locations can provide a valuable space buffer in the operation, allowing for any in transit pallets to be cleared off the receiving bays to the racks awaiting put away. They can also allow VNA trucks to continue retrieving items and to be stored. If the P&D locations are fully occupied, then the VNA trucks cannot operate. This buffer can provide greater operational flexibility.

Storing Pallets

Most UK warehouse generally have 2 standard pallet dimensions (mm) – 1200 x 1000 UK size pallet or 1200 x 800 Euro size pallet. Pallets can be stored either 1200 deep or 1200 wide.

Storing pallets 1200 deep should result in increased storage capacity, as the ratio of aisles to pallets increases compared to 1200 wide. Storing pallets 1200 deep also allows for storing UK or Euro size pallets in the same racks. This will need to be considered from the outset, and the mix of both pallet types calculated. Also, the warehouse numbering and labelling system will need to be considered.


Design Process

During the design phase it is not uncommon to create numerous concepts with different options for assembling the racks around the columns, and with the racks running either vertically or horizontally in the warehouse. The objective is to maximise the storage capacity, but with the racks running towards the despatch areas is preferred (see the example warehouse from earlier).

Designing a VNA warehouse is an iterative process, I provide a high degree of guidance and communication with my clients, we agree concepts and design assumptions at various stages. At the point when we have a concept layout we can engage with selected suppliers. Following the receipt of quotations, we can the select a supplier to validate the design, they will create their own engineering drawings and verify all dimensions. They will also ensure that all legislation and SEMA codes of practice are adhered to.

Section 4 WMS (Warehouse Management Systems) Evaluation

Another key part of the process is to evaluate the functionality and suitability of the WMS (Warehouse Management System) to support VNA operations. The following is a selection of the type of questions to be considered:

  • Does the system allow for dual cycling (this is also referred to as multi-tasking or interleaving)?

  • Does the system support the two-stage put away and retrieval process?

  • Will any consideration be made for the velocity of the SKU in determining the storage position in the aisle or the aisle allocated?

  • Does the system direct movement to the P&D location in real time based upon which aisles have a VNA truck operating in the aisle at that moment?

  • Does the WMS support the designated allocation process, i.e. FIFO, FEFO, etc?

Reasons for Inefficient VNA Operations

In an ideal world, there would be a smooth balance of work evenly distributed across the aisles and the trucks. The trucks would perform 100% in a dual cycle mode, and the aisle changes would be kept to an absolute minimum. The trucks operate at high speeds and coupled with the fact that they can simultaneously lift and move, and with the appropriate guidance and software they should operate very efficiently.

In the opening section it was stated that VNA operations do not always generate the desired throughput rates and are inefficient to operate. Warehouse managers are sometimes apprehensive when VNA racks and trucks are considered as a storage option.


The main reasons for inefficient operations are as follows:

  • Too many aisle changes – as the trucks are large and difficult to manoeuvre the time to change aisles should not be underestimated.

  • A low occurrence of dual cycling – if the system has the capability to allow for dual cycling, if the workload is such that there is an imbalance or inbound vs outbound movement dual cycling opportunities will be reduced.

  • The put away process is a two-stage process – one movement onto the P&D location using a reach truck, then a move from the P&D location into the rack using a VNA truck. The retrieval process is also a two stage move. This additional move requires extra labour and MHE. Balancing the availability of the reach truck team and the VNA teams’ activities is not always easy. Also, if the P&D racks are full then this can cause or delay on either part of the process.

  • Incorrect location of pallets in the area. The WMS storage strategy could result in faster moving items being stored in less accessible locations and could result in excessive travel distances.

Conclusion

Designing a fit for purpose VNA layout and operation is not a straightforward process. We are looking to maximise the utilisation of the space available and to provide efficient operations. We may even sacrifice efficient operations if overall costs are reduced when space costs are considered.


Please do not hesitate to ask for advice. Of course, I would be very happy to provide a quotation to provide any assistance required.

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