Case Study

Sainsbury’s Supermarkets – Waltham Factory – alternative fire engineering

July 23, 2002 12:00 am

The development comprised a warehouse incorporating office accommodation, welfare facilities and plant rooms but the building remained a simple structure. The aims of the client added to elements of high risk to the occupants necessitated an alternative fire engineering approach using performance based solutions, particularly in respect of the movement of smoke and the safe evacuation of occupants.

A practical approach to designing the fire engineering strategy for the most advanced grocery distribution operation in the World.

Project Name – Waltham Point Fulfilment Factory
Client Name – Sainsburys Supermarkets

Project Description and Application

Introduction
In 2001, LWF were appointed directly by Sainsburys Supermarkets to work with the design team on the most advanced distribution centre network the world has ever seen. The development at Waltham Point, one of nine proposed fulfilment factories across the UK is located immediately south of the M25 motorway at Waltham Abbey between junction 25 and 26 and provides 650,000 sq ft of floor space, covers 40 acres and cost £70 million.

Over a three year period the Sainsburys Supply Chain will be transformed to provide improved on the shelf availability with a paperless, stockless, accurate and flexible service, designed to meet the current and future needs of its business. With a peak weekly capacity of up to 2.4 million product cases, Waltham Point will employ 300 people and operate on a 24/7 basis, with picking and sorting taking place 20 hours a day.

The Project
The development comprises a warehouse 572m long by 100m wide with a clear internal height of 14m. Although office accommodation, welfare facilities and plant rooms are incorporated within the warehouse the building is effectively a simple structure. An automated mechanical handling and racking system has been designed specifically for the building and includes, high-level pallet storage and case picking, crane operations and manual handling. The warehouse will have a fully trained workforce of ambulant persons working in all areas. This internal fit out therefore creates a complicated fire safety situation.

From our initial review and discussions with the client, the insurers and design team it was apparent that three fundamental issues existed, these being:

The building design had to consider not only the requirements of the Building Regulations but also the aims and objectives of our Client and Insurers.
High risk to occupants due to easily ignitable fire load, occupants at a high elevation with required travel distances double those of prescriptive requirement.
The design must be transparent and manageable for the life of the operations.

To address all these issues necessitated an alternative fire engineering approach using performance based solutions particularly in respect of the movement of smoke and the safe evacuation of occupants.

The practical application and approach

Smoke venting
Our initial task was to challenge on behalf of the client the requirements of the Design Brief and seek reasoning for certain levels of fire protection such as smoke venting to keep contamination of stock to a minimum. Initially the Design Brief requested that the smoke levels be above the height of stored goods. However, considering the maximum height of the stored goods relative to the roof, was deemed unreasonable due to the possible variations in smoke quantity productions compared with the fixed parameters of smoke reservoir size and maximum possible venting area. Furthermore, it was agreed to challenge the design guides for natural smoke ventilation as these had already been designed prior to our involvement on a single fire scenario of 12MW. Unfortunately this didn’t take into account the many fire scenarios which may exist due to the number of permutations involving different racking configurations and possibilities of peak HRR. Results were therefore sought on fires with varying fire growth rates for a peak of 3, 6, 9 and 12 MW.

It was successfully established that the smoke ventilation system would only be effective for a small percentage of fires.

Unfortunately LWFs involvement came after the initial agreement with the Insurers and the installation of the natural smoke vents had been agreed, however the requirements for future developments were redefined with the Insurers resulting in a saving of approximately £250,000 on each of the remaining developments.

This therefore highlights the need to involve a Fire Engineer at the initial stages of any design.

Simplifying problems
Due to the nature and use of the building, the system of high level racking employed produced many different scenarios for both fire and smoke movement, the possibilities and the resulting effects of smoke movement, entrainment and temperatures were vast. Therefore, rather than adopt the philosophy that the task is difficult, we must use a sophisticated computer model to draw our conclusions, LWF sought a more simplistic approach. Having undertaken a quantitative assessment of the various hazards and the likelihood for ignition which were immense, we were able to establish the extent of the variables to be considered before choice of the most appropriate smoke production and movement tool could be chosen. After consideration of the variables and limitations of tools available, the zone model, CFAST, was chosen in conjunction with fundamental hand calculations. The simulations considered fires of varying growth rates, areas, peak HRR, and elevations. As predicted each scenario provided a different outcome with respect to the defined acceptance criteria. Thus, considering the acceptance criteria, no one scenario addressed all of the factors requiring consideration. This highlights the need for sensitivity assessments and full range of scenarios.

A full Available Safe Egress Time (ASET) Vs Required Safe Egress Time (RSET) assessment was undertaken where the RSET considered all the required factors including pre-movement of occupants. Once again rather than using the sophisticated visual graphic tools, hand calculations were used to continue the simplified approach adopted. As with smoke modelling a practical consideration of the evacuation simplified the scenario, as the number of converging streams of occupants was minimal.

Eventually, the differing fire scenarios were collated and assessed in comparison with occupant’s evacuation to demonstrate that the occupants could safely evacuate from the racking area at a high level in all probable fire scenarios.

Presentation of documents for the design and construction
Having produced a comprehensive report, which clearly identified all the compliant and non-compliant issues of the building regulations, a separate fire engineering report was produced following the principles of DD240 (BS 7974). When producing both reports it was strongly felt that clear and concise deliverables are the key to all successful projects. The most cutting edge fire engineering designs can fail if not properly presented and transcribed into a solution that is easily understood. Therefore great effort was put into the reports to ensure that the client, insurer, design team and all statutory authorities not only understood the strategy proposed but also bought into it.

As the final strategy included many different fire safety features, we produced a project-related report that addressed all the features proposed for the whole design team and identified roles and responsibilities during design, construction and commissioning. Each member of the design team were then fully responsible for co-ordinating, documenting and monitoring the on site activities of sub-contractors and the installation of the various fire safety features. History has shown that a significant proportion of projects do not meet the requirements of the fire safety solution, be it prescriptive or alternative, if there is no policing to ensure correct installation. LWF were consequently commissioned by the client to act as the project enforcer to ensure all systems are installed and commissioned to the exact requirements as well as defining the life time maintenance regime.

How the solution works

The final solution
As the worst case scenarios for smoke movement and evacuation were used to justify the complete fire strategy, the building does benefit from a high level interface between the aspirating smoke detection fitted throughout and property sprinklers protection fitted at roof level and at each level of the racking system. This results in the shut down of all the mechanical air handling plant including the chill fans, shut down of all refrigerant fans, closing of all doors and shutters, automatic release of any security doors.

The inherent benefits of the egress design were that points of choice were developed immediately and the natural ability of a large space to act as a reservoir for smoke while occupants move away from the area of risk considering human behaviour response to visual effects and a proposed level of staff training. The solution considered the practical application of the space and building. Limitations on storage of goods and locations were impractical hence the final solution gives the client flexibility whilst maintaining protection to Sainsbury’s two key assets, staff and stock.

Conclusion

Unfortunately for a building of this magnitude with high fire loads, excessive travel distances and occupants working at high level, meant that little relevant guidance was available, and that adherence to conventional codes would result in costly and ineffective measures being provided. This therefore reinforced the need to develop a fire strategy using a practical application from first principles and innovative flair to meet the client’s aims and objectives.

Fundamentally the strategy challenges the whole philosophy of natural smoke venting and its ability to offer a safer environment to the occupants in all scenarios, we have endeavoured to identify all weaknesses and provide a solution that is subject to less fluctuation in performance from uncontrollable circumstances.

Although Waltham Point will not be complete until early 2004, once operational it will handle 10,000 lines of high volume commodities all delivered and held on wooden pallets. It was evident that a comprehensive, but clear and simplistic fire strategy, defining all the actions to be taken to meet the final performance be developed.

Finally, having worked closely with a very proactive client, Sainsburys constantly challenged the design team as to the level of the features they proposed throughout the design. This questioning brought a very strong element of client interaction, as the clients needs were not just for life protection but also business continuity and asset protection. Therefore the design does not include any sophisticated levels of fire separation or convoluted paths of egress, the key to the solution is simplicity when considering all the characteristics of fire and smoke spread in the areas of risk and the optimum benefits achieved by a client who appreciates the practicalities of the fire solution designed for the life of the building.

3 Key Benefits

A risk and cost based solution has been produced, for a development for which prescriptive codes were found to be inadequate, that meets the client needs for safety and effective use of the warehouse.
As the single point of contact for all fire safety issues we have been able to develop alternative methods and cost effective solutions through performance based design, which allow the clients complex internal distribution system to function effectively.

The client has a fire engineered solution that provides not just for life protection but also business continuity and asset protection throughout the life cycle of the building.