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Case Study: Flying Flowers, Retreat Farm - holistic fire safety strategy, evacuation
In 2005, Lawrence Webster Forrest were appointed by Flying Flowers Ltd to advise on a fire escape strategy within an existing greenhouse building, Retreat Farm in Jersey. The main aspect requiring investigation was the means of safely evacuating the building in event of fire recognising that the internal lining to the roof panels do not meet the classifications as prescribed in Approved Document B. Indeed, where ADB recommended the achievement of a class 0 for the surface spread of flame, the product used within the premises, Kascoplex, fell into class 4 in accordance with BS 476-7: 1997 – Surface spread of flame.
The intention of this project was therefore to address the holistic fire safety strategy for the greenhouse building. The basis was the adoption of a fire engineering approach considering the inherent benefits and limitations of the building to assist in the determination of the suitability, or not, of the product Kascoplex.
The existing greenhouse had been provided over a footprint approximately 130 m long by 130 m wide and had been divided into two compartments. A series of pitched roofs provided a clear internal height of 3.2 m rising to a maximum height of 5 m at the top of the pitch. Several final exits were provided along the edge of the building perimeter.
The greenhouse consisted of various minor store and office areas, a ‘Plug Plant Growing Area’, a ‘Dry Good Storage Area’ and an area where all the flowers are sorted, packaged and then dispatched out of the warehouse for worldwide delivery.
In the ‘Packaging and Dispatch Area’ and in parts of the store areas Kascoplex had been newly installed in the roof construction due to several additional benefits within the functional operations of the business in comparison to glass.
Building Control requested fire properties (linings etc.) of the newly installed product, which identified that it was Class 4. Consequently the client, Flying Flowers involved LWF to identify the fire safety performance of the material Kascoplex and to determine if its installation creates an unacceptable level of risk in terms of life safety.
Method of Assessment
The approach taken by LWF considered the comparison between the Required Safe Egress Time (RSET) and the Available Safe Egress Time (ASET). The diagram presented below describes the chosen assessment process.
The practical application and approach: RSET vs ASET
RSET: The RSET is the required period of time occupants would need in a fire condition to evacuate. The RSET encompasses detection time, occupant’s pre-movement time, and movement time.
Detection time: Due to type and level of detection and the general layout of the building an estimated period of 1 minute was allowed for a fire to be detected. This was deemed to be a conservative assumption as it was expected that, due to available sensory cues, a member of staff would be aware of a fire in its incipient stages and activate the fire alarm before this time.
Occupant’s pre-movement time: The occupant’s pre-movement times was estimated using the profiles of the people and an adjusted form of alarm cue compared with that reported by Sime [SIME 1996]. The work of Sime is based on complex buildings where the pre-movement time is for those occupants not intimate with the fire, i.e. remote, and includes the time for people to investigate the situation and determine their nearest available exit. For the prevailing occupancy the pre-movement time resulted to almost 4 minutes.
Movement: For the assessment of the occupant movement the evacuation model STEPS (Simulation of Transient Evacuation and Pedestrian Movement) was used. STEPS is designed to simulate how people move in both normal and evacuation situations within building complexes. The evacuation model has been validated against the American Standard NFPA 130 resulting to comparable time-lines.
The modelling of the evacuation from the greenhouse areas a maximum occupancy of 140 people was considered, based on client information. Six final exits or exits to adjacent compartment respectively can be used for evacuation. However, the largest exit, exit 5, was discounted to consider a potential non-accessible exit in the event of a fire.
The evacuation modelling showed that the physical movement from the considered part of the greenhouse where Kascoplex is provided to a place of safety will take 34 seconds (see pictures below). Considering the detection time and the pre-movement time together, the RSET in total has been calculated to 5 minutes and 26 seconds.
ASET: The ASET is the available period of time before which the conditions within the premises under assessment will be untenable and render any escape hazardous to occupants. In order to establish the ASET, a three stage approach was adopted:
- Determine worst case fire scenario,
- Undertake a Computational Fluid Dynamic (CFD) analysis,
- Carry out physical testing of the material Kascoplex.
Step 1: Worst Case Scenario: Determination of the worst case scenario and a design fire necessitated fire-engineering judgement due to the limited information for this form of environment. Hence it was considered that the three trolleys housing the cardboard boxes utilised for the storage of flowers would present the greater risk as the trolleys would always be available presenting a readily ignitable fuel source. The design fire was calculated to be 6MW based on the area taken by the three trolleys having a maximum Heat Release Rate (HRR) of 500kW/m2.
Step 2: Computational Fluid Dynamics: CFD analysis was carried out to ascertain the compartment temperatures generated at high level from the fire and as such the temperatures Kascoplex would be subjected to. The fire scenario determined in the above paragraph was incorporated into the Fire Dynamic Simulator (FDS) software and positioned at a height of 1m above ground to consider the physical arrangements inside the building.
Step 3: physical testing of Kascoplex: Finally, an accredited testing house was engaged to undertake the physical testing of the product Kascoplex to the prescribed temperature curve determined by the CFD assessment. Two tests were undertaken, the first to introduce the temperature curve by heat diffusion to the product with the second test allowing for flame impingement. Both tests demonstrated that under the prescribed conditions the product Kascoplex failed after a test period of 10 minutes and 30 seconds.
The results of the egress assessment and the testing of the Kascoplex roof panels were combined in a time line comparison (see below). As the timeline indicated all occupants of the building were able to evacuate the area to a place of safety in a sufficient time period. The results showed that the safety factor between RSET and ASET was close to 2. This safety factor was deemed to be acceptable to cover potential elements of uncertainty referring to the assumptions made and potential design redundancies not having been considered within the modelling.
The design of the packaging and dispatch area in general met with prescriptive requirements with the exception of the internal lining of the ceiling panels. Due to the codes of practice being restrictive to the design, an alternative (fire engineering) solution was sought. Through fire engineering analysis, it has been demonstrated that the escape routes were maintained as tenable for a period of time that permits safe escape from the area. The fire engineering design was based on first principles and utilises accepted design practice and tools for assessment.
Building Control accepted the proposed fire engineering solution. The exiting design of the building with Kascoplex roof panels was retrospectively approved by Building Control without further amendments necessary.
1. As installed panels could remain in-situ, allowing the client to retain benefits for which they were originally installed. This had a cost saving implication and productivity was not affected.
2. The client has a fire engineering solution that provides not just for life protection but also business continuity and asset protection throughout the life cycle of the building.
3. As a single point of contact for all life safety issues we have been able to develop alternative methods and cost effective solutions through performance based design, which allows the clients work processes to function effectively.
Lawrence Webster Forrest, Legion House, Lower Road, Kenley, Surrey, CR8 5NH
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