The purpose of this Outline Fire Strategy Report is to facilitate preliminary discussions with the Approving Authorities and to outline the approach that will be taken to show compliance with Part B (Fire Safety) of Schedule 1 to the Building Regulations (2000 – 2006). This report will provide a brief summary on all the aspects of fire safety which correlate with the corresponding Building Regulations. However, it will focus primarily on issues pertaining to the Fire Detection (AFD) and Alarm system and linking factors, such as evacuation times and fire growth.
Since the 1960’s there has been a rapid increase in the construction of large single storey storage warehouse facilities in the United Kingdom (UK). One of the most notable of these was the 40,000m2 unsprinklered military storage facility constructed by the Ministry of Defence (MOD) at Donnington. In 1983 this storage facility was completely destroyed by fire despite the presence of an on-site fire service. The fire at Donnington and other unsprinklered warehouses circa 1980 raised concerns in the UK and the United States (US). They present an extremely high risk to firefighters due to size and fire loading, devastating was the incident in November 2007, which was a fire in a vegetable packing plant located in Warwickshire that resulted in the tragic loss of four firefighters.
Over the last 2 decades there has been an increase in the use of warehouses as a crucial part within the supply chain, this has been accelerated to deliver a just-in-time delivery system that has demanded more sophisticated processes. Where property protection sprinkler systems are installed with additional enhanced features, significant improvements upon the 80% reliability can be achieved..
3 Description of the issue to be solved
When minimum fire protection measures are provided (i.e. manual fire alarm, 2m floor to ceiling height, etc), ADB suggests maximum travel distances in warehouse accommodation of 25m in a single direction of travel and 45m where two means of escape are provided. The proposed building contains a number of inherent fire safety features such as high ceilings, automatic fire detection, sprinklers and an open plan environment. Such features assist in prolonging the onset of hazardous conditions and increase the chance of a person becoming aware of a fire in the initial stages of its development independent of the alarm being raised by others. Therefore, the basic recommendations set out in ADB are considered to be unduly restrictive fir the proposed building. The 2.5 min notional escape time used in current guides came from the Empire Palace Theatre Fire in 1911, as it was the time taken for the audience to escape, which was measured due to the Orchestra playing ‘God saves the King’ during the evacuation. Many other requirements within the prescriptive code developed in the same way with no specific foundation to support them.
4 Methods and schemes
To comply with the functional requirements the Secretary of State has approved a series of Approved Documents, which aims to provide practical guidance with respect to the functional requirements of schedule 1 and Regulation 7 of the Building regulations 2000 (SI 2000/2531) for England. The approved document for the above function is Approved Document B (Fire Safety), Volume 2 – Buildings other than dwelling houses (ADB); however these functional requirements can be achieved in a number of different ways. This report will focus on B1.
4.1 Approved document B and solution of fire engineering
ADB states the following: ‘Fire safety engineering may provide an alternative approach to fire safety. It may be the only realistic way to achieve an acceptable standard of fire safety in some large and intricate buildings and in buildings containing diverse uses’. Therefore, where the proposed development does not fully comply with the recommendations of ADB it is intended to incorporate the latest guidance available as part of an alternative fire safety engineering approach. This approach will be base don the recommendations of BS7974 with the overarching aim of achieving the optimum design solution and Building Regulation Approvals.
A fire alarm system is designed to detect and raise the alarm in a fire situation for the following purposes:
As part of the life safety system (L)
For the protection of property (P)
For a warehouse the type of system is determined by the current addition of BS5839-1; Fire detection and fire alarm systems for buildings (Code of practice for system design, installation, commissioning and maintenance). Table A.1 of BS5839 Part 1 identifies the appropriate alarm system for the building.
If we consider the building as a warehouse the British Standard states that the premises will require the highest standard of property protection a ‘Category P1 system’, where automatic fire detectors are installed in all areas of the building. The system is intended to satisfy the requirements of fire insurers as it considers a life safety system not appropriate for the building type as the occupiers are awake and will detect and raise the alarm. This is against the recommended type of system under BS9999 as it states for a fast growing fire with a risk rating of A3 (reduced from A4 due to sprinklers being fitted) requires a L2 alarm system. A mostly unmanned warehouse cannot rely upon a manual response and an automatic fire detection system and/or sprinkler system will be warranted. Historically, the type and extent of fire detection is often dictated by external influences rather than by a measured assessment of the risk and the determination of the most appropriate and effective form of detection. These influences may include the requirements of legislation as enforced by the Fire Authority and Building Control Body, or the requirements of interested parties, such as the Insurer. The alarm will also be capable of operating extinguishing and smoke control systems. Actuation will close fire dampers, open smoke vents and close heating, ventilation and air conditioning systems or switch them over to extraction mode
When detecting in large open areas such as atria, warehouses etc, the detection device is likely to be some distance vertically and horizontally from the fire source. Consequently, detection methods should be applicable for such circumstances. There are a number of considerations for protecting such environments, these can include smoke and heat dilution as the fire plume rises towards the fire detectors, the effects of stratification and air currents taking the smoke and gasses away from the detectors. When attempting to assess the probable time to detection, there would need to be a detailed assessment of the environmental conditions affecting fire growth, as the environment is likely to play a major part in the success or failure of the detection system. In this case:
tgrowth will be based upon typical fire models for the range of likely combustibles;
tmove will be based upon a number of assumptions of how the environment will affect the way the detected products will travel towards the detectors.
Parameters to be considered here are the effects of HVAC, the continued entrainment of air as the smoke plume rises (with the possible eddy current created by cooling of the smoke), and the effects of stratification.
Warehouses offer particularly unusual challenged to these types of detector since point type smoke detectors are not recommended for use above 10.5m except in unusual circumstances. The reason for this is the well-known phenomenon of smoke ‘stratification’, where smoke from any given size fire only produces sufficient thermal buoyancy to raise smoke to a given level (often well away from detectors), rendering conventional smoke detection virtually useless in this application. Beam detectors however negate the problems associated with point detectors.
Beam smoke detectors make use of the obscuration property of smoke to detect a fire. They may consist of a combined transmitter and receiver unit with reflective element, or a separate transmitter and receiver unit. They are normally mounted on walls at either side of a monitored area, such that there is a clear line of sight between one unit and the other. Typically, an infrared beam is transmitted along this length. In the event of smoke passing through the beam, the receiver measures the resultant attenuation. The value “CL”, expressed in dB, is used to identify the reduction in intensity of the light beam, defined by the following equation:
I0= received intensity without reduction in intensity
I = received intensity after reduction in intensity
The main requirements are that:
– The detectors have sufficient immunity to false alarm conditions such that CL, min is less than 0.5 dB;
– The variation in response between successive operations should be limited such that the ratio between the maximum value of CL, max recorded and the minimum value CL, min is less than 1.6. Although this data is used in testing detectors, it may also be appropriate in assessing on-site conditions.
By contrast, high sensitivity aspirating systems may often have their sampling pipes arranged to sample from multiple heights within the protected area, overcoming this problem. However they are cost prohibitive within this scheme.
5 Analysis and computations
To demonstrate the building complies with the functional requirement B1 a fire safety engineering analysis of the performance based design will need to be adopted as recommended in BS7974. Making use of basic physical provisions to ensure life safety, but design evaluation depends upon a time based comparison of the time available for the occupants to escape (Available Safe Escape Time, (ASET)) and the escape time (Required Safe Escape Time, (RSET)). This will allow the design to deviate form the restrictions imposed by the prescribed rules Research on Evacuation Times.
Standard fire safety recommendations for means of escape relate to travel time, i.e. the time taken to travel to and through exits to a place of relative safety. However, it is widely acknowledged that the time taken for occupants to start to evacuate can be considerably longer than the time taken to actually move out of the building. An extract of Table C.1 from BS7974-6 is partially replicated in Figure 4. This demonstrates the potential variation in pre-movement times of evacuees which is dependent upon the level of management and fire alarm provision. The management of this building will adopt a proactive fire safety management approach to comply with standard fire safety regulations for the workplace.
A1-A2: Automatic fire detection and alarm throughout building
A3: Automatic fire detection and alarm only in certain areas, or manual call points only throughout
There has been a number of real fire tests carried out on high rack storage arrangements by insurance companies. Tests carried out by Underwriters Laboratories Inc. concluded that fire size is unlikely to exceed 5MW where in-rack sprinklers have been provided. Smaller tests on single racks of clothing would indicate a typical peak heat release rate of 2.5MW for isolated arrangements. BRE 368 would also suggest a peak heat release rate of 2.5MW for sprinklered fires using fast response sprinkler heads. Within this project an assumption of a 5 MW fire is made
5.2 Calculation procedures
How occupants respond to a fire will depend initially on the cue they received. The components of occupants’ evacuation times are recognition time, response time and travel time. Both recognition and response times are known as pre-movement time. The former represents the time taken to appreciate that the alarm is a fire alarm relating to the occupant (as opposed to a security alarm, car alarm etc).
During this time occupants will continue with activities as they were prior to the activation of the alarm, e.g. staff members undertaking duties, etc. The latter represents the duration taken by occupants to begin to move towards an escape route, e.g. investigative behaviour shutting down machinery etc.
In warehouse buildings occupants are generally familiar with their surroundings and are trained in fire emergency evacuation protocols. In the proposed storage facility and associated offices, occupants should be familiar with the sounding of the fire alarm and as such would commence evacuation reasonably promptly. Where a reasonable standard of fire detection is provided, BS7974-6 recommends that familiar occupants of a building can be expected to take between 1mins and 2mins before they start to evacuate. The characteristics and determination of evacuation behaviour can be simplified in terms of the following broad categories of behaviour; the first is Pre-Movement Behaviour and the second is Travel Behaviour. The recommendations of BS7974-6 have been used to determine the Required Safe Egress time (RSET) for the proposed and code compliant layouts.
RSET can be calculated using the following formula:
rtrset = rtdet + rtalarm + rtpre + rttravel
rtrset = Total time required for escape (secs)
rtdet = Time to detection (secs)
rtalarm = Time to alarm (secs)
rtpre = Pre-movement time (secs)
rttravel = Travel time (secs)
Travel Time is the time required for the occupants to walk to an exit leading to a place of safety. Walking time may be expressed as a distribution of individual times or as a single time such as an average time required.
As stated earlier we will assume the maximum travel distance within the warehouse is 60m with no dead end situations allowing for two directions of escape from all areas due to the number of exits provided from the warehouse. We will take the speed of from Annex D PD7974-6 2004 as 1.4m/s.
ttrav(walking) = ——————————- = ———– = 43(s)
Speed of travel 1.4 m
6 Comparison of results with acceptance criteria
6.1 Summary of the results of this case study
To meet with code recommendations a Type ‘M’ manual fire alarm system would be necessary in a standard warehouse building. However, the provision of an automatic fire detection and alarm system is an integral part of the overall fire safety package from a fire engineering perspective.
The building will be provided with an automatic fire alarm and detection system meeting the recommendations of BS 5839-1 Category L2. The system will consist of beam detectors within the warehouse area.
Manual call points will be provided at each storey exit and throughout the premises to give warning to the occupants in case of an event other than fire, a voice alarm system will also be installed in accordance with the recommendations of BS 5839-8. The sound level of the alarm system should generally be at least:
60dB (A) in open plan areas>60m2, e.g. offices (excluding a 500mm perimeter boundary)
60dB (A) in staircases and rooms less than 60m2
5dB (A) above the ambient noise level.
Table B.1 – Typical occupational noise levels (Lp)
7.1 Fire protection requirements
It is proposed to provide a high standard of automatic fire detection and alarm system throughout the warehousing compartment. A manual system will cover the office area of the premises. The standard of detection and alarm in the building will affect the general time to warning for occupants and provide an enhanced level of fire safety through early warning and shorter evacuation times. Activation of the sprinkler system will also lead to the fire alarm sounding throughout the building. To ensure a conservative estimation of fire alarm activation our analysis has been based on an anticipated sprinkler system activation time within 2 minutes.
Many code compliant warehouse facilities with code compliant travel distances only require manual fire detection to meet with code recommendations. Therefore the time to detection would depend on the time taken for occupants in the room of fire origin to raise the alarm. However, should the room of fire origin be unoccupied then the time to alarm for people who may be in rooms remote from the fire would depend on them or other occupants becoming aware of cues of the fire scenario, e.g. sounds of burning material, smell of smoke or seeing smoke through a vision panel.
Although BS7974-6 recommends a time to detection of at least 15 minutes for such scenarios, a reasonable worst case for a typical code compliant open plan warehouse could be 3 – 5 minutes. The fire alarm and detection system will be an addressable analogue system. Multi-state detectors will be capable of giving normal, fire and alternate signals dependent upon the required sensitivity. The control panel will be sited in a low fire risk area (office). It shall have suitable ambient light and sound levels for both staff and FRS use. The fully addressable system will give floor/zone and the specific address of the signal’s location. There will be 100 second fault monitoring. It will be provided with staff alarm, as well as test, silence and reset buttons.
Plan of warehouse
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