Application of fire safety engineering
Mr. Sunil Samuel Calvin, Managing Director, M/S Alpha National Fire Safety Services Pvt. Ltd., Mumbai
Holds Bachelor’s Degree in Chemical Engineering and Post-Graduate Diploma in Fire Safety and Environmental Engineering. Has over two decades of experience in Design, Erection, Testing and Commissioning of Fire Protection systems on a turnkey basis for various industries in India, Germany, Mexico and Cyprus.
‘7 Students, engineer carried to death in city restaurant fire.’ Seven students and an engineer out for lunch were burnt to death in a Chinese restaurant in Kurla (W) on Friday, 16th Oct. 2015. This might become a story for others but a nightmare and lifelong remembrance for the family members.
Post analysis of the incidence shows that the single filmy staircase leading to upper illegal floor gave away trapping the eight. It is very important for all of us to understand the concept of fire safety engineering in building areas where we spend most of our valuable hours.
2.0 WHAT DO YOU MEAN BY SAFETY ?
The concept of Safety “The State of being safe and protected from danger or harm.”
3.0 THE PHILOSOPHY OF FIRE SAFETY
- Fire is a dynamic event which is unchecked can grow from small beginning into a life threatening highly destructive configuration.
- In case of fire in buildings, there are limits of tolerability for the survival of people and of the building structure exposed to the fire. They may be expressed as
- Life Safety : the time available for escape
- Structural stability : the time before collapse.
- The act of escaping from a fire is also a dynamic event and this too can be expressed on terms of time i.e. the time required for escape.
- The aim of fire safety is to ensure that people have sufficient time to escape and before the stability of the building is put at risk
- The time available for escape must be always be longer than time required for escape.
- In passive fire safety, this is achieved by limiting the distance people may have to travel to reach an exit.
- Fire safety engineering typically uses smoke control system to keep exit routes clear of heat and smoke.
4.0 QUALITATIVE DESIGN REVIEW
At the early stages of the building design process, the proposal will be subjected to a qualitative design review (QDR). QDR is a technique in which the impact of possible fire hazards on the need to maintain the risk to people or the building structure to an acceptable level can be determined. The design needs of the proposal can then be assessed quantitatively against the design objectives of the designer. Any change in the design throughout the process needs to be agreed with QDR team.
The QDR team on a major project might include the following;
- Fire Safety Engineer
- Services Engineer
- Structural Engineer
- Member of operational management
- Member of Approvals Agency and / or Insurer
The QDR should take into account of :
- The use of the building
- The number and characteristics of proposed occupants (life safety schemes)
- The architectural design of the building
- The fire safety objectives
- The fire hazards and possible consequences
- Results of trial fire safety designs.
- Possible fire scenarios for analysis
- Proposed acceptance criteria
The outcome of the QDR will determine the viability of the proposed scheme. Where viability cannot be established, the review should re-address the matters considered until either a satisfactory result is reached. It should be ensured that only realistic data is used and that the results of trial fire safety designs or fire scenario predictions are not distorted to meet other objectives or constraints.
The satisfactory completion of the QDR should typically provide the following information;
- The results of the architectural review
- A clear statement of the fire safety objectives
- Identification of the significant hazards and their possible consequences
- Evaluation of one or more of the trial designs
- The acceptance criteria and suggested methods of analysis
- The specifications of the fire scenarios for analysis
At this stage, it should be possible to decide whether qualitative analysis is necessary to demonstrate that the design meets the fire safety objectives.
5.0 QUALITATIVE ANALYSIS
This analysis may be divided into a number of parts and each of these parts is covered by the sub-systems to be provided in the series of Published Documents that will eventually accompany BS 7974.
In order that a satisfactory qualification study can be undertaken, the trial fire safety design should consider the following;
- Automatic suppression
- Automatic systems
- Smoke control
- Alarm and warning systems
- Evacuation strategy
- Means of escape
- First aid fire fighting
- Fire service facilities
- Management of fire safety
- Use of materials in potential fire area
- The general configuration of materials
These sub-systems are intended to provide guidance on the type of calculations that may be carried out in support of a fire engineering study and to present the general principles and procedures appropriate to the aspect of fire safety engineering covered by that part of the analysis. The various aspects of the analysis may be risk assessment.
The alternative strategy is to treat fire as a series of random events and assess the possible outcome in a probabilistic manner in order to estimate the likelihood of a particular, unwanted event occurring.
Deterministic procedures quantify fire growth, fire spread, smoke movement and the consequences of these for the building and its occupants. The evaluation of a set of circumstances that will provide a single outcome, i.e. whether the design will either be successful or not.
Several techniques are available for evaluating the development and effects of fire and the movement of people. The interaction of fire, buildings and people can give rise to a very complex system.
6.0 DESIGN FIRE SCENARIO
A fire is an exothermic reaction caused when heat comes into contact with a flammable material. Except in the case where a material is volatile at normal temperatures, e.g. petrol, the application of heat causes the flammable substance to give off a flammable vapour, which, in the presence of sufficient heat is ignited. The heat from the fire then causes further vapours to be given off thus sustaining the fire until either it is extinguished or all of the fuel is consumed.
The fire will create a convective plume rising above it. In the main the plume will be air, drawn into the fire by the convective process and heated. It will also contain oxygen-depleted air and products from the combustion process, e.g. smoke. The mass of the convective plume (mass flux) will be proportional to the temperature of the fire and the area of its base.
As the plume rises, it will expand as it mixes with any additional air. The plume will also cool as it rises further from the fire. If the rising column comes against a horizontal surface, it will spread horizontally in all directions. This is known as a ceiling jet. The temperature of the plume or ceiling jet at any point will be according to the temperature of the fire and its height from the fire. Eventually the plume will dissipate into the atmosphere, the heavier products of combustion falling back to ground.
When designing a ventilation scheme, the designer will take into account the flammability of fuel sources likely to be involved in fire and their probable rate of heat release. From this a fire scenario (design fire) will be assumed from which the temperature of a steady state fire and the mass flux can be calculated. The ventilation system will be designed to accommodate those values.
For similar fuel sources, the intensity of a fire is usually directly proportional to the amount of oxygen present. Ventilated fires always have a plentiful supply of oxygen due to the inflow of replacement air. The ability of that air to reach the fire will depend on the configuration of the burning fuel. For instance, stacked chairs will burn vigorously because the air can get into the fire from all sides and under the fuel. Fires against walls deny access to air on the wall side, whilst fires in the corner between two walls denies access on two sides. This results in a great flame length up the wall.
7.0 FIRE SAFETY ENGINEERING ACCEPTANCE CRITERION
As has already been stated, fire safety engineering offers a legitimate, alternative approach to the present system of building control. The aim is to achieve a satisfactory level of safety (acceptance of risk) on a building.
There can be no suggestion, in such a solution, that the existing codes are inadequate and that the engineered solution being offered is the only `correct’ procedure.
The accepted principle here is that the developer designer of the building wished to express themselves conceptually, but were unable to do so using either the generally accepted design criteria or restrictions placed on the use of any specific construction material. The `adjustment’ of certain design or construction principles there is intended to allow the original concept to be achieved.
A fire-engineered solution would normally be considered during the planning design stages of any building or structure. Indeed, this may be one of the first considerations to be given to the actual detail because it may have been determined at the conceptual stage that the innovative approach being adopted will not fit the more traditional or conventional techniques.
It will be at this early state that the fire engineering consideration and evaluation given to the desired design parameters, will enable adjustments to be made, whilst perhaps maintaining the integrity of the original design. As with all engineering studies it will be necessary to take account of all the possible variations, before coming to an agreed position.
In considering the acceptability, or otherwise of an engineered solution, a number of basic parameters must be considered. There usually concentrate on the ‘acceptability’ or otherwise of the risk. There may be a number of valid reasons as to why a ‘risk’ may be acceptable these would include:
- Ignorance – if the existence of the risk is unknown or not fully understood, it may accepted for many years. An example of this may be the fire behavior of certain construction products, e.g. large insulating (sanchwich) panels. In such cases when the risk became apparent, action was taken to reduce the risk.
- Negligible – if the risk is so low that they can be considered negligible they often can be accepted. However, the difficulty here is to define `negligible’.
If the risk identified is considered to be above what would be classed as `negligible’ then two further classifications may be applied. The increased level of risk (above that considered negligible) is either considered tolerable or intolerable.
- Tolerable – any high, medium or low risk is usually considered tolerable when the benefits of the situation causing the risk are considered to overweigh the level of risk. For example, there are risks inherent in travel by air, but there is universal acceptance that the advantages of such mode of travel far outweigh any perceived level of risk. Such acceptance relies upon a simple measure in that it is accepted that every effort has been made to reduce the level of risk as far, as is possible. A principle, known as `As low as reasonably practicable’.
- Intolerable – if the risk is not considered tolerable, then by definition, it becomes intolerable. Such risks cannot be tolerated under any circumstance.
The present system on on-going control in respect of fire safety in occupied buildings required an ongoing evaluation of the circumstances by those responsible for enforcement – usually the fire brigade, in respect of the ‘fire certificate’ issued under the 1971 Fire Prevention Act. Under the Workplace Regulations (as amended), the fire brigade will enforce, but the assessment of the fire risk will be completed by the employer / responsible person.
Fire safety engineering is an integral part of our day to day life. Right step should be taken by the fire safety engineer when devising an engineered scheme for a building and as appropriate gives minimum criteria beyond which the validity of the design may be questionable.
No one single solution offers the required consideration. It is therefore necessary to adjust the individual aspect of any considered solution until the closest ‘fit’ can be achieved.