Objectives of Fire safe Building Design

Objectives of fire safe building design

Effective fire safe design begins with conscious analysis and decision-making early in the design process. To effectively incorporate the building fire defenses into the design the fire safety objective must first be identified.

The acceptable levels of safety and the focus of fire safety analysis and design process objectives are concentrated in the following areas.

1.      Life safety
2.          Property protection
3.          Continuity of operation
4.          Environmental protection

Fire Safety Design Strategy

Fire safety objective can be met if fire ignition can be prevented or if, given ignition, the fire can be managed. Evaluating a design for building fire safety represent a systematic approach to the fire safety strategy. These strategies can be identified as follows:

1. Prevent fire ignitionn

The first opportunity to achieve fire safety in a building is through fire prevention, which involves separating potential heat sources from potential fuels. Major building fires are started by heat sources and ignitable materials that are brought into the building, not built into it. This means the design of the building, from the architects and builder’s standpoint provides limited potential leverage on building future fire experience.

2. Control combustion Process

The control combustion process is concerned with slowing the fire to provide other fire safety measures with sufficient time to be effective. The building fire safety system can be organized around fire growth and its resulting production of combustion i.e. flame, heat smoke ad gases. The ease of generation and movement of these products is influenced by counter measure provided by the building. The effectiveness of the building fire safety system determines the speed, quantity and paths of movement of these products of combustion.

The main factors that influence the control of combustion process are

1.      Fuel load
2.           Interior finish of the room
3.        Air supply
4.        Size and shape and construction of the room
5.        Fire load

3. Control fire by construction

Fire Resistance Barriers such as walls, partition and floors, separate building spaces. These barriers also delay or prevent fire from propagation from one place to another. In addition, barriers are important features in any fire fighting operation because they dictate the size of fire.

The effectiveness of a barrier depends upon its inherent fire resistance construction and its penetration. Use of flame retardant paints, fire stop barriers, firewalls, fire doors and windows. Other few methods for controlling the fires by construction in building.

4. Fire detection and alarms

Fire detection is needed so that automatic or manual fires suppression will be initiated, any other active fire protection system will be activated and occupants will have time to move to safe locations typically outside the building.

5. Automatic suppression

To accomplish automatic suppression, both detecting the fire and applying sufficient suppressant are necessary. The major elements of determining the automatic extinguishing system are

1.        Its presence or absence
2.        If present, its reliability
3.        If reliability its designing and extinguishing effectiveness.

6. Manual suppression

The major aspect of this part of building design include

i.	Fire department’s distance from building
ii.	Initial agent application
iii.	Fire extinguishment
iv.	Ventilation
v.	Water storage and use
vi.	Water removable
vii.	Barrier effectiveness

7. Managing the exposed

Fire impact can be lessened by managing the exposed i.e. people property, operations, environment or heritage depending on the design aspects being considered. The exposed people or property may be safeguarded either moving them to a safe area or refuge or by defending them in a place. The design for life safety may involve one or combination of

1.        Evacuation of the occupants
2.        Defending the occupants in place
3.        Providing an effective area of refuge
4.        Practicing evacuation drill

Safety requirements for high raised buildings

The susceptibility of buildings to fire depends on several factors like:

1.        Type and size of building,
2.        method of construction,
3.        combustibility of materials of construction,
4.        the type of occupancy,
5.        age of the building,
6.        degree of fire resistance,
7.        the type of building services,
8.        fire load. of the building,
9.        fire prevention, and fire protection arrangements of the building,
10.           scores of others, including the human factor.

 However, for purposes of analysis of the various fire hazards in buildings it is common to divide these hazards into:

(i) Internal hazards- which arise inside the building and which concern the safety of the occupants (Personal Hazard or more widely known as Life Hazard): and which concerns the safety of the structure and the contents; and

(ii) External hazards - which arise as a result of fires in surrounding property (Exposure Hazard)

The relative degree of each of these hazards will vary according to the type of occupancy of the building - An Assembly occupancy will be having predominently life hazards, whereas a Storage occupancy will have primarily a damage hazard, ie., hazard to the structure and contents.

Fire and Damage Hazards to the Building that needs to be considered for safety requirements for high raised buildings:

The other internal hazard arising from fires within the building is the damage hazard or the hazard to the building structure and contents.

Many factors can have a bearing on this form of internal hazard such as:

1.        The type of building construction,
2.        Fire resistance of the elements of structure,
3.        Combustibility of materials of construction,
4.         Unbroken large spaces,
5.        Fire load of the building,
6.        Open vertical and horizontal shafts,
7.        Bad house-keeping and storage practices,
8.        Unsafe processes,
9.        Non-availability of fire doors and other types of fire barriers
10.           Inadequate fire fighting/fire protection measures,
11.          Unsafe building services,
12.          Unfavourable human elements etc.. 

Hazards from building services

1.     Electrical installations

2.     Heating

3.     Ventilation

4.     Air conditioning

5.     Refuse disposal

6.     Plumbing,

7.     Communications,

8.     Transportation and conveyance systems.

Design Considerations of Egress of a building:

Designing a means of egress involves more than numbers, flow rates and densities. Good exits facilitate everyone to leave the fire area in the shortest possible time by prompt and efficient use of them. If the fire is discovered promptly and occupants alerted also equally promptly, early evacuation can be done. However, evacuation times are directly related to the fire hazard; higher the hazard, shorter the exit time.

Provision of two separate means of exits for every floor including basements is a fundamental requirement, except in very few deserving cases.

In general, life safety from fire requires the following principles to be adopted:(Some of these may have already been covered earlier):

1.     Sufficient number of unobstructed exits of adequate capacity and properly designed, with easy access;
2.     Safeguarding of exits against fire and smoke during the length of time they are designed for use;
3.     Availability of alternate exit and means of access to it, in case one exit is unusable due to fire;
4.    Sub division of areas to provide sufficient areas of refuge for occupants where evacuation may be delayed;
5.    Adequate protection of vertical openings to minimize hazards from fire and smoke;
6.    Efficient fire alarm systems for alerting occupants and others concerned in case of fire;
7.   Adequate lighting of exits and rescue paths;
8.   Adequate exit indication signs to help evacuation;
9.    Ensuring trouble free evacuation through the escape route by safeguarding of equipment and vulnerable areas from fire and smoke hazards;
10.    Fully rehearsed exit drill procedures to ensure orderly and smooth exit;
11.    Institution of panic control measures;
12. Control fire hazards from interior finish material.

EXITS

Exits are a critical part of overall life safety. Several basic concepts are involved in determining the number, arrangement, and construction of exits for a specific area.

The details of some of the means of exit or egress or escape is a continuous path of travel from any point in a building or structure to the open air outside at ground level. It consists of three constituents which are:

(i) the exit access;

(ii) the exit ;

(iii) the exit discharge

The main requirements for the means of exit consisting of the three constituents as given above are:

1.  All means of exit, including staircases, lift lobbies and corridors, shall be adequately ventilated;
2. Exits not properly ventilated can cause suffocation to people being evacuated because a large number of people would be present in such enclosed place with no natural ventilation till they get out of it and reach open air;
3. Every building meant for human occupancy shall be provided with exits sufficient to permit safe escape of occupants, in case of fire or other emergency;
4. An exit may be a doorway, corridor, passageway(s) to an internal staircase, or external staircase, or to a verandah or terrace(s), which have access to the street, or to the roof of a building or a refuge area. An exit may also include a horizontal exit leading to an adjoining building at the same level;
5. Every exit, exit access or exit discharge shall be continuously maintained free of all obstructions or impediments to full use in the case of fire or other emergency.
6. Even if adequate exits are provided at the initial stage, often at the time of renovation/alteration, knowingly or unknowingly, people do not give same attention to exit requirements. In view of the above this requirement assumes great significance.
7.  No building shall be so altered as to reduce the number, width or protection of exits to less than that required.
8. People escaping from areas filled with fire and smoke will be all anxiety to reach open air where they can breath normally and become tension-free at the earliest.
9. All exits shall provide continuous means of egress to the exterior of a building or to an exterior open space leading to a street.
10. Exits shall be clearly visible and the route to reach such exits shall be clearly marked and signs posted to guide the occupants of the floor concerned. Signs shall be illuminated and wired to independent electrical circuits on an alternative source of supply.
11. The sizes and colour of the exit signs shall be in accordance with established international practice.
12. Normal colour used for exits is green. Illumination of exits and exit route signs, even when electricity is turned off, is very important to ensure orderly evacuation of occupants without chaos.
13. The floors of areas covered for the means of exits shall be illuminated to values not less than 10 lux at floor level.
14. This should cover all portions of exit access, exits and exit discharge.
15.  Fire doors with 2 hour fire resistance shall be provided at appropriate places along the escape route and particularly at the entrance to lift lobby and stairwell, where a funnel or flue effect may be created inducing an upward spread of fire, to prevent spread of fire and smoke.

     The fire load

Fire load is usually expressed in terms of the “wood-equivalent” weight of combustible building contents per unit building- floor area in psf. The actual weight of combustible contents is adjusted to the wood-equivalent weight based on the estimated potential heat of contents normalized to the potential heat of wood (8000 Btu/lb). Alternatively, the fire load could be expressed in terms of the potential heat of building contents per unit building-floor area in Btu/ft2.

Knowing the fire load of rooms in a burning structure is important information for fire safety:-

1. It indicates how destructive fires in different rooms or compartments can be.
2. Gives an idea of how likely a fire is to spread from one area to another.
3. Firefighters use this information to identify the most vulnerable or dangerous areas of burning buildings.
4. It is also one consideration taken into account when a building is being constructed. For example, concrete does not contribute to fire load because it does not burn, and so is often used to construct rooms or building where highly flammable materials are kept.
5. Fire codes and building codes often include regulations restricting where and how highly flammable materials such as fuel may be stored, because they greatly contribute to the local fire load and present a heightened risk of an out-of-control fire if kept in places with fire-control measures that were not designed to deal with the amount of heat they can generate.

Fire load can be caluclated as follows:-

Weight of the material in Kg (mass) X Calorific value of the material

Unit of fire load is in Kilo Joules.

For example 1 tonne of propane @ a calorific value of 47.3x10³ KJ/KG

= 1000 x 47.3 x 10³ = 47300 KJ

Structural Fire Protection

The structural elements of a building need to be designed to withstand the effects of building compartment fires, so as to minimize the likelihood of structural collapse. The use of steel framed construction and in particular the use of fireproofing materials is known to provide fire resistance levels for the steel elements of buildings.

Structural fire protection – what is it and why do we need it?

Steel structures, consisting of structural steel members, connections, fasteners and frames, act together in resisting imposed actions (loads, pressures, displacements, strains, etc). Load bearing steel framed buildings or building structures need to cater for all the loads that building can experience, including the dead load of the building itself, live loads applied to the building in use, wind and snow loading. We need to pay special attention to the fireproofing of steel framed buildings to ensure that during pre-determined fire scenarios that the steel does not get above critical temperatures

Fireproofing is achieved by applying fireproofing covering materials to thermally protect the underlying steel from the heat of the fire. The type and amount of fire proofing material is dependent on a number of variables, some of which include:

1.     size of steel members in question

2.     fire severity

3.     fire rating or duration of exposure to a given fire

4.     time of application of the fireproofing covering

5.     budget

6.     aesthetics

          7.    location of project

For the purposes of this article, fireproofing materials have been allocated in three (3) distinct generic categories which include:

1. conventional spray applied materials

2. board materials

3. intumescent paints

  

Structural integrity

Structural integrity and failure is an aspect of engineering which deals with the ability of a structure to support a designed load (weight, force, etc...) without breaking, tearing apart, or collapsing, and includes the study of breakage that has previously occurred in order to prevent failures in future designs.

Structural integrity is a performance characteristic which is applied to a component, a single structure, or a structure consisting of different components. Structural integrity is the quality of an item to hold together under a load, including its own weight, resisting breakage or bending. It assures that the construction will perform its designed function, during reasonable use, for as long as the designed life of the structure. Items are constructed with structural integrity to ensure that catastrophic failure does not occur, which can result in injuries, severe damage, death, or monetary losses.

Structural failure refers to the loss of structural integrity, which is the loss of the load-carrying capacity of a component or member within a structure, or of the structure itself. Structural failure is initiated when the material is stressed to its strength limit, thus causing fracture or excessive deformations. In a well-designed system, a localized failure should not cause immediate or even progressive collapse of the entire structure. Ultimate failure strength is one of the limit states that must be accounted for in structural engineering and structural design.

To construct an item with structural integrity, an engineer must first consider the mechanical properties of a material, such as toughness, strength, weight, hardness, and elasticity, and then determine a suitable size, thickness, or shape that will withstand the desired load for a long life. A material with high strength may resist bending, but, without adequate toughness, it may have to be very large to support a load and prevent breaking. However, a material with low strength will likely bend under a load even though its high toughness prevents fracture.

Failure of a structure can occur from many types of problems. Most of these problems are unique to the type of structure or to the various industries. However, most can be traced to one of five main causes.

·         The first, whether due to size, shape, or the choice of material, is that the structure is not strong and tough enough to support the load. If the structure or component is not strong enough, catastrophic failure can occur when the overstressed construction reaches a critical stress level.

·         The second is instability, whether due to geometry, design or material choice, causing the structure to fail from fatigue or corrosion. These types of failure often occur at stress points, such as squared corners or from bolt holes being too close to the material's edge, causing cracks to slowly form and then progress through cyclic loading. Failure general occurs when the cracks reach a critical length, causing breakage to happen suddenly under normal loading conditions.

·         The third type of failure is caused by manufacturing errors. This may be due to improper selection of materials, incorrect sizing, improper heat treating, failing to adhere to the design, or shoddy workmanship. These types of failure can occur at any time, and are usually unpredictable.

·         The fourth is also unpredictable, from the use of defective materials. The material may have been improperly manufactured, or may have been damaged from prior use.

·         The fifth cause of failure is from lack of consideration of unexpected problems. Vandalism, sabotage, and natural disasters can all overstress a structure to the point of failure. Improper training of those who use and maintain the construction can also overstress it, leading to potential failures.

Building fires due to electrical failures:

Modes of ignition

The examination of failures can be approached in several different ways:

(a) identifying the act(s) or omission(s) leading to failure

(b) classifying failures by the functional nature of the device or part thereof that failed

(c) studying the basic physics of failures.

A consideration of the failure mechanisms reveals that there are only a few main ways that electrical insulation, or combustibles close by to electric distribution components, can be ignited, although there are diverse aspects to each:

(1) arcing

(2) excessive ohmic heating, without arcing

(3) external heating.

Some ignition types involve a combination of mechanisms, so they must not be viewed as mutuallyexclusive causes of fire.

Arcing

The causes of arcs can be many, but the primary ones are:

(1) carbonization of insulation (arc tracking)

(2) externally induced ionization of air (created by flames or an earlier arc)

(3) short circuits

CARBONIZATION OF INSULATION

In 120/240 VAC circuits, it is not difficult to cause sustained arcing if there is a carbonized conductive path. This is sometimes called arcing-across-char. Insulating materials vary widely in their susceptibility to arc tracking. A large fraction of wiring in 120/240 V circuits is insulated with PVC, When PVC is exposed to temperatures of 200 . 300ºC, it chars and the char is a semiconductor. Not surprisingly, this can lead to leakage currents and arcing.

Following sequence of steps were identified:

1. Initial current flow occurs due a carbonized layer.
2. The current flow increases and results in local arcing
3. The arcing causes melting of metal and expulsion of the molten pieces.
4. Once the molten pieces are expelled, current flow drops
5. Continued current flow through carbonized material eventually leads again to a sizeable current flow.

 EXTERNALLY-INDUCED IONIZATION OF AIR

The intrinsic dielectric strength of air is high (roughly 3 MV m-1, for all except very small gaps), but breakdown can occur at much lower values if the air space is ionized by some means. Two such means are flames and pre-existing arcs. If a serious arc-fault occurs in a distribution bus, a large amount of ionized gases will be ejected. These can travel a certain distance, and if they encounter another circuit, they can readily cause a breakdown and new arcing at the second location.

SHORT CIRCUITS

The term short circuit is commonly applied in the situation where a low-resistance, high-current fault suddenly develops in a circuit.

This can take two forms:

(1) a bolted short where a good metal-to-metal contact is made across a full-thickness section of metal;

(2) an arcing short, where initial metal-to-metal contact is not sustained and current flow through an arc.

An arcing short results from a momentary contact of two conductors. This causes melting of the

material around the contact area. Magnetic forces tend to push the conductors apart, and the liquid bridge between them then gets broken. Sparking may be observed as the conductors come apart.

Excessive ohmic heating

The causes of excessive ohmic heating can be subdivided into:

(1) gross overloads

(2) excessive thermal insulation

(3) stray currents and ground faults

(4) overvoltage

(5) poor connections

External heating

Most cases of external heating involve the wire or wiring device as .victim. of fire and not as the

initiator of fire. But some situations do exist where external heating of wiring serves as the initiating event. In many cases, arcing occurs after sufficient overheating.The electrical

failure of two cables as a function of oven heating. Electrical failure was considered to be a short circuit or a low-resistance condition developed across the line; experiments were not conducted to actually elicit ignitions. A cable with cross-linked polyethylene insulation failed at 270ºC, while a cable with polyethylene/PVC wire insulation and PVC jacket failed at 250ºC. A NIST study on lighting fixtures examined the effect of over-temperatures on 60ºC-rated normal building wiring. When overlamping of a fixture created 202-205ºC temperatures in the electrical junction box, failure occurred in less than 65 h. The wire insulation became brittle, cracked, fell away from the conductors, and this led to a short circuit.