5. FIRE SAFETY IN BUILDINGSGeneral Issues

Occurrence and Causes of Fire Occurrence of fire in a building is included

into the category of extraordinary actions, similar to explosions, earthquakes et al. Such an exceptional event may occur with a certain probability during the life of a given building. Although this probability is relatively small for each building considered individually, the number of constructions world- wide affected by the fire every year is quite large and social economical consequences are extremely important.

The fire action has certain peculiarities, which makes it completely different from other actions. A fire once initiated feeds itself until all combustible materials existing inside the respective space self-ignite due to high temperature. This phenomenon is currently known as flash-over, meaning a generalised fire inside a certain enclosed space or volume.

Generally speaking, fires are caused either by lack of knowledge on the matter or by inadequate or insufficient measures applied for fire protection.More specifically, among the causes that generate or amplify fire in buildings one should mention in the first place:

- excessive or/and inadequate storage of combustible materials;

- leakage or spilling of fuel or other flammable liquids present under normal service conditions in buildings;

- explosion of gases or other chemical matters;

- fireworks;

- deficient electrical installations or appliances;

- negligence and deficiencies in buildings utilisation;

- lightning;

-self-ignition of crops, woods or other combustible matters outside the buildings, due to excessive heat and drought;

- criminal actions.

Temperature Variation vs. Time During a Fire

Inside a compartment set on fire, it has been found that temperature variation vs. time generally follows a pattern emphasising three phases, as represented schematically in Fig. 4.1.

During the first phase, when temperature

currently does not exceed 200C, the amount of heat produced is very small with respect to the volume of the compartment set on fire. This phase of fire may last from a few minutes to several hours or even days .The propagation of burning process contributes in the beginning to a slow raising of temperature, which eventually is accelerated up to a critical level of self-ignition of materials and compartment contents. The fire becomes generalised (flash-over) and the process enters its second phase. At the beginning of the second phase there is a sharp raise in temperature (510 times), which frequently reaches 1500C or more and remains quite constant for a while, until the compartment thermal load is consumed.

From the viewpoint of investigation of buildings behaviour when exposed to fire action, this phase is of most interest. After this moment, the fire process enters its third phase, which usually lasts for a long period of time. The temperature drops rather quickly in the beginning, but decreases very slowly afterwards.

Fig.4.1. Temperature vs. Time Relationship During a Real Fire Smoke A Primary Threat to Life A fire always generates combustion gases and smoke; the latter consists of fine solid or liquid particles, remains of combustion drawn by hot gases. Despite the common belief that flames are the immediate danger, it is smoke the primary threat to life in actual fires. Choking, blinding and, very possibly, lethally hot or poisonous, the smoke can quickly disorientate and kill people far from the seat of a fire. Sleeping people in residential buildings may never live to wake up and try to escape. The movement of smoke in buildings set on fire is extremely important. Many cases have been recorded when a relatively small fire had produced a large amount of smoke, hampering evacuation of people and generating panic and casualties.

Overall Considerations on Measures Aimed at Ensuring Protection Against Fire

Generally speaking, there are two broad categories of measures to be taken in order to provide adequate fire protection.

Passive measures consist in devising and applying appropriate design provision concerning the layout, planning and detailing of buildings, so that:

- the possibility of fire occurrence be reduced as much as possible;

fire be contained in the area ( compartment ) where it has occurred and its spreading be prevented. The limitation of fire propagation is primarily based on the capacity of fire resistance provided by certain building elements such as floors, walls, doors, shutters, hatches etc;

people be allowed to get out of the building set on fire, through purposely created escape routes;

interventions from exterior be facilitated in order to fight the fire.

Active measures consist in providing special devices, equipment and installations aimed at:

- detecting incipient fire( activated by smoke, heat or flames);

- triggering fire alarm, inside and outside the building;

keeping the fire in a confined area, either by automatically triggered devices(e.g. sprinklers, drenchers using water, chemical foams, inert gasesetc) or by people-handled means (e.g. hoses connected to hydrants, portable extinguishers with foam, carbon dioxide, aerosols etc), till the arrival of fire-intervention squads.

One must stress the fact, confirmed by experience acquired during past fires, that to rely primarily upon the fire service to save people from buildings does not represent good politics and a correct approach. This is in no way to discount the fire services vital work, but simply to recognise that, if a building has to rely on outside help only to save its occupants, then it has failed.

Properties and Behaviour ofConstruction Materials and

Elements Exposed to HIGH

TEMPERATUREs

The resistance against fire represents the most important exigency addressed to both structural and non-structural elements, with respect to the conditions imposed by fire outbreak in a building. It expresses the capacity of the respective element to maintain, during the fire, its specific functions (e.g. mechanical resistance, partitioning, insulation etc.). A quantification of fire resistance is currently based on the so-called fire resistance rating (or limit of fire resistance), meaning the period of time (measured in hours or minutes) during which the element under consideration is able to maintain its specific functions, when subjected to fire action. Obviously, this assessment is relevant only if the fire severity is stipulated.

To this aim, a standard fire testing procedure has been worked out, making use of a standard curve for temperature-time relationship. The standard curve introduced by I.S.O. is intended to express an equivalence between the quantity of heat developed during a standard fire and that corresponding to actual fires having the same duration The combustibility of construction materials and elements is given by their capacity to ignite and keep on burning, thus contributing to increase the heat quantity produced by the fire.

Depending on their behaviour to fire, construction materials and elements can be non-combustible ( class Co ) or combustible. These latter are further classified according to present Romanian regulations into the following classes of combustibility:

- practically non-flammable (C1);

- hardly flammable (C2 );

- slightly flammable (C3 );

- easily flammable (C4 );

Based on furnace tests and experience, and applying some specific I.S.O. criteria, any combustible construction material and element is assigned to one of the above classes of combustibility. Rather empirically, materials that ignite when in direct contact with the fire, burn as long as this contact is maintained and self-extinguish after cease of contact, are currently included into class C1 or C2. Similarly, materials that ignite under the action of fire and heat, burn with flames, are charred and smoulder after removal of heat source, are included into class C3 or C4.

Fire Risk and Degree of Fire Resistance

Exigencies and Performance Criteria Related to Fire Safety of Buildings The assessment of buildings performance concerning fire safety can be done based on the following main exigencies, which have to be addressed to either the whole building system, to its subsystems structure, enclosure, partitions, or to its elements (e.g. exterior and interior walls, floors, roof, etc);

- stability

- tightness;

- thermal insulation.

Stability means the capacity of the structure as a whole and of the structural elements to avoid collapse as well as excessive deformations during the phase of heating and, respectively, of cooling. The latter is currently estimated to last about 24 hours after the consumption of thermal load.

Tightness is expressed by the capacity of partitioning elements(walls, floors) to prevent flames and hot gases to penetrate into building compartments adjacent to the one where a fire has broken out. This generally implies absence of cracks or dislocations. Thermal insulation represents in this case the capacity of partitioning elements to reduce excessive heat transfer from the face oriented towards the fire to the opposite one. Accordingly, the temperature on this latter face should not exceed 150200C, in order to avoid self - ignition of combustible materials. Specific technical regulations set up different values for fire rating (or limit of fire resistance) are corresponding to each of the exigencies previously mentioned.

When considering the building as a whole, the following two exigencies are of paramount importance: reduction of the risk of fire breaking out and evolving inside the building;

reduction of the possibility of fire propagation between neighbouring buildings.

Basic design Provisions

Life safety is obviously a major concern in building design. From the viewpoint of the building structure, the most relevant elements of fire resistance are:

- combustibility of the structure. If structural materials are combustible, they contribute fuel to the fire as well as hasten the collapse of the structure;

- loss of strength and stiffness at high temperature. This consists of a race against time, from the moment of fire burst to the failure of the structure a long interval increasing the chance for the occupants to escape;

- containment of fire. Fire usually starts at a single location, and preventing its spread is a strong requirement.

protection of Neighbouring Buildings

There is the risk that fire may spread from one building to another. Many of today provisions aiming at reducing as much as possible this risk stem from past disaster experience showing what can happen without proper fire-spread control.

The threat can be by flames and hot gases directly. It can be also by flying brands, which have been known to ignite buildings as much as half a kilometre from a fire outbreak. It can be by radiation as well; timber facades can be ignited at 50 m distance, and even if neighbouring buildings are not ignited directly, they can be heated to the point where ignition by flying brands is more likely to occur.

The chance of fire spread between an exposing and an exposed building depends on many factors, such as:

- their distance apart;

fire severity, potentially depending on what and how much there is to burn, i.e. on the thermal load. This, in turn, depends on the exposing buildings construction type and use, on its size or, more exactly, the maximum size of any compartment within the building in which fire occurs. Logically, compartmentation within a building increase its effective compartmentation from its neighbours;

fire resistance of the enclosures and, most immediately, the exposing enclosure, including the extent to which its area is perforated by doors, windows (and sometimes roof-lights);

- combustibility of both buildings enclosure surfaces.

In accordance with Romanian regulations, in order to avoid the propagation of fire during an appropriately established period of time, as well as the potential damage to neighbouring buildings in case of collapse of the building on fire, the distances between buildings must not be less than the values given in Table 4.3.

Table 4.3. minimum Distances Between Buildings Required for Fire Safety

Degree of fire resistance of the

building under consideration

Minimum distances(m) with respect to

buildings having the degree of fire resistance

I ,IIIIIIV, V

I , II 6 8 10

III 8 10 12

IV, V 10 12 15

Overall Planning Issues Fire safety-based planning of buildings aims at creating proper condition for:

- evacuation ( escape) of buildings occupants;

- limitation of fire propagation and smoke;

- efficient intervention from outside.

Evacuation of buildings users, cannot be strictly limited to indoor circulation; it is also dependent on building location, having to account for the possibility of people to move away from the building on fire. Limitation of fire propagation and smoke eviction means provision of obstacles inside the building (e.g. walls, floors, water curtains etc.), able to ensure the containment of fire within well defined compartments and proper ventilation systems. It also means avoidance of fire propagation from one building to another, either by obstacles similar to the indoor ones or by provision of safe distances between buildings.

On the other hand, certain correlations are necessary to be established between building function, maximum number of storeys and degree of fire resistance.

Prompt and efficient intervention of emergency services from outside implies easy access of fire engines and ambulances close to the building set on fire, as well as easy access of fire fighting teams inside the building. they

normally follow functional circulation paths (lobbies, stairs, corridors) that also serve as escape routes for buildings occupants.

a

b

Fig.4.15. Escape Provision by Having Alternative Routes:

a unsafe building;

b much safer building.Fig.4.16. Provision of Alternative Escape Exits from Large Rooms and Spaces

Fig.4.17. Example of Hotel Floor Where Compartimentation Ensures That the Two Stairways Are Truly Alternatives to Escapers And That Any Fire Outbreak Is Confined to Its Floor of Origin

Fig.4.18. Stair Tower Compartmented as Fully Protected Refuge

Fig.4.19. Some Measures Preventing Fire from Bypassing Internal Compartments/Divisions Via the Enclosure The evacuation of smoke and hot gases can be achieved by:

- organised natural draught;

- mechanical draught;

- pressurisation of the protected space with respect to the space on fire;

- combination of the above procedures.

Smoke evacuation by organised natural draught implies communication with the outside, either directly (through air-intake openings and vents) or indirectly (through ducts), so achieved that appropriate air circulation within the protected space is obtained, with subsequent smoke evacuation.

The admission of air can be ensured through:

- faade wall openings;

- doors in the exterior walls of protected rooms;

- pressurisation or efficient aeration of rooms and corridors;

- not enclosed stairways;

- air-intake holes, either linked or not to ducts.

Fig.4.20. Successive Phases of Smoke Spreading and Stratification in a One Storey Building Without Indoor Compartimentation

Fig.4.21. Smoke Spreading and Accumulation in a Large Size Room

Fig.4.22. Smoke Spreading in a Multi-Storey Building

Fig.4.23. Examples of Smoke Evacuation in Single Storey Public Buildings, Through Organised Natural Draught

Fig.4.24. Smoke Evacuation Through Organised Natural Draught Applied for Enclosed Spaces Over 10000 m2, Without Partition Walls

The evacuation of smoke can be provided through:

- faade wall openings, kept either open or closed by automatically triggered

openable devices in case of fire;

- ducts or fireproof shafts;

- automatically triggered hatches, located either at roof level or in the upper third of rooms exterior walls.

Some typical examples for single-storey buildings with public functions are schematically shown in Figs. 4.23. and 4.24. The latter should be applied where the enclosed space to be protected has a built area over 10000m2 and there are no partition walls.

Smoke evacuation by mechanical draught implies mechanical removal of

smoke, together with either natural or mechanical admission of fresh air, so organised that an efficient air circulation within the protected space is obtained, accompanied by elimination of smoke.

Alternatively, mechanical devices can be provided to create pressurisation of the smoke-protected space.

Fig.4.25. Smoke Evacuation from Enclosed Staircases, Without Windows, Through Organised Natural Draughts: hatch provided at roof levels

hatch provided in topmost-storey staircase wall

Fig.4.26. Smoke Evacuation in Multi-Storey Buildings, Through Mechanical Draught:

a mechanical admission of air into staircase and natural evacuation of smoke from adjoining spaces;

b natural admission of air into staircase and mechanical evacuation of smoke from adjoining spaces.

Fig.4.27. Mechanical Smoke Evacuation in High -Rise and Tall Buildings:

a mechanical admission of air into staircase, buffer rooms and

common horizontal circulation plus mechanical evacuation from buffer rooms and common horizontal circulation;

b mechanical admission of air into staircases and buffer rooms, plus mechanical evacuation of smoke from common horizontal circulation.

Additional Provisions Automatic fire detectors and a proper alarm, are by no means essential to fire safety in buildings.

Fire-extinguishing is the most important precaution and, certainly, it is the one that comes first to many peoples minds when thinking of fire safety.

Extinguishers are normally water-filled but modern types contain carbon dioxide for flammable liquids. Hoses can be supplied directly off the mains, possibly pump-assisted to give adequate head. Alternatively, there can be water-storage tanking within the building, either located high enough for gravity feed or, again, pumped.

Sprinkler systems first developed in the 1870s in the USA for protecting high-hazard industrial buildings are increasingly used today, wherever the fire risk to life and property is high enough to justify their considerable expense. They are reliable, highly effective and, in any unsupervised area, have the enormous advantage of coming on automatically.

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