Evolution of fire: differs, is related to: shape and size of the room; thermal load (density and concentration of thermal load); exterior openings; nature and position of inflammable materials; fire source; room position; ventilation, etc.Fig.1 Fire evolution

2. Real fire / normalized (standard) fire:

To understand the fire behavior of buildings is to understand: - fire development; - spreading mechanisms; - physical and chemical transformations in the environment. The standard temperature-time curve depicts the increase of temperature during the ignition time.The use of this standards graph allows to estimate the fire duration, the maximal temperatures during the fire action, as well as the released energy during the fire.Conventionally, the fire development in a room has three stages: initial stage, combustion stage or integral fire development and extinction stage:- Initial stage (a-b), lasts aproximatively for 20-30 minutes; the temperature is relatively small. The fire starts with the ignition of a single item, it can be smothered, or it can grow into an advanced fire.

Fig. 2 Temperature time curve

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- Combustion stage (b-c), can last 10-30 minutes or more. The temperature grows fast, attaining its maximal value in point c. The heat generated by flames and hot gas is transmited, by the means of convection and radiation, to the flammable materials surrounding the fire source, producing their ignition. The fire spreads in stages, until it engulfs the entire fire compartment. A very dangerous occurence in this stage is the breaking or melting of windows, increasing the ventilation area and, as such, the air intake, and allowing the fire transfer to other, yet undamaged, parts of the building. This stage, of full fire development, is marked by a generalized ignition, defined by an instantaneous spread of flames on all combustible surfaces from the room.

- Extinction stage (c-d) is the stage when the fire decreases in intensity, due to the lack of oxygen (in airtight environments), or the depletion of combustible materials inside the fire compartment, or or due to human intervention.The temperature after the fire extinction decreases over a long period of time, and the heat continues to affect the structure of the building, even leading to its collapse.

For the evaluation of the fire resistance of various structures and parts, a normalized temperature time curve was determined through various tests and calculations corresponding to a normalized / standard fire. This temperature-time graph (ISO-834) varies according to the conventional relation: f - 0 = 345 lg(8 t +1)where: f fire temperature at time t, in C; 0 initial fire compartment temperature, in C; t time, expressed in minutes, measured from the starting of fire.The ISO-834 standard curve conventionally replicates the actual fire temperature increase.For , very important is The concept of fire aggressiveness (destructive potential) is another tool for evaluating the fire behavior of buildings or parts of buildings which, correlated with the requirement and performance criteria that apply to a given situation, can provide a corect evaluation of the required fire resistance of a building and its parts. The destructive potential of a fire also relates to the severity of the fire. The severity of fire can be assessed with: the expected fire development, the presence of combustible materials, the division of fire compartments, the presence and abundance of the air intake.

This approach consideres the temperature of burnt gas in the room as an indicator of the destructive potential of fire, while the room boundaries are considered passive, solely reacting to the extrinsic destructive conditions and not influencing them. Fig. 3 ISO Standardized temperature time curve

3. Factors that influence the development and the destructive potential of a fire :

Fires vary in intensity and duration in relation to specific factors. For example, the severity and duration of a fire are related to the caloric power of the combustible material, on the oxygen flux: if the air intake is limited in a fairly well closed space, the temperature wont grow extensively, but the duration of the fire will be longer instead; in the case of broken or molten windows the temperature grows fast, attaining high values, but the duration of fire will be short (because of the fast alteration of all combustible materials).

a) The influence of the thermal load: The thermal load is the ratio between the global caloric power of the combustible materials and the surface of the fire compartment. It encompasses all combustible elements from the building, including finishes, furniture and, in some cases, stored combustible substances.When the thermal load is not uniformly distributed in the room, the concentration of combustible materials will be considered separately, taking the proximity of any structural parts into account, as it increases the danger of building collapse in case of fire. Fire duration depends directly on the thermal load. The collaps hazard increases proportionally to the fire duration and time of cooling down.

b) The influence of ventilation:The air intake is the quantity of the incoming air per time unit. It influences not only the temperature inside the fire compartment, but also the burning rate of combustible materials, being one of the main parameters which characterize a real fire. Experience shows that the oxygen flux during a fire depends on openings, as well as on the hot gas evacutaion rate; when the openings dont allow for the evacuation of hot gas from burning, the combustion remains slow and in certain cases can be stopped.

c) The influence of the thermal inertia of the surrounding surfaces: The area between temperature-time curve and the horizontal axis of the graph can be interpreted as a measure of the fire destructive potential. Thus, if for two fires, the surfaces under the curves dont reach over a basic limit, both fires have the same severity, even if the temperatures of hot gas are different. The temperature of hot gas is not the main parameter of the fire destructive potential. The temperature inside a room during a fire is the result of a significant and complex interaction between the generated hot gas and the surrounding surfaces of the room. Thus, the destructive potential of a fire is defined by its influence on these surfaces, and is assessed through the normalized thermal load, which considers the reaction of the rooms surrounding surfaces.

Fig. 4. Fire severity

The thermal load is the total amount of heat received by the rooms surrounding surfaces, measured per surface unit.

The normalized thermal load is obtained by reference to the thermal inertia of the rooms surrounding surfaces.

A fire with high thermal load in a room having surrounding surfaces with a high thermal inertia equals in severity to a fire with a lower thermal load, in a room with a low thermal inertia.Two fires can be assumed to have the same destructive potential when the areas between the time-temperature curves and the abscissa are equal, regardless of the temperature of the hot gas.

d) The influence of the distribution of the thermal load:The fact that the thermal solicitation is much higher if the thermal load is concentrated than if it is uniform distributed in a fire division has been determined experimentally.Recent studies and experiments emphasise the main factors which influence the destructive potential of fire.

Fig. 5 Factors that influence the destructive potential of fire

The fire severity depends also on the nature of the combustible: charring (cellulose based) or non-charring materials (most plastics). The calculus for cellulose combustible materials proved that the destructive potential of fire (expressed by the normalized thermal load) can increase slower (than liniar) with the fire load, can decrease while the room is ventilated, and, also, if the thermal inertia of the rooms surfaces is increasing.

A standard fire test represents a fire simulation for a room, following one temperature-time curve. The property of rooms surrounding surfaces to resist the destructive potential of fire is assesed by standard fire tests using samples of the materials in the room. The normalized thermal load for each sample relates only to the testing duration.

4. Fire safety goals

Protection of building occupants against fire effects:- The notification of emergency services in short time after fire starts;- Protection of emergency exits;- Provide areas of refuge where is necessary.Fire prevention or delay of fire development and spread: - Check out the fire properties of combustible materials;- The use of an adequate fire compartment;- The use of automatic fire extinction systems.

The minimization of fire impact:- fire compartments, according to function or maximum surface;- building structural integrity;- continuity of activity in common areas of building.

Support for extinction activity (of firefighters) - Identification of fire location; - Good transit flow towards refuge areas - Access, control, communication and water resources for firefighters.

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