Introduction:
With the rapid economic development and urbanization, more and more high-rise residential buildings are developing, especially in densely populated areas. High-rise buildings have unique challenges, such as long evacuation time and distance, smoke movement and control, fire department accessibility, etc. In recent years, more residential building fires were reported prompting concerns over fire safety in this type of buildings. In some of residential buildings, large amounts of combustibles are stored in units of small area. With the adoption of some design features, like green building and open kitchen, new fire risks might be posed.
Identifying design fires is one of the most critical tasks in a performance-based fire safety design. There are many methods used to characterize design fires. An appropriate and relevant design fire representative of the building considered, heat release rate for a design fire as the single most important variable in fire hazard . It provides information on heat output of the fire varying with time. Once the heat release rate is known, the fire resulting environment can be calculated as time progresses. Then when the untenable conditions are to reach, damage to the structure can be estimated. Characteristics of combustibles, ventilation openings, compartment geometry, material properties of enclosure boundary and environmental factors are important factors in determining how severe a fire is. In order to characterize fires, fire risk analysis, information on fire load is very important. Fire load survey is the direct way to collect associated information which can provide a basis for the design fire. There are many surveys made for office buildings & other buildings but due to privacy issues, it is more difficult to make a fire load survey in residential buildings than in other buildings. Since heat release rate for design fires is closely related with the corresponding fire load data, the heat release rate for design fires and fire load survey techniques for high-rise residential buildings are important.
Design fires:
The design fire involves things like heat output, smoke production and toxic gases generation of a fire. Information of toxic gases is of great importance in fire hazard assessment, while due to the complexity of combustion process it is difficult to quantify the type and amount of gaseous products generated in real fire. With the aid of oxygen consumption principle, heat release rate can be measured in full-scale or bench-scale tests. Heat released varying with time becomes the important representation for a design fire. The process of compartment fires is usually divided into several stages: ignition, growth, flashover, fully developed fire, decay. Taking flashover as the demarcation point, the fire can also be divided into pre-flashover stage and post-flashover stage. The development of a fire depends upon a range of variables, such as the ignition source, properties and arrangement of fire loads, the size and location of compartment openings, and the material properties of enclosure boundary. Admittedly, actions of occupants and fire protection features installed affect the size and rate of development of a fire. There are many ways to develop a design fire. A steady state fire is the simplest way to specify a design fire which is based on the expected largest size of a fire under a certain scenario. In some cases, an arbitrarily given value is used as the constant heat release rate. If this value is much less than the actual value in real situation, it can be very dangerous. Time-dependent design fire is closer to a real fire. There are many mathematical models of this kind of fires. One that is widely known is the T-square fire for growth stage which is based on extensive experimental data. It consists of fires of ultrafast, fast, medium and slow in terms of growth coefficient according to the time reaching 1 MW. The standard fire curves as ISO834 and parametric design fire curve as that specified in Euro code 1 Part 2 are fully developed fires which are usually used to evaluate the fire performance of building structure elements.
Heat release rate
It gives useful information such as the fire size, rate of smoke production, the possible fire environment, and other relevant data for hazard assessment. The main factors that control the fire development at growth stage and fully developed stage are different.
- Heat release rate in the growth stage
In the fire growth stage, there is sufficient oxygen for combustion and the fire is so-called fuel-controlled. The characteristics, distribution and arrangement of the fuel dominate the heat release by the fire. After initiation of a fire, whether the surrounding items will be ignited depends on the radiation heat flux received by the exposed items and how easily they can be ignited. When the fire load is close to each other, the fire may become intense more quickly; otherwise it may self-extinguish due to burn up of the fire source. Types of combustible materials will typically be of primary concern. Heavy wooden furniture might be difficult to ignite, but once ignited, it lasts a relatively long time. On the other hand, a vertically hanging curtain can be ignited with ease and pose a rapid flame spread, burning very shortly. There are many experimental data for heat release rate of single items. The principle of superposition is sometimes used to combine heat release rate of combustibles. The ignition temperature or critical heat flux are usually taken as ignition criterion.
- Heat release rate for fully-developed fire:
After flashover, all the exposed surfaces of combustibles in the compartment are involved in the fire. Excessive combustible gases are emitted and there is not enough air for combustion. Therefore, the fire becomes ventilation controlled. During post-flashover stage, the concern is on the structural stability. The time-temperature curves are used to estimate the fire resistant performance of structural elements. The energy release rate of the fire can be roughly determined by the air inflow from the openings. It is important to note that in tall buildings, the wind effect and stack effect might have substantial effect on the heat release of the fire, especially for floors at a high level.
Few fire load surveys:
An appropriate and rational design fire should be based on the fire load of the buildings of consideration and characteristics of the fire load depend largely on occupancy and building use. Fire load, expressed in MJ, is defined as the total energy released by combustion of all combustible materials in the enclosure. It is customary to divide the fire load into two categories: moveable / content fire load and fixed/ non movable /permanent fire load . Fire load density is the fire load averaged by characteristic area of the compartment. Normally, it is expressed as the fire load per unit floor area. As per data available, the mean fire load varied from 278 MJ/m2 to 852 MJ/m2. It is noteworthy that they found that the mean and maximum fire load decreased as the room floor area increased and the fire load had no direct relation with the height of the building. The mean fire load density was 600 MJ/m2 with a standard deviation of 200 MJ/m2. The average fire load densities were found to be 807 MJ/m2 for kitchens; 393 MJ/m2 for dining rooms; 288 MJ/m2 for basement living rooms; 534 MJ/m2 for Primary bedrooms; 594 MJ/m2 for secondary bedrooms.
Methodology of fire load survey:
Several methods were employed in fire load surveys in buildings. These are weighing, inventory, combination of weighing and inventory, questionnaires, and website review. The weighing and inventory methods were most often used by different surveyors. The NFPA Standard 557 also proposes that a fire load survey can be conducted by either the weighing or the inventory technique or a combination of them. Each survey method has its own advantages and disadvantages.
1) The weighing technique:
Weighing is the most direct and simply way to obtain the mass of items. This method needs surveyors to physically enter the building to weigh the mass of combustible objects. It is easy to weigh small items, such as books, chairs. Large items like heavy furniture are difficult to weigh. Besides, this technique is applicable to movable fire load, while it is impractical for fixed combustibles. Some items consist of not only combustible materials but also non-combustible materials. Then, the proportion of the combustible part should be determined.
2) The inventory technique:
In the inventory technique, there are two ways to obtain the mass of an item. One way is to measure the dimensions of the object and then the mass is determined by multiplying the measured volume with the corresponding density. The other way is to weigh the commonly encountered items in advance or get their weight from the manufacturers and then create an inventory list. Thus, the mass of surveyed items will be estimated based on the inventory list. This technique can be used for items that cannot be weighed. The problem is that sometimes it is difficult to determine the volume of irregular shaped items and the density of various types of materials. Error will arise when the surveyed item is mismatched with the ones from the inventory list.
3) The questionnaire technique
A printed paper questionnaire or an electronic one is designed first consisting of questions to be answered by occupants. The questionnaires will then be distributed by traditional methods or through the Internet. Occupants’ participation might be very poor . Whether a satisfactory survey result can be achieved depends largely on the design of the questionnaire and occupants’ understanding of and commitment to it.
4) Website review technique:
Fire load survey can be made through website information. Sale information listed on the real estate websites usually offer the floor plan of the house or apartment with dimensions of the major rooms. Digital photographs of various rooms provided can give a general picture of typical furnishings and their layouts in a particular building category. In this technique, fire load is estimated according to these photographs. The application of this method is limited because it is not easy to find a real estate website providing enough information you want.
And these photographs usually cannot show all the combustible items in a room, for example, contents in cabinets or things hidden by others and there is a big uncertainty in estimating the mass and calorific value of these items just through the photos. The surveyor’s judgment and experience will have great impact on the result.
5) Fire load survey methods recommended for residential buildings:
For residential buildings, occupants are usually reluctant to allow strangers into their personal space, not to mention weigh the content at their home item by item. In view of this, questionnaire might be a good choice. If the samples size is large enough and the questionnaire is designed carefully, the disadvantage of questionnaire technique might be compensated. But the later data processing work may be troublesome.
The content of fire load survey:
Till now, there is no consensus on what data should be collected in a survey. Though many surveys have been conducted on different building categories, the range of the survey vary greatly. Some, only reported the mass of various combustible items present and the corresponding floor area, while some also reported on the window size and composition of the fire load. Besides, surface area of fire load or percentage of floor area covered by furniture was included in a few surveys. Since the design fire is affected by many factors, it is better to get more information about the combustion on a practical basis. It is suggested that the following basic information should be surveyed:
1) Information about the building surveyed should include, but not limited to, the height and layout of the residential unit, and the use and geometry of each room.
2) The location and size of openings. When the fire becomes ventilation-controlled, the size and location of the openings play an important role in determining the heat release rate of a fire. Openings include windows that may be broken in a fire and doors that might be left open. In high-rise buildings, the orientation of the window is of special importance due to expected strong wind.
3) The fire load
Both movable fire load and fixed fire load need to be recorded. Information collected for combustible items include, but not limited to the type, quantities, and the proportion that are combustible.
4) Fire protection system installed
Fire detection and alarm system, fire suppression system and other fire protection systems will intervene with the process of a fire. Therefore, systems installed and their configurations should be recorded. The survey data should be analyzed carefully. Statistics such as the mean fire load density, standard deviation, and a cumulative probability distribution should be given. The main composition and typical arrangement of the combustibles should also be reported.
Conclusions:
With the number of residential fires increasing in recent years, more attention has been paid to high-rise residential buildings. Though there are many difficulties in conducting fire load surveys in terms of time, labor and organization, it is the more effective way to collect basic information that can help to develop the design fire. Methods suitable for residential buildings and fire load survey content are discussed. Full preparation work should be done to ensure a high quality of survey data. When determining the heat release rate for a fire, the dominant factors in different stages should be considered. Magnitude of wind effect on high-rise building fires and its incorporation into the design fire is also important criteria.
By Mr. Suvek Salankar, BE(Fire) , NESC, Nagpur.