The amount of heat liberated in combustion of any content or part of the building of a floor area is referred to as fire-load. It is represented in kilojoules per square metre (kJ/m2).
The fire-load is the ratio of the weight of all combustible materials (by their respective calorific values) to the floor area under consideration. For example, let a floor area of 120 m2 contain 18 × 103 N of combustible material having calorific value of 1.5 × 103 J/N, then the
The fire-load is used as a measure of grading of occupancies by BIS (BIS 1641–1968). Accordingly the classifications are as follows:
- Low fire-load
- Moderate fire-load
- High fire-load
Table 29.1 shows the classification of occupancies.
Table 29.1 Grading of occupancies by fire-load
29.4.3 BIS Grading
Bureau of Indian Standards (BIS 1641–1968) has graded the structural elements into five grades with respect to ‘time in hours for resisting standard fire’, as shown in Table 29.2.
Table 29.2 Fire resistant grades
National Building Code graded type of construction into four categories as Type 1 to 4 as given in Table 29.3.
Based on the availability of firefighting equipment in the premises or the public fire brigade availability, the duration of fire-load of 2.10 × 106 to 4.60 × 106 is usually considered as less than 3 hours. Hence, all the normal buildings are considered to come under Type 1 construction. Further care should be taken for ventilation and escape of gases.
Table 29.3 Types of construction and hours of resistance
29.4.4 General Safety Requirements Against Fire
All building should satisfy certain safety requirements against fire, smoke and fumes.
1. Maximum Height
The height of a building is restricted depending on the number of storeys, the number of occupancy and the type of construction. Furthermore, all the above factors in turn depend on the width of the road in front of the building, floor area ratio and the local firefighting facility available.
2. Open Space
In general, every room for use by human beings should abutt on an interior or exterior open space or on an open verandah. The open spaces inside or outside should be able to provide sufficient lighting and ventilation. Further, the open space adjoining a road should be well inside giving scope for widening of the road.
3. Mixed Occupancy
When a building is used for more than one type of occupancy, for example, residential, godown, shops, etc., it should conform to the requirements for the most hazardous of the occupancies. Such mixed occupancy should be avoided as there is more risk for life of occupants. If mixed occupancy is separated by walls of 4-hour fire resistance, then the occupancy can be treated individually and safety measures can be taken.
4. Openings in Separating Walls and Floors
The openings in separating walls and floors should be designed in such a way that necessary protection is guaranteed to all such factors which may spread fire. For types 1–3 construction a door way or opening in a separating wall may be limited to about 6 m2 (i.e., height 2.75 m and width 2.1 m). Such wall openings should be provided with fire-resisting doors or steel rolling shutters. All openings in the floors shall be protected by vertical enclosures. In Type 4 construction, openings in the separating walls or floors should be fitted with 2-hour fire-resisting assemblies.
5. Enclosure on all openings
Wherever openings are permitted, they should not exceed three-fourths the area of the wall in the case of external wall and should be protected with fire-resisting assembles or enclosures. Such assembles and enclosures shall also be capable of preventing the spread of human or smokes.
6. Power Installations
Electrical power installations and gas connections for kitchen, if any, should be done as per norms and requirements from the point of view of fire safety.
7. Materials of Construction
The structural elements of the building such as floors, partitions, roofs, walls, etc., should be invariably constructed with fire-resisting materials. In general non-combustible materials like stones, bricks, concrete, metal, glass, clay products, etc., should be used in construction. Combustible materials such as wood and wood products, fibreboards, strawboards, etc., should be avoided or used only for the most essential places.
29.4.5 Emergency Fire Safety Measures
Apart from the steps taken in construction of buildings the following general measures of fire safety have to be adopted.
1. Alarm Systems
Alarm systems are installed with a view to give an alarm and to call for assistance from neighbours in case of fire. As per the saying ‘prevention is better than cure’, the first five minutes of fire should be stopped instead of fighting to extinguish the fire for five hours. Further, safety alarm also gives enough time and warning for the occupants to save important materials and to reach to a safe place.
The alarm system may be manual or automatic. The manual alarm system may consist of a horn bell or siren by which the occupants can be alerted. The automatic alarm system is usually installed in large industrial building which is unoccupied at night. The automatic fire alarm, apart from sending information to the nearest control point, also alerts the nearest fire brigade station.
2. Fire-extinguishing Arrangements
Various types of extinguishing arrangements are provided to extinguish the fire depending on the importance of the building.
(i) Portable Fire Extinguishers
The purpose of portable fire extinguishers is intended for immediate use in case of an outbreak of fire. The portable extinguishers in common use are carbon dioxide type, foam machines, large foam generators, etc. Carbon dioxide type extinguishers are the most common for small fires. Sometimes small fires can be extinguished by keeping buckets of water, sand and asbestos blankets.
(ii) Fire Hydrants
Fire hydrants may be installed inside or outside the building. But they should be located in a suitable position such that water is made available easily. For large and close buildings the fire hydrants should be located 90–120 m apart. For open areas the distance may be 300 m or more. One hydrant for an area of 4000–10000 m2 is provided depending on the population and importance of the region. Generally, hydrants are installed at all street corners.
(iii) Automatic Sprinkler System
This consists of pipes and sprinkles. They are installed in such a way as to operate automatically by the heat of fire and sprinkles water on the fire. This arrangement is suitable for the internal protection of premises. This arrangement is provided in industries which produce combustible materials like textile mills, paper mills, gas industries, etc.
(iv) Escape Routes
Adequate passages to escape in times of emergency have to be made in the building. This is more important in public buildings like theatres, town halls, schools, restaurants, etc. In case of buildings more than 25 m, it is recommended to provide at least one fire tower as the escape route. All escape routes over roofs and strairs should be protected with railings.
29.5 PROTECTION FROM LIGHTNING
Lightning protection should be provided in the following areas:
- In areas where lightning can occur often.
- Buildings located in exposed areas.
- Height of building is more compared to the surrounding buildings and places.
The lightning-protection system consists of an unbroken chain of conductors from the roof of a building to the ground. This provides an easy path for the heavy electrical power released by the lightning to discharge to the earth in the shortest time possible.
The conductor should be pure copper. The conductors should be of shortest length without sharp bends, kinks, etc. The area of influence of a lightning conductor is assumed to be a cone with the top most point of the conductor as the apex and a radius related to the height of the apex. This radius may be taken as equal to the height of the conductor on a safe side.
29.6 EARTHQUAKE-RESISTANT BUILDINGS
Causes of Earthquakes
Earthquakes may be caused by natural reasons or due to man-made activities. Natural causes are tectonic forces or volcanic eruption and man-made activities such as reservoir-associated forces.
1. Tectonic Earthquakes
Earthquakes are mainly caused due to sudden movement along faults which in turn due to tectonic origin. Such earthquakes generally result from sudden yielding to strain produced on the rocks by accumulation of stresses. Because of this the rock break along the weakest plane or otherwise and produces relative displacement of the rocks. Along the fault-planes the movement occurs after overcoming the frictional resistance along the fault-plane. Earthquakes due to fault line failure is an established fact (Parbin Singh, 2012).
2. Volcanic Earthquakes
Earthquakes associated with volcanoes are more localised. Compared to failure along faulting planes, the extent of damage and the intensity of wave produced are generally less. Volcanic earthquakes may be caused due to one of the following mechanisms:
- Explosion of volcano may take place due to the relax and expansion of gases and lavas.
- Faulting may also occur within a volcano and thereby causing high pressures in the chamber of molten rock.
- Centre of volcano may collapse and thereby extrusion of gases and molten matter.
3. Reservoir-associated Earthquakes
Only during the second half of the twentieth century, a new class of earthquake associated with reservoir has been recognised. It is believed to have caused due to impounding of water in artificially created reservoirs. Areas which were region of seismic activity (discussed elsewhere) have shown sign of disaster due to earthquake. Seismic shocks associated with filling of water in reservoirs have also been recorded in different parts of the world.
Reasons for such earthquakes have been identified due to (i) Sagging effect of the load and (ii) Increased pore pressures (Parbin Singh, 2012).
Magnitude of Earthquake
Magnitude of an earthquake is a measure of the amount of ground shaking based on the amplitude of elastic wave it generates. Richter’s magnitude scale, named after Prof. Charles Richter, a geologist is most often used. The Richter scale starts from 2, and there is no upper limit. Table 29.4 gives the description of an earthquake in relation to its magnitude on the Richter scale.
Table 29.4 Magnitude of an earthquake
The Richter scale is a logarithmic one; that is, an earthquake of magnitude 4 causes 10 times as much ground movement as one of magnitude 3, 100 times as much as one of magnitude 2, and so on. The Richter scale is widely used throughout the world.
Seismograph is an instrument designed to record earth motion set up by seismic waves. The actual record of motion produced by a seismograph is called a seismogram. Seismograph is designed to record both the horizontal and vertical component of ground motion.
Seismic Zones of India
Varying geological conditions at different locations of the country may have at any time damaging earthquakes to occur. Thus there is a need for seismic zone map of the country so as to design structures taking into effect the magnitude of earthquake likely to occur at a particular location.
Figure 29.6 Location of epicentre from travel-time records (Source: IS: 1893–1984)
The zone map (IS: 1893–1984) sub-divides India into five zones, I, II, III, IV and V (Fig. 29.6). The corresponding intensity and acceleration are shown in Table 29.5 which is based on Mercali scale. Mercali scale is shown in Table 29.6.
Table 29.5 Intensity of earthquake
Table 29.6 Mercali scale
Seismic zone maps are to be revised periodically with the better understanding gained with time. For instance, the Koyna earthquake classified under Zone I in 1966 was changed to Zone IV in 1970.
Epicentre is the point on the earth’s surface vertically above the focus of an earthquake. Shaking is highest at the epicentre and gradually decreases outwards. The difference in primary waves (P – waves) and secondary waves (S – waves) may be used to determine the epicentre.
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