Chpater 29 PROTECTION OF BUILDINGS – Building Construction Materials and Techniques

29

PROTECTION OF BUILDINGS

29.1 INTRODUCTION

Although buildings are constructed based on the space requirement and safety needs, they are also to be protected from natural calamities such as action of termites, dampness, fire, lightening, earthquake, etc. Steps taken during construction itself to protect the buildings from the above calamities the durability and life of the building will be increased.

Termites are commonly found in tropical and sub-tropical countries. These termites are nothing but white ants which live in a colony and destroy textiles, woodwork, paper products, etc. As doors and windows and other cubboards and furnitures are of wood, it is essential to safeguard these from termite. Necessary steps should be taken during construction of the building.

One of the essential requirements of a building is that it should be dry. Dampness in building may occur due to several reasons such as improper design, faulty construction, use of poor quality materials, ground water position, etc. Dampness not only affects the life span of the building but also creates unhygienic conditions to the occupants. Damp prevention is considered to be one of the important items of work in the construction of a building.

Fire protection is another important requirement to ensure the safety of the building. Protection of a building against fire can be attained by the use of special materials and construction techniques which aim at giving adequate resistance to the spread of fire within the building. The building components should be fire resistant, at least for a few hours, so that the occupants can live safely. Also, there should be sufficient appliances available to stop fire from spreading.

Tall buildings and buildings constructed in open areas are to be protected from lightening failing which the building will be subjected to large quantity of electricity. This may cause electrification of occupants. Hence, necessary lightening-protection system should be provided in the building.

Buildings are also to be protected from the dangerous natural calamity, the earthquake.

Protection from the above calamities is discussed in the following sections.

29.2 ANTI-TERMITE TREATMENT

29.2.1 Termites

Termites, popularly known as white ants, are found in groups in tropical and sub-tropical countries. They are very fast in eating wood and other cellulosic materials as food. They also damage non-cellulosic materials like plastics, leather, etc. The term termite-proofing is used to indicate the treatment which is given to a building. This is done to prevent or control the growth of termite in a building. Termites are of two types, viz., dry wood termites and subterranean termites.

29.2.2 Principles of Termite-Proofing

While making a building termite-proof, the following points have to be observed:

  1. It should be observed that no part of a building is bridged to untreated soil.
  2. If any fill material is found to contain termite colonies, it should be eradicated.
  3. Joint fillers or metal strips may be used to make floor-joints as termite-proof.
  4. Termites can not enter through dense concrete. Hence, foundations should be carried out with good quality concrete or superior quality materials with good workmanship.
  5. The site should be cleared before taking up the construction from all old tree stumps, dead wood, etc.
  6. Superstructure materials should be treated with suitable preservatives so as to prevent termite.
  7. Selection of the method to be adopted for termite-proofing should be carefully selected.
  8. Pre-treatment is cheaper compared to post-treatment and should be resorted to.

29.2.3 Methods of Termite-Proofing

Termite-proofing methods may be grouped under the following two categories:

  1. Soil treatment with chemicals
  2. Structural barriers

1. Soil Treatment with Chemicals

(i) Chemicals for Treatment

Following are the emulsifiable chemicals recommended in BIS code:

  1. Chloropyrifos concentrate 1% by weight
  2. Heptachlor concentrate 0.5% by weight
  3. Chloride concentrate 1.0% by weight

Chloropyrifos has been widely used. They are available with agrochemical agencies which are used to control termites in crops. This chemical with 1% concentration is mixed in water for soil treatment and in kerosene oil for treatment of wood. As the solution is toxic, it should be washed immediately by soap and water, if it comes into contact with part of the body.

(ii) Pre-Construction Treatment

Treatment should be started when foundation trenches and pits are excavated to the size and level. Treatment should not be done when the soil is wet due to rain or sub-soil water. Further, the treated area should not be disturbed. The treatment is performed in soil in five stages as discussed below.

In the first stage, treatment of wall trenches and basement excavation are done. If termite hills are noticed at the site during site clearance chemicals should be sprayed first. All side surfaces of trenches/pits and bottom of wall trenches/pits and basement excavations should be treated to a height of 30 cm from the bottom. The usage of solution should be at the rate of 5 l/m2 of surface area (Fig. 29.1).

In the second stage, treatment of refill in contact with foundation is attended. All the refill earth in the excavation immediately in contact with both sides of the wall footing should be treated (Fig. 29.1) for a distance of 30 cm. Similarly all the four sides of a column should be treated and treatment should be at the rate of 3–5 l per linear metre vertical surface of the wall.

Figure 29.1 Anti-termite treatment of load bearing wall foundation

The third stage is treatment of soil below floors. The earth fill below the floors up to the plinth level has to be treated after the fill has been made. This is performed by making holes 5–7.5 cm deep at 15 cm centres in a grid pattern. The holes are filled with the solution at the rate of 5 l/m2 of treated surface (Fig. 29.1).

The fourth stage consists of treatment of junction of floor and wall. Before laying the sub-grade, channels of 3 cm wide and 3 cm deep are dug along the junctions of floor and wall. Along the channel, holes at 15 cm apart are made and treated at 15 l/m2 of the wall surface and the chemical is allowed to seep through the bottom. The soil is tampered back in position after the operation (Fig. 29.1).

The fifth stage is treatment of soil along external perimeter of the building. After the completion of building, holes are made along the external perimeter at intervals of 15 cm and to a depth of 30 cm. These holes are filled with the chemical emulsion at the rate of 5 l/m length of wall (Fig. 29.1).

Apart from treating the soil, it is also important to treat the expansion joints, soil around the pipes and conduits and wooden surfaces for a complete protection from termite.

2. Structural Barriers

In order to prevent the termites’ entry through walls, impenetratable physical structural barriers may be provided at the plinth level continuously. Such a structural barrier is generally of concrete or sometimes metal. The cement concrete may be from 50 to 75 mm thick. It is recommended to project about 50–75 mm internally and externally. The metallic materials may be sheets of non-corrodible one, such as copper or galvanised iron with a thickness of 0.80 mm.

29.3 DAMP Prevention

Presence of hydroscopic moisture on a surface is called dampness. In general dampness causes unhygienic conditions, affects the health and comfort of inhabitants. Further, it deteriorates the stability of the damped surface. Thus, it is an essential feature in construction to prevent dampness.

29.3.1 Sources of Dampness

Dampness in a building may be caused due to natural causes or structural causes.

1. Natural Causes

Natural causes may be due to:

  1. Penetration of rain
  2. Rise of moisture from ground
  3. Moisture condensation
  4. Drainage condition of the site
  5. Orientation of the building

(i) Penetration of Rain

Rain may penetrate a building through the top of the walls, through the surface of the walls and through the roof. Rain may penetrate the unprotected parapet walls of a building during heavy rains.

(ii) Rising of Moisture from the Ground

The ground or sub-soil on which a building is constructed may give an access to water to enter the structure and cause dampness. Gravel and sandy soil are highly permeable and allow water to pass through them easily. But clay and clayey silt or clayey sand are less permeable but cause dampness due to heavy capillary rise.

(iii) Moisture Condensation

Whenever the warm air in the atmosphere is cooled, the process of condensation takes place. Because of this condensation, moisture is deposited on the areas of walls, floors and ceilings.

(iv) Drainage Conditions of the Site

The elevation of a ground on which a building is constructed has significance on dampness condition. Buildings built on a higher ground can be drained easily and are, hence, less liable for dampness. Low-lying sites can not be drained easily and may cause water logging. The condition will get still worse if impervious soil is present beneath the structure. Hence, it is an essential condition to be looked into before starting any construction.

(v) Orientation of the Building

Depending on the climatic conditions and monsoon walls of buildings may be subjected to constant splash of rain water. Hence, the construction of a building should be accordingly planned.

2. Structural Causes

Structural causes may be due to faulty design of the building, faulty construction, poor workmanship or use of inferior quality material in construction.

(i) Faulty Design of Structure

Improper mix design of concrete not only has less strength but of high permeability causing dampness. Improper mortar proportions may also cause water penetration. Non-provision may also cause water penetration. Non-provision of a damp-proof course in the design and improper plinth-protection measures may cause dampness in the building.

(ii) Faulty Construction of Structure

Improper construction of various parts, for example, fixtures in a building, joints in the roofs, throating of sills and copings, joining of walls, etc., may cause dampness by entry of water.

(iii) Poor Workmanship and Materials

Skilled workers should be used for construction of walls, roofs, floorings, etc., and electrical and plumbing works. Further, good quality materials should be used to get effective construction.

29.3.2 Effects of Dampness

In general, presence of dampness results in poor functional performance, shabby appearance and structural weakness. Following are the effects of dampness:

  1. Presence of damp conditions causes efflorescence on the surfaces leading to disintegration of bricks, stones, tiles, etc., and in reduction of strength.
  2. Plastering may get softened or crumbled due to dampness.
  3. Paints on surfaces may get blistered, bleached and disfigured.
  4. Dampness may cause corrosion of metals used in the construction.
  5. Materials used as floor coverings, such as tiles, marble stones, etc., may be damaged due to loss of adhesion with the floor bases.
  6. Doors and windows and other timber works may get warped due to dampness.
  7. All electrical fittings get deteriorated and liable to cause short circuits.
  8. Dampness promotes the growth of termites and hence deteriorates materials and causes unhygienic conditions.
  9. Dampness added with warmth and darkness leads to breeding of germs which may cause some disease.
  10. Dampness creates an unhygienic working condition for the occupants.

29.3.3 Methods of Damp-proofing

Damp-proofing courses (DPC) of suitable materials are provided at appropriate locations for their effective use (Sharma, 1988). DPC prevents basically the entry of the water from ground in buildings. The best position for DPC is the plinth level in buildings.

Following general principles should be adopted while providing DPC in buildings:

  1. DPC should cover the full thickness of the wall.
  2. Mortar bed on which the DPC is laid should be level and there should not be any projection.
  3. In places where a vertical DPC is provided, it is to be laid continuously with a horizontal DPC and a fillet.
  4. DPC course should be continuous and should form as a bearer from the entry of moisture.
  5. DPC should not be exposed in total.

29.3.4 DPC Treatment in Buildings

Provision of damp proof course at plinth level is shown in Fig. 29.2, whereas Fig. 29.3 shows the DPC in basement.

Figure 29.2 DPC above ground level

Figure 29.3 DPC in basement

Prevention of damp along parapet walls and flat roof are shown in Figs. 29.4 and 29.5.

Figure 29.4 DPC in parapet wall

Figure 29.5 DPC in flat roof

29.3.5 Materials used for DPC

Materials generally used for DPC are flexible materials like, hot bitumen, bituminous felts, bituminous sheets, polythene sheets, metal sheets of lead, copper, etc.; semi-rigid materials like mastic asphalt or combination of materials or layers and rigid materials like first-class bricks, stones, slates in courses and cement-concrete stones, slates in courses and cement-concrete or mortar layers, etc.

29.3.6 Damp-proof Surface Treatment

In this method the surface exposed to moisture is treated by providing a thin film of water-repellent material over the surface. Such a surface treatment may be external or internal. Generally the external treatment is more effective in damp prevention when compared to internal treatment.

Surface treatments include pointing, plastering, painting, distempering, etc., Lime-cement plaster mix (1 cement:1 lime:6 sand proportion) is more effective.

Materials used for surface treatment are sodium or potassium silicates, aluminium or zinc sulphates, barium hydroxide and magnesium sulphate, soft soap, linseed oil, coal tar, soap, bitumen, remix and gums, etc., applied in alternate layers with suitable combination depending on the climatic conditions.

29.3.7 Integral Damp-proofing Treatment

In this process certain compounds are added along with concrete or mortar while mixing which when used in construction act as barriers to moisture penetration.

The added materials function based on different principles. Based on the mechanical principle, materials like chalk, talk, fullers earth, etc., fill in the pores present in the concrete or mortar and make the concrete or mortar and set as a waterproofing agent. Based on the chemical reaction principle, the materials like alkalines, silicates, aluminium sulphates, calcium chloride, etc., react chemically as water-resistant. Based on the repulsion principle, the materials like soaps, petroleum oils, fatty acid compounds such as stearates of calcium, sodium, ammonium etc., which when added with concrete or mortar react with it and become water repellent.

29.4 FIRE PROTECTION

When some materials get ignited, the material catches fire and spreads. If there are opening in walls and floors the fire spreads to more area. If there are no openings, the temperature of the structure is increased by fire. In buildings, staircases and lift shaft act as flues for fire and increases the possibility of spreading of fire.

There are natural and man-made causes for fire to occur. They may be caused due to faulty workmanship in electrical wiring, leakages in heating and cooking equipment, flammable liquids, careless throwing of cigarette bits and matches, lightening, spontaneous combustion, etc.

The fire spreads over different materials and produces different gases of which some are poisonous. The gases produced are carbon monoxide, carbon dioxide, hydrogen sulphide, nitrogen dioxide, etc. Thus to protect the goods and activities within a building or structure and of adjacent buildings fire-protection has to be resorted to. Fire resistance of a material is the time during which a structure fulfils its function with reference to safety when a fire prevents with a particular intensity.

29.4.1 Fire-Resisting Properties of Building Materials

With reference to fire, materials may be classified as combustible materials and non-combustible materials.

Combustible materials are the materials which combine exothermally with oxygen and give rise to flame at a particular range of temperature. Examples of such materials are wood, wooden products, animal products, and man-made products like fibreboard, strawboard, etc.

Non-combustible materials are those which when decomposed by heat will do so endothermically. These materials do not catch fire by or decomposed at a particular range of temperature. Examples of such materials are metal, stone, glass, concrete, clay products, gypsum products, asbestos products, etc.

The building materials have varying fire-resistant properties which are discussed below.

1. Bricks

Bricks in general have good fire-resistant property. Particularly first class bricks are fireproof and can withstand heat for a considerable length of time. As bricks are made out of clay, which is a poor conductor, can withstand heat as high as 1300°C. Special type of bricks called fire-bricks are best for use in fire-resistance constructions. In total, brick masonry is most suitable to withstand fire hazards.

2. Terracotta

Terracotta is also a clay product which has better fire-resisting properties than bricks. As the cost is high, it is used only in restricted places.

3. Stone

Although stone may resist high temperature but deteriorate due to sudden cooling. Thus stone should be used only for a limited use in buildings with reference to fire-resistance. Granite although very strong crumbles or cracks when subjected to heat. Compact sand stone has better fire-resistant capacity. But lime stone is not all desirable as fire-resistant material.

4. Concrete

As concrete is a bad conductor of heat it has high fire-resistance capability. The extent of fire resistance depends on the aggregate, density and position of reinforcement in RCC. Use of foamed slag, blast furnace slag, crushed brick, cinder, crushed limestone, etc., form the best aggregate for fire-resisting concrete.

5. Mortar

Mortar is a cheap and best incombustible material. Cement mortar is better fire-resistant than lime mortar as lime plaster is susceptible by calcination. In order to increase the fire-resistant property, the thickness may be increased. Cement mortar with surki or pozzolana shows very high fire-resistance capability.

6. Asbestos Cement

Asbestos cement is formed by combing fibrous mineral with cement. This material shows a very high fire-resistant property. The products of asbestos cement are widely used for the construction of fire-resistive partitions, roofs, etc. Any structural member formed by combing asbestos cement offer great resistance to fire, less susceptible for cracking, or disintegration at high temperatures.

7. Steel

Steel is incombustible at moderate temperatures but shows very low fire resistance at high temperatures. At high temperatures the yield stress reduces and deforms when quenched with water in the process of extinguishing a fire. That is, all the exposed steel should be protected against fire by covering them with materials like bricks, terracotta, etc.

8. Wrought-Iron and Cast-Iron

These materials have the same behaviour as that of steel. The only difference being that it has less elasticity and retains compression and tension compared to steel. Cast-iron should not be used as a fire-resistant material as it is susceptible for deterioration when subjected to cooling.

9. Aluminium

It is a poor performance material as a fire-resistant and recommended only in places where low fire risks are expected. It is a very good conductor of heat but possesses adequate tensile strength.

10. Glass

Glass is a poor conductor of heat and undergoes a very small compression or expansion. Thus it is a good fire-resistant material. But sudden change in temperature leads to cracks or fracture. But a reinforced glass possesses high melting point and thus recommended for fire-resistant doors, skylight, windows, etc.

11. Timber

In general timber is a combustible material. But it has a special property of self insulation by forming a charred face when exposed to fire which forms a protective cover. Use of timber in large sizes offers a better fire-resistance. Timber may be made fire-resistant by impregnating it with fire-retarding chemicals such as ammonium phosphate and sulphate, boric acid, zinc chloride, etc.

29.4.2 Fire-Load

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:

  1. Low fire-load
  2. Moderate fire-load
  3. 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:

  1. In areas where lightning can occur often.
  2. Buildings located in exposed areas.
  3. 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

29.6.1 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:

  1. Explosion of volcano may take place due to the relax and expansion of gases and lavas.
  2. Faulting may also occur within a volcano and thereby causing high pressures in the chamber of molten rock.
  3. 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).

29.6.2 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.

29.6.3 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.

29.6.4 Effects of Earthquake on Structures

During an earthquake, ground motions occur in a random fashion, both horizontally and vertically, in all directions, radiating from the epicentre. The ground accelerations cause structures to vibrate and induce inertial forces on them. Hence, structures to be constructed in earthquake-prone areas need to be suitably designed and detailed to ensure stability, strength and serviceability.

The magnitude of the forces induced in a structure due to a given ground acceleration will depend, amongst other things, on the mass of the structure, the material and type of construction, the damping, the ductility and energy dissipation. Ductility of a material is the ability of a structure or member to undergo inelastic deformations beyond the initial yield deformation with no decrease in the load resistance.

Thus by enhancing ductility and energy dissipation capacity in structure, the induced seismic forces can be reduced and also the probability of collapse reduced (Pillai and Menon, 2012). Further, it is desirable to avoid discontinuities in mass or stiffness in plan or elevation. Torsional effects should particularly be accounted for in buildings with asymmetry in plan or elevation.

29.6.5 General Requirement

Bureau of Indian Standard have specified the minimum design requirements for earthquake–resistance design in IS codes: 1893 (Part I): 2002, IS: 4326: 1993 and IS: 13920: 1993. These requirements have been stipulated after considering the characteristics and probability of occurrence of earthquakes, the characteristics of the structure and the foundation and the amount of damage that is considered tolerable. Codal provisions from other countries are also available.

The criteria adopted by codes for fixing the level of the design seismic loading are generally as follows:

  1. Structures should be able to resist minor earthquakes without damages.
  2. Structures should be able to resist moderate earthquakes without significant structural damage, and
  3. Structures should be able to resist major earthquakes without collapse, but with some structural and non-structural damages.

29.6.6 Major Design Considerations

1. Objective

Bureau of Indian Standards IS: 13920 : 1993 recommends for special design to ensure adequate toughness and ductility (with ability to undergo large inelastic reversible deformation) for individual members such as beams, columns and walls and their connections and to prevent other non-ductile types of failure.

As a general rule, to maintain overall ductile behaviour of structure with minimal damage, it is necessary to provide the following combinations (Pillai and Menon, 2012):

  1. Strong foundations and weak superstructure.
  2. Members stronger in shear than in flexure.
  3. Strong columns, and beams with little over-strength.

2. Means of Providing Ductility

Some of the main design considerations in providing ductility include:

  1. Using a low tensile steel ratio (with relatively low grade steel) and/or using compression steel.
  2. Providing adequate stirrups to ensure that shear failure does not precede flexural failure.
  3. Confining concrete and compressions steel by closely spaced hoops or spirals, and
  4. Proper detailing with regard to connections, anchorage, splicing, minimum reinforcement, etc.

3. Requirements of Stability and Stiffness

Under a severe earthquake, large lateral deformation and oscillations are induced resulting in formation of reversible plastic hinges at various locations. Thus a structural system should be designed to ensure that the formation of plastic hinges at suitable locations may, at worst condition, result in the failure of the individual element rather than progressive collapse.

Apart from the stability, the structure should have sufficient stiffness to limit the lateral deflection or drift. As per code the inter-storey drift is to be limited to 0.004 times the storey height to account for stiffness.

4. Requirements of Materials

As mentioned earlier use of relatively low grade steel is recommended. Further, lower the grade of steel, higher is the ratio of the ultimate tensile strength ( fu) to the yield strength ( fy). A high ratio of fu/fy is desirable, as it results in an increased length of plastic hinge and thereby an increased plastic rotation capacity.

For all buildings, which are more than three storeys, in height, have to use M20 as a minimum grade of concrete. Low density concrete lead to poor performance under reversed cyclic loading, whereas very high strength concrete is associated with lower ultimate compressive strain which adversely affects ductility.

5. Foundation

It is most important in the design to ensure that the foundation of a structure does not fail before the possible failure of superstructure. The maximum seismic forces transmitted to the foundation shall be governed by the later loads at which actual yielding takes place in the structural elements transforming the later loads to the foundation. Thus to ensure a safe foundation, it has to be ensured the foundation is stronger than the superstructure. Such a design concept is necessary to provide for ductile behaviour of the superstructure without serious damage to the foundation.

6. Flexural Yielding in Frames and Walls

As reinforced concrete is less ductile in compression and shear, dissipation of seismic energy is best achieved by flexural yielding. Thus it is necessary to avoid weakness of structure in compression and shear in relation to flexure.

In a structure composed of ductile movement-resisting frames and/or shear (flexural) walls, the desired inelastic (ductile) response is developed by formation of plastic hinges (flexural yielding) in the members, as shown in Fig. 29.7.

Figure 29.7 Formation of plastic hinges in a ductile structure

In ductile frames, plastic hinges may form in the beams or in the columns (Fig. 29. 7a). It is always desirable to design the frame such that the plastic hinges form only in the beams rather in columns. The reasons for such a condition are as follows:

  1. Plastic hinges in beams have larger rotation capacities than in columns.
  2. Mechanisms involving beam hinges have larger capacity – absorptive capacity on account of the larger number of beam hinges (with large rotation capacities) possible.
  3. Eventual collapse of a beam generally results in a localised failure, whereas collapse of a column may lead to a ‘global’ failure, and
  4. Columns are more difficult to straighten and repair than beams, in the event of residual deformation and damage.

Ductility and strength assessment of an entire structure requires non-linear analysis, considering material and geometric non-linearities.

SALIENT POINTS
  1. Protection of buildings in general have to be made against termite, wetness, fire and lightning.
  2. Termites, popularly known as white ants, are found in groups in tropical and sub-tropical countries.
  3. Methods of termite-proofing are: (i) Soil treatment with chemicals and (ii) Structural barriers.
  4. Chemicals used for termite treatment are: (i) Chloropyrifos concentrate (1% by weight), (ii) Heptachlor concentrate (0.5% by weight) and (iii) Chloride concentrate (10% by weight).
  5. Presence of hydroscopic moisture on a surface is called dampness. In general dampness causes unhygienic conditions.
  6. Natural causes for dampness may be due to (i) penetration of rain, (ii) rise of moisture from ground, (iii) moisture condensation, (iv) drainage condition of the site and (v) orientation of the building.
  7. Structural causes for dampness are: (i) faulty design of structure, (ii) faulty construction of structure and (iii) poor workmanship and materials.
  8. General principles should be adopted while providing DPC in buildings are: (i) DPC should cover the full thickness of the wall, (ii) mortar bed on which the DPC is laid should be level and there should not be any projection, (iii) In places where a vertical DPC is provided it is to be laid continuously with a horizontal DPC and a fillet, (iv) DPC course should be continuous and should form as a bearer from the entry of moisture and (v) DPC should not be exposed in total.
  9. Materials used for DPC are flexible materials like hot bitumen, bituminous felts, bituminous sheets, polythene sheets, metal sheets of lead, copper, etc.; semi-rigid materials like mastic asphalt or combination of materials or layers; and rigid materials like first-class bricks, stones, slates in courses and cement-concrete or mortar layers, etc.
  10. Combustible materials are the materials which combine exothermally with oxygen and give rise to flame at a particular range of temperature.
  11. Non-combustible materials are those which when decomposed by heat will do so endothermically.
  12. The amount of heat liberated in combustion of any content or part of a building of a floor area is referred to as fire-load.
  13. Fire-load is the ratio of the weight of all combustible materials (by their respective calorific values) to the floor area under consideration.
  14. General measure of fire safety to be adopted are: (i) Alarm systems and (ii) Fire-extinguishing arrangements.
  15. Lightning protection should be provided (i) in areas where lightning can occur often, (ii) if building is located in exposed areas and (iii) if height of building is more compared to the surrounding buildings and places.
  16. 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.
  17. Earthquakes are mainly caused due to sudden movement along faults which is due to tectonic origin.
  18. Earthquakes associated with volcanoes are more localised.
  19. Reservoir-associated earthquakes have been identified due to (i) sagging effect of the load and (ii) increased pore pressures.
  20. Magnitude of an earthquake is a measure of the amount of ground shaking based on the amplitude of elastic wave it generates.
  21. Richter’s magnitude scale starts from two and there is no upper limit.
  22. Epicentre is the point on the earth’s surface.
  23. Seismic map of India is based on Mercali’s scale.
REVIEW QUESTIONS
  1. Give the name of chemicals used for anti-termite treatment.
  2. Describe the general principles of termite treatment.
  3. What are the methods adopted for termite-proofing?
  4. What is dampness in buildings?
  5. Discuss the natural causes for dampness. What are their effects?
  6. What are the general principles to be observed in all the methods of damp-proofing?
  7. Explain the requirements of an ideal damp-proofing material.
  8. What are the materials used for damp prevention?
  9. Prepare a list of materials which are commonly used for a damp-proofing course. Briefly explain each of them.
  10. Briefly discuss the methods generally adopted in preventing the defects of dampness.
  11. What is integral damp-proofing treatment?
  12. It is intended to construct a basement of 4 metre height and of 5 m × 8 m in size. There is a likelihood of dampness occurring in the inside of the basement. Explain briefly the various methods of damp prevention in this situation if the walls are to be brick masonry and concrete floor.
  13. What do you understand by fireproof construction?
  14. Discuss fire-resisting properties of different materials.
  15. Define fire-load.
  16. Discuss grading of occupancies by fire-load.
  17. What is fire-grading? Explain.
  18. Bring out the special measures to be adopted for safety against fire in case of theatres.
  19. Enumerate general safety requirements against fire.
  20. Briefly explain the emergency fire safety measures to be adopted.
  21. What steps do you take to protect a building from lightning?
  22. How earthquakes are caused?
  23. Explain reservoir-associated earthquake.
  24. What is Richter scale? How is it measured?
  25. Explain the earthquake zones of India.
  26. Explain the major design consideration to protect buildings from earthquake.