Building services

Table of Contents for
 Basic Civil Engineering

Cover image for Basic Civil EngineeringBasic Civil Engineeringby Satheesh GopiPublished by Pearson Education India, 2009
  1. Cover (less than 1 min)
  2. Title Page (less than 1 min)
  3. Contents (5 mins)
  4. About the Author (less than 1 min)
  5. Preface (1 mins)
  6. Part I: Materials for Construction (less than 1 min)
  7. Part II: Building Construction (less than 1 min)
  8. Part III: Basic Surveying (less than 1 min)
  9. Part IV: Other Major Topics in Civil Engineering (less than 1 min)
  10. Copyright (less than 1 min)

Chapter 24

Building Services

24.1 AIR CONDITIONING

24.1.1 Principles of air conditioning

Air conditioning is the process of treating air so as to control simultaneously its temperature, humidity, purity and distribution to meet the requirements of the conditioned space, such as comfort and health of human beings and other needs of the situation.

24.1.2 Purposes of air conditioning

  1. To improve the quality of the products in industrial processes such as artificial silk and cotton cloth. In the case of other industries, it helps in providing comfortable working conditions for the workers, resulting in the increase of production.
  2. In commercial premises such as theatres, offices, banks, shops and restaurants, air conditioning is done to improve the working atmosphere and maintain comfort within these concerns.
  3. To give comfort to the residents of private buildings. The air-conditioning system in this case serves a small number of persons.
  4. In travel by air, railway, road and water, air conditioning imparts facility and comfort by conditioning the quality of air in aeroplanes, railway coaches, road cars, buses, ships, etc.
The principle involved in air conditioning is:
  1. Sucking the air through a filtering media.
  2. Cooling it (in summer) or heating it (in winter).
  3. Dehumidifying, if it is to be cooled, or humidifying, if it is to be heated.
  4. Forcing it into the rooms for proper circulation.
  5. The used air is collected through an exhaust and mixed with some outside fresh air and sucked again through the filtering medium, thus completing the cycle.

24.1.3 Principle of comfort air conditioning

Comfort feeling is a good indication of healthy atmosphere and it depends upon the temperature, air motion or air velocity and humidity changes for different seasons of the year.
The principle of air conditioning should involve proper control of temperature, humidity and air velocity
  1. Temperature control: Comfortable zone is the temperature range suitable for majority of the people. The comfortable zones are different for summer and winter due to the clothing worn in these two seasons. The effective temperature zone for summer is 20–23°C and for winter is 18–22°C. A temperature of 21–25°C is required for comfort conditions regardless of the outside temperature.
  2. Air velocity control: Air velocity control is also an important factor. The increase in velocity results in the decrease of inside effective temperature below the outside temperature. Therefore, the velocity of air is generally taken as 6–9 m/sec, which is considered as relatively still air.
  3. Humidity control: Dry air imparts great strain for the human body. Due to this reason moisture is added to the heated air (i.e., humidification) in case of winter air conditioning and moisture is extracted from the cooled air (i.e., dehumidification) in case of summer air conditioning. An average value of relative humidity between 40 per cent and 60 per cent is considered desirable. During the summer season 40–50 per cent is comfortable and for winter 50–60 per cent is suggested.

24.1.4 Systems of air conditioning

Depending on the location of air-conditioning equipment, the system of air conditioning is classified as follows:
  1. Central system: In this system, all the equipments pertaining to air conditioning are installed at one focal or central point and then the conditioned air is distributed to all the rooms or enclosures by ducts. This type of system requires less space for installation and the maintenance is also easy. It proves to be economical. Due to the presence of ducts, it requires large space.
  2. Self-contained or unit system: In this system, special portable attractive cabinets which fit in with the decoration of modern rooms are placed inside the room near the ceiling or window. They are self-contained in every respect and conditioned air is formed inside the unit itself. The conditioned air is then directly thrown into the room without the help of any ducts.
  3. Semi-contained or unitary central system: In this system, every room is provided with an air-conditioning unit and the room unit obtains its supply from the central system. Such a system results in the smaller size of ducts. Another form of this system is adopted in which conditioned air may be supplied from a central unit but the heating or cooling may be carried out in the room itself.
  4. Combined system: A combined system may consist of (i) central and self-contained system (ii) central and semi-contained system and (iii) self-contained and semi-contained system
The choice of a particular system of air conditioning depends upon several factors such as the size of structure, method of heating, volume and type of air conditioning unit, period of year for which air conditioning is required and number of rooms to be served.
24.2 FIRE PROTECTION
It should be the objective of every engineer and architect, while planning and designing the buildings, that the structures offer sufficient resistance against fire so as to afford protection to the occupants in the event of fire. This objective is achieved by adequate planning, use of fire-resisting materials and construction techniques and by providing quick and safe means of escape in the building.
The building should be so planned or oriented that the elements of construction or building components can withstand the fire for a given time depending upon the size and use of building and the various compartments should be isolated so as to minimize the spread of fire. Suitable separation is necessary to prevent fire, gases and smoke from spreading rapidly through corridors, staircases, lift shafts, etc. Adequate separation from adjacent buildings should also be planned.
All the structural elements, such as floors, walls, columns and beams, should be made of fire-resisting materials so that life, goods or contents and activities within the building can be protected.
The construction of structural elements, namely walls, floors, columns, lintels, arches, etc., should be made in such a way that they should function at least for the time, which may be sufficient for the occupants to escape safely in times of fire. Escape elements like stairways and staircases, corridors, lobbies and entrances should also be constructed out of fire-resistant materials and be well separated from the rest of the building.
Adequate means of escape are provided for the occupants, to leave the building quickly and safely, in times of outbreak of fire. This objective is attained by providing an exit from within a building by way of definite escape ways, corridors and stairs to a street or an open space or roof of an adjoining building from where access to escape may be found. The desired degree of fire resistance largely depends upon the use of the buildings. In India, the types of building construction and fire zones in a city are classified on the basis of fire-resistance and fire-hazard characteristics, respectively.

24.2.1 Fire-resisting materials

24.2.1.1 Timber

Timber, though itself a combustible material, offers sufficient resistance to fire when used in adequate sizes. Timber also possesses the properties of self-insulation and slow burning. Timber on exposure to fire first gets charred; this charring provides a protective coating to the inner portions of the timber and prevents it from rapid combustion, even if subjected to a temperature up to 500°C. At still higher temperatures, under continued exposure, it is dehydrated giving rise to combustible volatile gases, which readily catch fire. Additional fire resistance is achieved by impregnating timber with large quantities of fire-retarding chemicals, like ammonium phosphate and sulphate, borax and boric acid and zinc chloride, because these chemicals retard the rate of temperature rise during fire. To make a timber structure more fire resistant the following points should be given due consideration:
  1. Instead of using a number of smaller sections for joists and floor beams, thicker sections at a wider spacing should be used.
  2. The number of corners and the area of exposed surface should be reduced to a minimum. All sharp edges should be rounded.
  3. Timber should not be treated with oil paints or varnishes, which are liable to catch fire. Instead of this, timber ceilings and partitions should be treated with asbestos or ferrous oxide paints if needed.
  4. In a multi-floor timber structure, there should be a minimum number of floor openings and no through opening in multi-floor levels should be provided. A through opening spreads the fire in the vertical direction and behaves like a chimney and induces draught.
  5. Adequate fire steps or barriers should be provided in the floors and walls.

24.2.1.2 Stone

The use of stone in a fire-resisting construction should be restricted to a minimum as this material cannot resist the effects of sudden cooling. After becoming hot, if it is cooled, it breaks into pieces. Granite, when subjected to excessive heat, crumbles to sand or cracks and turns to pieces with a series of explosions and disintegration. The use of limestone is not at all desirable as it gets crumbled and ruined (turns to quick lime) under the effects of fire. The compact sandstone has better resistance against fire than limestone as it can stand the exposure to moderate fire without serious cracks.

24.2.1.3 Bricks

First-class bricks are practically fire proof as they can withstand the exposure of fire for a considerable length of time. Being poor conductors of heat the bricks can withstand high temperatures up to 1300°C without causing serious effects. Firebricks are best for use in fire-resisting construction. The degree of fire resistance of bricks depends upon factors like size of bricks, composition of brick clay and method of construction. Though brick has its own structural limitations for use in buildings, brick masonry has been proved to be most suitable for safeguarding the structure against fire hazards.

24.2.1.4 Terra-cotta

Like bricks, it is also a clay product which possesses better fire-resisting qualities than bricks. Being costlier, its use is restricted in the construction of fire-resisting floors.

24.2.1.5 Steel

Steel, although an incombustible material, has a very low fire-resistance value. With increase in temperature, it gets softened and, hence, there is reduction in resistance to the effects of tension and compression. At about 600°C, its yield stress is reduced to only one-third of its value at normal temperatures. When the members made of steel come in contact with water used for extinguishing the fire, they tend to contract, twist or distort and thus the stability of the entire structure is endangered. It has been observed in practise that unprotected steel beams sag and steel columns buckle, resulting in collapse of the structure. It is, therefore, necessary in the fire-resisting characteristics of a structure to protect all the structural steel members with some covering of insulating material. This can be achieved by covering the steel members completely with materials like bricks, burnt clay blocks, terra-cotta or concrete.

24.2.1.6 Wrought iron and cast iron

Wrought iron behaves almost in a similar way as steel when subjected to fire except that it has lesser elasticity and lower strength in compression and tension as compared to steel. Cast iron is rarely used from the fire-resisting point of view in construction, as on sudden cooling it gets contracted and breaks into pieces or fragments. For using cast iron in fire-resistive construction, it should be protected by a suitable covering of bricks, concrete, etc.

24.2.1.7 Aluminium

In some advanced countries, aluminium is being used for reinforcement purposes in multi-storeyed structures because of its light weight and anti-corrosion properties. However, it has a very poor performance as a fire-resisting material and its use (as alloy) should be restricted to those structures which have low fire risks. It is a good conductor of heat and possesses enough tensile strength.

24.2.1.8 Concrete

In general, it is a bad conductor of heat and possesses good fire-resisting characteristics. The actual degree of fire resistance of this material depends upon the nature of aggregates used and its density. In the case of RCC and prestressed construction, it also depends upon the position of steel in concrete. It is found that ordinary concrete, when exposed to fire, gets dehydrated and results in shrinkage cracks. (This happens because on heating aggregates in concrete expand whereas cement shrinks and these two opposite actions lead to the development of cracks.) Coarse aggregates like foamed slag, blast furnace slag, crushed brick, crushed lime-stone and cinder are best suited for concrete from the viewpoint of fire resistance. Aggregates like flint, gravel and granite possess poor fire-resisting characteristics. It has been observed that in the event of average fire the concrete surface gets disintegrated for a depth of about 25 cm because of the fact that the mortar in concrete gets dehydrated by the fire. Hence, in the case of reinforced concrete fire-resistive construction, a cover of sufficient thickness should be provided (cracks generally originate from the reinforcement). RCC structures are considered superior to steel-framed structures since less steel is used and that too are well protected by the mass concrete.

24.2.1.9 Glass

Because of its low thermal conductivity, this material undergoes very small volume changes during expansion or contraction and, hence, is considered as a good fire-resisting material. However, sudden and extreme changes in temperature result in fracture or cracks. When glass is reinforced with steel wire netting, e.g., in wire glass, its fire resistance is considerably increased and its tendency to fracture with sudden changes in temperature gets minimized. Reinforced glass has a higher melting point and, hence, is commonly used for making fire-resisting doors, skylight, windows, etc. in construction work.

24.2.1.10 Asbestos cement

This material, which is formed by combining fibrous mineral with Portland cement, has a great fire-resistive value. Asbestos cement products are largely used for the construction of fire-resistive partitions, roofs, etc. Being poor conductors of heat and incombustible material, the structural members blended with asbestos cement offer great resistance to cracking, swelling or disintegration when subjected to fire.

24.2.2 Causes of fires and their prevention

Fire may not occur under any one of the following conditions:
  1. Absence of a component necessary for combustion.
  2. Improper ratio of combustible material to oxygen for the formation of a combustion system.
  3. Heat source available is insufficient to ignite a combustion system.
  4. Heat source is not available for a sufficient time to ignite a combustion system
It is possible to establish the different causes of fire which occur as a result of various optimum combinations of combustion systems and heat sources necessary for a fire to start. Fire prevention and limitation of fire spread are achieved by taking action with respect to the combustion systems and heat sources so as to avoid conditions under which fires can originate. Proper design and planning of buildings are important in fire safety.

24.2.3 Fire protection of buildings

All the structural components of a building should be constructed in such a way and of such materials that they withstand, as an integral member of the structure, for the period desired according to the type of construction, in the event of fire (i.e., 1–4 hours for type 4 buildings to type 1 buildings).
The load-bearing walls or columns of masonry should be thicker in section to resist fire for a longer time. Bricks are more preferred to stones and all walls should be plastered with fire-resistive mortar. All steel members should be embedded in dense concrete or some other fire-resistant material. There should be sufficient cover for all the embedded steel material (a minimum of 5 cm). RCC floors and jack arch floor with steel joists embedded in concrete are more preferred. The wall openings provided which act as escape passages should be properly protected, otherwise they help in the on spread of fire. So, all doors and windows are to be provided with fire-proof shutters and frames. Thick wooden members and steel shutters offer resistance to a certain extent. In addition, to the internal stair, fire escapes should be provided which in the form of external stairs. These should be straight flight type with a minimum width of 75 cm and 20 cm rise and 15 cm tread. Non-combustible hand rails should be provided for a minimum height of 100 cm. Ramps with gradient not more than 1 in 10 can also be provided. Alarm systems and fire extinguishing systems are also to be provided. Generally, one fire hydrant per 4,000–10,000 m2 area is provided depending on the density of population and the importance of the region.
24.3 VENTILATION

24.3.1 Necessity of ventilation

  1. To prevent an undue concentration of body odours, fumes, dust and other industrial by products.
  2. To prevent an undue concentration of bacteria-carrying particles.
  3. To remove products of combustion, and, in some cases, to remove body heat and the heat liberated by the operation of electrical and mechanical equipment.
  4. To create air movement, so as to remove the vitiated air or replace it by fresh air.
  5. To create healthy living conditions by preventing the undue accumulation of carbon dioxide and moisture and depletion of the oxygen content of the air. For comfortable working conditions, the content of carbon dioxide should be limited to about 0.6 per cent volume (in air).
  6. To maintain conditions suitable to the contents of the space.
  7. To prevent flammable concentration of gas vapour or dust in case of industrial buildings.

24.3.2 Functional requirements of a ventilation system

A ventilation system should meet the following functional requirements.
  1. Rate of supply of fresh air
  2. Air movements or air changes
  3. Temperature of air
  4. Humidity
  5. Purity of air

24.3.2.1 Rate of supply of fresh air

The quantity of fresh air to be supplied to a room depends upon the use of the building to which it is subjected. The rate of supply of fresh air is decided by considering several factors such as the number of occupants, type of work and age of occupants.
Type of buildingMinimum rate of fresh air supply to buildings
(m3 per person per hour)
Assembly halls, canteens, shops, restaurants28
Factories and workshops 
i) work rooms23
Residential buildings 
ii) living rooms3 air changes per hour
iii) kitchens6 air changes per hour
iv) bathrooms6 air changes per hour
v) halls and passages1 air changes per hour
Hospitals 
i) wards3 air changes per hour
ii) theatres10 air changes per hour

24.3.2.2 Air movements (or air changes)

At workplaces, air has to be moved or changed to cause proper ventilation of the space. The minimum and maximum rates of air change per hour are 1 and 60 respectively. If the rate of air change is less than one per hour, then it will not create any effect on the ventilation system. While, on the other hand, if the rate of air change is more than 60 per hour, it may lead to discomfort due to high velocities of air. For effective working of the ventilation system, 5-6 air changes per hour are considered alright. Moreover, the air movements should be uniform and should not allow the formation of pockets of stagnant air at any spot in the room.
In naturally ventilated buildings, cross ventilation is provided to secure air movement, whereas in the case of mechanically ventilated buildings air movement is obtained by either increasing the rate of fresh air supply or by recirculation of a part of air in water. The air movement or rate of air change will depend upon the velocity of incoming fresh air, disposition of inlets, type of activity in the premises, number of occupants, etc. The air movement should be varied both in velocity and direction and this can be achieved by means of fans.

24.3.2.3 Temperature of air

It is desirable that the incoming air for ventilation should be cool in summer and be warm in winter before it enters the room. Whenever the velocity of the incoming air is high, its temperature should not be lower than the room temperature. The usual temperature difference between inside and outside should not be more than 8°C.
The effective temperature should, therefore, be maintained with regard to the comfortable conditions for different seasons of the year. This effective temperature indicates a most suitable temperature to the majority of people, considering the comfort of human body under the probable conditions of humidity and air motion. The value of effective temperature depends upon the type of activity, geographical conditions, amount of heat loss from the body, age of occupants, etc.

24.3.2.4 Humidity

A relative humidity within the range of 30–70 per cent at a working temperature of 21°C is considered desirable and, therefore, should be maintained. When work is required to be done at a higher temperature, low humidity and greater air movements are necessary for removing a greater portion of heat from the body. The value of relative humidity can be obtained by comparing dry bulb and wet bulb temperatures.
Any water vapour within a given space or volume is known as humidity and its ratio with the water vapour it had contained had it been saturated is known as relative humidity. The relative humidity depends only upon the vapour pressure of water vapour and dry bulb temperature. The relative humidity of saturated air is 100 per cent.

24.3.2.5 Purity of air

The purity of air plays a significant role in the comfort of people affected by a ventilation system. Hence, it is essential that the ventilating air should be free from any impurities. To get pure ventilated air, the entry point of the ventilation system should not be situated in the neighbourhood of chimneys, latrines, urinals, etc.

24.3.3 Systems of ventilation

The systems of ventilation are basically divided into the following two categories:
  1. Natural ventilation or aeration
  2. Mechanical ventilation or artificial ventilation

24.3.3.1 Natural ventilation

In this system of ventilation, the outside air is supplied into a building through windows, doors, ventilators and other openings due to the wind outside and convention effects arising from temperature or vapour pressure differences or both between the inside and the outside of the building.
This system of ventilation may be developed where precise control over the air conditions and the rate of air changes are not required. Natural ventilation is usually considered suitable for houses and flats (i.e., small buildings) and it cannot be adopted for big offices, assembly halls, theatres, auditoriums, large factory workshops, etc. This system is very economical and the desired ventilation can be achieved by providing sufficient windows and other openings which open to the external air. An opening area equal to not less than one-twentieth of the floor area of the room should be provided in view of proper ventilation. The top of this opening area should be not more than 45 cm below the ceiling.
The rate of ventilation by natural means through doors, windows and other openings depends upon the following effects.

Wind effect (or wind action)

In this, ventilation is affected by the direction and velocity of wind outside and the sizes and position of the openings. Wind creates pressure differences and when it blows against a building a positive pressure is created on the windward side and leeward side. Suction will occur on the other side and the wind will blow from the windward side to the other side if there is an opening.
When wind blows at right angles to one of the rectangular faces of the building or an exposed site, a positive pressure is produced on the windward face and a negative pressure on the leeward face. If the wind direction is at 45° to one of the faces, positive pressure will be produced on the two windward faces and negative pressure on the two leeward faces.
The rate, at which the air change or airflow will occur, depends upon the pressure difference between the inside and the outside. The greater the wind speed the greater will be the pressure difference and sometimes the air changes can occur quickly.

Stack effect

In this, the ventilation rate is affected by the convection effects arising from temperature or vapour pressure difference or both, between the inside and outside of the room, and the difference of height between the outlet and inlet openings. If the air temperature inside is higher than that of outside, the warmer air tries to rise and pass through the opening in the upper part of the building.
At the same time, the incoming cooler air from outside through the opening at the lower elevation replaces it. The rate of air flow, in addition to the temperature or pressure difference and height difference, also depends upon the ratio between the areas of the two openings.

General considerations and rules for natural ventilation

The following considerations and rules should be followed for promoting natural ventilation in buildings:
  1. Inlet openings in the buildings should be well distributed and should be located on the windward side at a low level. Outlet openings should be located on the leeward side near the ceiling in the side walls and in the roofs.
  2. Inlet and outlet openings should preferably be of equal size for greatest air flow, but when the outlet is in the form of a roof opening the inlet should be larger in size.
  3. Where the wind direction is variable, openings should be provided in all walls with suitable means of closing them.
  4. Inlet openings should not be obstructed by adjoining buildings, trees, signboards, partitions or other obstructions in the path of air flow.
  5. Increased height of the room gives better ventilation due to stack effect.
  6. The long narrow rooms should be ventilated by providing suitable openings in short sides.
  7. The rate of air change in the room mainly depends on the design of the opening location of the inlet and outlet and the difference in temperature between the inside and outside air. Generally, the outside air is cooler than the inside air. Hence, the cooler air enters from the bottom and after becoming hot during its stay in the room leaves from the top. It would, therefore, be advantageous to provide ventilators as close to the ceilings as possible.
  8. The efficiency of roof ventilators depends on their location, wind direction and the height of the building.
  9. It is found that the ventilation through windows can be improved by using them in combination with a radiator, deflector and exhaust duct.
  10. For cross ventilation, the position of outlets should be just opposite to inlets. The openings over the doors of back walls create good conditions for cross ventilation.
  11. Windows of living rooms should either open directly to an open space or the open space created in buildings by providing adequate courtyards.
  12. If the room is to be used for burning gas or fuel, enough quantity of air should be supplied by natural ventilation for meeting the demands of burning as well as for ventilation of the room.

24.3.3.2 Mechanical or artificial ventilation

In this system of ventilation, outside air is supplied into a building either by positive ventilation or by infiltration by reduction of pressure inside due to exhaust of air, or by a combination of positive ventilation and exhaust of air. The supply of outside air by means of a mechanical device such as a fan is termed as ‘positive ventilation’, whereas the removal of air and its disposal outside by such a device is termed as ‘exhaust of air’. For positive ventilation, centrally located supply fans of centrifugal type, and for exhaust of air, wall- or roof-located exhaust fans of propeller type are normally used. So, this ventilation involves the use of some mechanical arrangement for providing enough ventilation to the room.
Mechanical ventilation is recommended in all the cases where a satisfactory standard of ventilation in respect of air quantity, quality or controllability cannot be obtained by natural means. A mechanical system is capable of meeting the requirements of air quantity and quality (of air) regarding humidity, temperature, etc and produces the comfortable conditions at all times during the year. Though this system is comparatively costly, it results in the considerable increase in the efficiency of the persons under the command of this system. This system is adopted for big offices, banks, assembly halls, auditoriums, theatres, large factories, workshops, places of entertainment, etc. This system may be regarded as generally desirable in all rooms occupied by more than 50 persons, where the space per occupant is less than 3 cu.m.
The following methods of mechanical or artificial ventilation are in common use.
  1. Extract or exhaust systems
  2. Supply or plenum systems
  3. Combination of exhaust and supply systems or balanced systems
  4. Air conditioning

Extract or exhaust systems

In this system, a partial vacuum is created in the inside of the room by exhausting or removing the vitiated inside air by means of propeller type fans. The extraction of air from inside permits the fresh air to flow from outside to inside and thus it becomes possible to provide fresh air to flow from outside into the room through doors and windows.
These fans for exhaust are installed at suitable places in the outside walls or roofs and they are further connected to different rooms through a system of duct-work.
These exhaust systems are best confined to situations where it is essential to create an air flow towards the ventilated rooms, such as in kitchens, lavatories and industrial plants. This system is useful for removing smoke, odours, fumes, dust, etc from the above-mentioned rooms. In this system, the ducts are placed near the place of formation of smoke, fumes, odours, dust, etc.

Supply or plenum systems

As the name implies, in this system the space is filled with air by means of fans, but no special provision is made to remove it. In plenum ventilation, the air inlet is selected in the side of the building where the air is purest. In this opening, screens or filters may be fixed and fine stream of water may be impugned in the path of the incoming air. The disinfection of incoming air is achieved by adding ozone at the point of inlet. Thus, by this system of mechanical ventilation, it is possible to control the quality, humidity and temperature of incoming air.
Ventilation by plenum process may be downward or upward. In downward ventilation, the incoming air is allowed to enter at the ceiling height and is taken out through outlets situated at the floor level. In upward ventilation, the fresh air is allowed to enter at the floor level and the outlet is provided at the ceiling height.
These ventilation systems are costly and are used for factories, big offices, theatres, etc. and also for supplying air to the air-conditioned buildings.

Combination of exhaust and supply systems or balanced systems

This balanced system is a combination of the above-said two systems and makes use of fans to supply and extract air (i.e., input fans and exhaust fans). This system enables full control over the air movement and conditions to be obtained and should be used where accurate performance is desired. In most buildings, it is desirable to extract only 75 per cent of the quantity of air supplied so that positive pressure is maintained within the rooms. This is essential to prevent the entry of hot air when the doors are opened and also to prevent the infiltration of dust and air-borne contaminants. Moreover, the recirculation of air is possible in this system.

Air conditioning

This is the most effective system of artificial ventilation, in which provision is kept for humidifying or dehumidifying, heating or cooling, filtrations, etc. of the air to meet the possible requirements.
24.4 LIFTS AND ESCALATORS

24.4.1 Elevators or lifts

Elevators are used in buildings having more than four storeys. They are used for providing vertical transportation of passengers or freight. They can be either electric traction elevators or hydraulic elevators. Electric traction elevators are used exclusively in tall buildings. Hydraulic elevators are generally used for low-rise freight service which rises up to about six storeys. Hydraulic elevators may also be used for low-rise passenger service.
The different components of an electric traction elevator are the car or cab, hoist wire ropes, driving machine control equipment, counterweight, hoistway rails, penthouse and pit. The car is the load carrying element of the elevator and a cage of light metal supported on a structural frame, to the top of which the wire ropes are attached. The ropes raise and lower the car in the shaft. They pass over a grooved, motor-driven sheave and are fastened to the counter weights. The paths of both the counter weights and the car are controlled by separate sets of T-shaped guide rails. The control and operating machinery may be located in a penthouse above the shaft or in the basement. Safety springs or buffers are placed in the pit, to bring the car or counterweight to a safe stop. For elevators serving more than three floors, means should be provided for venting smoke and hot gases from the hoist ways to the outer air in case of fire. Vents may be located in the enclosure just below the uppermost floor, with direct openings to the outside or with non-combustible duct connections to the outside. Vent area should be at least 35 per cent of the hoistway cross-sectional area.

24.4.1.1 Design considerations

The key considerations which affect elevator system design are:
  1. Number of floors to be served
  2. Floor-to-floor distance
  3. Population of each floor
  4. Location of building
  5. Specialist services within the building
  6. Type of building occupancy
  7. Maximum peak demand in passenger per 5-minute period

24.4.1.2 Design parameters

There are numerous parameters which can be used to judge elevator system performance. The principal one is based on quality of service and quantity of service.
The ‘quality of service (or interval)’ is related fundamentally to the time interval a passenger has to wait. It can also be said as the expected interval (in seconds) between the arrivals of elevators in the main floor. For a large building, the quality of service can be categorized as
  1. average interval 20–25 seconds – excellent
  2. average interval 35–40 seconds – fair
  3. average interval 45 seconds – poor
The ‘quantity of service (or handling capacity)’ of a system is expressed in the elevator industry design terms as a function of expected building population. A large building with single tenancy usually provides heavier peak flows than those with multiple tenancy.
The following handling capacity should be used as a basis for design to meet up morning peak. Single tenancy – 15–25 per cent of the total building population entering in a 5-minute period. Multiple tenancy – 10–15 per cent of the total building population entering in a 5-minute period.

24.4.1.3 Location of elevators

The most efficient method of locating elevators to serve an individual building is to group them together.
A group has a lower average interval between car arrivals than a single elevator.
Groups should be located:
  1. For easy access to and from the main building entrance.
  2. Centrally for general ease of passenger journey.
If a building has areas which give long distance to the central group elevator, then it may be efficient to provide an additional elevator for local areas.

24.4.2 Ramps

They are sloping surfaces used to provide an easy connection between the floors. They are especially useful when large number of people or vehicles have to be moved from floor to floor. They are usually provided at places such as garages, railway stations, stadiums, town hall, office buildings and exhibition halls. Sometimes, they are provided in special-purpose buildings such as schools for physically handicapped children. They should be constructed with a non-slippery surface.
Ramps are generally given a slope of 15 per cent. But a slope of 10 per cent is usually preferred. The space required for ramps is more. The ramp need not be straight for the whole distance. It can be curved, zigzagged or spiralled. Ramps and landings should be designed for a live load of at least 21.2 kg/cm2. Minimum width of pedestrian ramps is 75 cm for heights between landings not exceeding 3.6 m. Landings should be at least as wide as the ramps. Powered ramps, or moving walks, carrying standing passengers may operate on slopes up to 8° at speeds up to 60 m/min and/or slopes up to 15° at speeds up to 47 m/min.

24.4.3 Escalators

These are powered stairs. They are used when it is necessary to move large number of people from floor to floor. These stairs have continuous operation without the need of operators. These escalators are in the form of an inclined bridge spanning between the floors. The components of an escalator are a steel trussed framework, hand rails and an endless belt with steps. At the upper ends of an escalator are a pair of motor-driven sprocket wheels and worm gear driving machine. At the lower end is a matching pair of sprocket wheels. Two precision-made roller chains travel over the sprockets pulling the endless belt of steps. Escalators are reversible in direction. They are generally operated at a speed of 30 or 40 m/min. Slope of stairs is standardized at 30°. For a given speed of travel, the width of steps determines the capacity of the powered stairs.
Escalators should be installed where traffic is heaviest and where it is convenient for passengers. In the design of a new building, adequate space should be allotted for powered stairs. Structural framing should be made adequately to support them.
Escalators are generally installed in pairs. One of them is used for carrying upgoing traffic and the other for traffic moving down. The arrangement of escalators in each storey can be either parallel or criss cross. Criss-cross arrangement is more compact. It reduces walking distance between stairs at various floors to a minimum. This is why criss-cross arrangement is preferred over parallel arrangement.
Number of floorsSpeed
4-5
0.5–0.75 m/s
6–12
0.75–1.5 m/s
13–20
above 1.5 m/s
REVIEW QUESTIONS
  1. What are the purposes of air conditioning a building?
  2. What are the principles of comfort air conditioning?
  3. What are the systems of air conditioning?
  4. What is the necessity of fire protection of a building?
  5. Briefly discuss fire-resisting materials.
  6. What are the main causes of fire and how are they prevented?
  7. What are the necessities of ventilation?
  8. What are the functional requirements of a ventilation system?
  9. What are the general considerations and rules for natural ventilation?
  10. What are the functional requirements of a lift in a building?
  11. What is the difference between a lift and an escalator?

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