METHODS OF SITE EXPLORATION.

The various methods of site exploration may be grouped as follows:

1. Open excavations. Trial pits are tho cheapest method of exploration in shallow deposits, since these can be used in all types of soils. In this method ....

2. Borings. The following are the various boring methods commonly used:(i) Auger boring.(ii) Auger and shell boring.(iii) Wash boring.(iv) Percussion boring.(v) Rotary boring ....

3. Sub-surface soundings. The sounding methods consist of measuring the resistance of the soil with depth by means of penetrometer under static or dynamic loading. The penetrorneter may consist of a sampling spoon, a cone or other shaped tool. The resistance to penetration is empirically correlated with some of the engineering properties of soil, such as density index, consistency, bearing capacity etc. The value of these tests lie in the amount of experience behind them ....

4. Geo-physical methods. Geo-physical methods are used when the depth of exploration is very large, and also when the speed of investigation is of primary importance. Geo-physical investigations involve the detecttion of significant differences in the physical properties of geological formations.

SITE INVESTIGATION AND SUBSOIL EXPLORATION: Design and Construction of Foundations.

Since the foundations have to transfer the load to the sub-soil, surface conditions at any given site must be adequately explored to obtain ínformation required for the design and construction of  foundations.

Sub-soil exploration is done for the following purposes:

(a) For New Structures

1. The selcction of type and depth of foundation.
2. The determination of bearing capacity of the selected foundation.
3. The prediction of settlement of the selected foundation.
4. The determination of the ground water level.
5. The evaluation of the earth pressure against walls, basements, abutments etc.
6. The provision agains constructional difficltties.
7. Thte suitability of soil and degree of compaction of soil.

(b) For Existing Structures

1. The investigation of the safety of the structure.
2. The prediction of settlement.
3. The determination of remedial measures if the structure is unsafe or will suffcr detrimental settlement.

SITE RECONNAISSANCE

An inspection of the site and study of topographical features is often helpful in setting useful information about the soil and ground water conditions and in deciding the future programme of exploration. On going over the site, a study of the following features may be useful : local topography, excavations, cuttings, quarries, escarpments evidence of erosion or land slides, fills, water level in wells and drainage pattern for the building site. If there has been an earlier use of the site, information should be gathered, if any particular about the underground workings, if anv, and about the location of fills and excavations.

SITE EXPLORATION

The object of the site exploration is to provide reliable, specific and detailed information about the soil and ground water conditions of the site which may be required for a safe and economic design of foundations.

For this purpose, an exploration of the region likely to be affected by the proposed works should yield precise information about the following

(i) the order of occurrence and extent of soil and rock strata.
(ii) the nature and engineering properties of the soil and rock formation, and
(iii) the location of ground water and its variation.

Depth of exploration

Exploration, in general, should be carried out to a depth upto which the increase in pressure due to structural loading is likely to cause perceptible settlement or shear failure of foundations. Such a deph, known as significant depth, depends upon the type of structure, its weight, size, shape and disposition of the loaded areas, and the soil profile and its properties. The significant depth may be assumed to be equal to one-and-a-half to two times the width (smaller of the lateral dimension) of the loaded area.

The depth of exploration at the start of the work may be decided according to the following guide rules, which may need modification as exploration proceeds:

1. Isolated spread footing or raft: One and a half times ihe width.
2. Adjacent footings with clear spacing less than twice the width: One and a half times the length.
3. Pile foundation: 10 to 30 metres, or more, or at least one and a half times the width of the structure.
4. Base of the retaining wall: One and a half times the base width or one and a half times the exposed height of face of wall, whichever is greater.
5. Floating basement: Depth of construction.
6. Weathering considertions: 1.5 m in general and 3.5 m in black cotton soils.

DEEP FOUNDATIONS: Well Foundations (or caissons).

Well foundations or caissons are box like structure-circular or rectangular-which are sunk from the surface of either land or water to the desired depth. They are much large in diameter than the peir foundations or drilled caissons. Caisson foundations are used for major foundation works, such as for:

(i) bridge piers and abutments in rivers, lakes etc,
(ii) Wharves, quay walls, docks.
(iii) Brceak waters and other structures for shore protections.
(iv) Large water front structures such as pump houses, subjected to heavy vertical and horizontal loads.

Well foundations or caissons are hollow from inside, which may be filled with sand, and are plugged at the bottom. The load is transferred though the perimeter wall, called steining (Fig. 2.10).

Well foundations ar not used for buildings.

                                                       FIG. 2.10.  WELL FOUNDATION.

DEEP FOUNDATIONS: Pier Foundation (drilled caisson foundation).

A Pier foundation consisis of a cylindrical column of large diameter to support and transfer large super-imposed loads to the firm strata below. The difference betwcen pile foundaiion and pier foundation lies in the method of construction. Though pile foundations transfer the load through friction and/or bearing, pier foundations transfer the load only through bearing. Generally, pier foundation is shallower in depth than the pile foundation. Pier foundation is preferred in a location where the top strata consists of decomposed rock overlying a strata of sound rock. In such a condition, it becomes difficult to drive the bearing piles through decomposed rock. In the case of stiff clays, which offer large resistance to the driving of a bearing pile, pier foundation can be conveniently constructed.

Pier foundations may be of the following types:

(i) Masonry or concrete pier
(ii) Drilled caissons.

These are shown in Fig. 2.9.

When a good bearing stratum exists upto 5 m below ground level, brick, masonry or concrete foundation piers in excavated pits may be used [Fig. 2.9 (a)]. The size and spacing of the piers depends upon the depth of hard bed, nature of overlying soil and super-imposed loads.

The terms drilled caissons, foundation pier or sub-pier are interchangeably used by engineers to denote a cylindrical foundation.

A drilled caisson is largly a compressed member subjected to an axial load at the top and reaction at the bottom. Drilled caissons are generally drilled with the mechanical means. Drilled caissons may be of three types : (i) concrete caisson with enlarged bottom [Fig. 2.9 (b)], (ii) caisson of steel pipe with concrete filled in the pipe [Fíg. 2.9 (c)]  and (iii) caisson with concrete and steel core in steel pipe [Fig. 2.9 (d)].

FIG. 2.9  PIER FOUNDATIONS.

DEEP FOUNDATIONS: Pile Foundation.

Pile foundation is that type of deep foundation in which the loads are taken to a low level by means of vertical members which may be of timber, concrete or steel. Pile foundation may be adopted (i) instead of a raft foundation where no firm bearing strata exists at any reasonable depth and the loading is uneven, (ii) when a firm bearing strata does exist but at a depth such as to make strip or spread footing uneconomical, and (iii) when pumping of sub-soil water would be too costly or timbering to excavations too difficult to permit the construction of normal foundations.

Piles used for building foundation may be of four types:

(i) End bearing pile [Fig. 2.8 (a)]
(ii) Friction pile [Fig. 2.8 (b)]
(iii) Combined end bearing and friction pile [Fig. 2.8 (c)] and
(iv) Compaction piles [Fig. 2.8 (d)]

End bearing piles [Fig. 2.8 (a)] are used to transfer load through water or soft soil to a suitable bearing stratum. Such piles are used to carry heavy loads safely to hard strata. Multi-storeyed buildings are invanably founded on end bearing piles, so that the settlements are minimised.

FIG.  2.8  PILE FOUNDATIONS.

Friction piles [Fig. 2.8 (b)] are used to transfer loads to a depth of a friction-load-carrying material by means of skin friction along the length of the pile. Such piles are generally used in granular soil where the depth of hard stratum is very great.

Fig. 2.8 (c) shows a pile which transfers the super-imposed load both through side friction as well as end bearing. Such piles are more common, specially when the end bearing piles pass through granular soíls.

Compaction piles [(Fig. 2.8 (d)] are used to compact loose granular soils, thus increasing their bearing capacity. The compaction piles themselves do not carry a load. Hence they may be of weaker material (such as timber, bamboo sticks etc.)-sometimes of sand only. The pile tube, driven to compact the soil, is gradually taken out and sand is filled in its place thus forming a ‘sand pile’.

SHALLOW FOUNDATION - Mat Foundation (Raft Foundation).

A raft or mat is a combined footing that covers the entire area beneath a structure and supports all the walls and columns. When the allowable soil pressure is low, or the building loads are heavy, the use of spread footings would cover more than one half the area and it may prove more economical to use mat or raft foundation. They are also used where the soil mass contains compressible lenses or the soil is sufficiently erratic so that the differential settlement would be diflicuht to control. The mat or raft tends to bridge over the erratic deposits and eliminates the difíerential setlements. Raft foundation is also used to reduce settlement above highly compressible soils. by making the weight of structure and raft approximately equal to the weight of the soil excavated.

Rafts may be divided into three types, based on their design and construction:

(i)   Solid slab system
(i)   Beam slab system
(iii) Celular system

All the three types are basically the same, consisting of a large, generally unbroken area of slab covering the whole or the large part of the structure. The thickness of the slab and the size of beams (if any) will be governed by the spacing and loading of the column and degree of rigidity required in the raft.

SHALLOW FOUNDATION - Strap Footings.

If the independent footings of two columns are connected by a beam, it is called a strap footing. A strap footing may be used where the distance between the columns is so great that a combined trapezoidal footing becomes quite narrow, with high bending moments. In that case, each column is provided with its independent footings and a beam is used to connect the two footings. The strap beam does not remain in contact with soil, and thus does not transfer

any pressure to the soil. The strap, assumed to be infinitely stiff, serves to transfer the column loads on to the soil with equal and uniform soil pressure under both footings.

Fig. 2.7 shows the strap footing for two columns A and B. Column A is so near to an existing wall that the footing of the wall does not permit the independent footing of column A lo spread out towards the wall, though it has freedom in other directions.

FIG. 2.7 STRAP FOOTING.

SHALLOW FOUNDATION - Combined Footings: Rectagular, Trapezoidal and Column-Wall Footings.

A spread footing which supports two or more columns is termed as combined footing. The combined footings may be of the following kinds:

(i) Rectangular combined footing [Fig. 2.5 (a)]
(ii) Trapezoidal combined footing [Fig 15(b)]
(iii) Combined column-wall footings [Fig. 25(a),(b)]

Combined footings are invanably construcied of reinforced concrete.

FIG 2.5  COMBINED FOOTINGS FOR COLUMNS.

The combined footing for columns will be rectangular in shape if they carry equal loads. The design of rigid rectangular combined

footing should be done in such a way that centre of gravity of columu loads coincide with the centroid of the footing area. If the columns carry unequal loads, the footing is of trapezoidal shape, as shown in Fig. 25 (b).
Sometimes, it may be required to provide a combined footings for columns and a wall. Such combined footings are shown in Fig  2.6 (a) [when the columns carry equal loads] and in Fig. 2.6 (b) [when the columns carry unequal loads].

FIG. 2.6 COMBINED FOOTINGS FOR COLUMNS AND WALL

SHALLOW FOUNDATION - Spread Footings: Single footing, Stepped footing, Sloped footing, Wall footing, Grillage foundation.

Spread footings are those which spread the super-imposed load of wall or column over a larger area. Spread footings support either a colunm or wall. Spread footings may be of the following kinds:

(i)   Single footing [ Fig. 2.2(a)] for a column
(ii)  Stepped footing [ Fig. 2.2(b)] for a column
(iii) Sloped footing [ Fig. 2.2(c)] for a column
(iv)  Wall footing without step [ Fig. 2.3(a)]
(v)   Stepped footing for wall [ Fig. 2.3(b)]
(vi)  Grillage foundation [ Fig. 2.4]

Fig. 2.2 SPREAD FOOTINGS FOR COLUMNS.

Fig. 2.2 (a) shows a single footing for a column, in which 

the loaded area (b x b) of the column has been spread to the size 

B x B through a single spread. The base is generally made of concrete. 

Fig. 2.2 (b) shows the stepped footing for a heavily loaded column, 

which requires greater spread. The base of the column is made of 

concrete. Fig. 2.2 (c) shows the case in which the concrete base does 

not have uniform thickness, but is made sloped, with greater thickness 

at its junction with the column and smaler thickness at the ends.

FIG. 2.3  SPREAD FOOTING FOR WALL : STRIP FOOTING.

Fig. 2.3 (a) shows the spread footing for a wall, consisting 

of concrete base without any steps. Usually, masonry walls have 

stepped footings as shown in Fig. 2.3 (b), with a concrete base.

FIG.  2.4  GRILLAGE FOUNDATION.

Fig. 2.4  shows a steel grillge foundation for a steel stanchion carrying heavy load. It is a special type of isolated footing generally provided for heavily loaded steel stanchions and used in these locations where bearing capacity of soil is poor. The depth of such a foundation is limited to 1 to 1.5 m. The load of the stanchion is distributed or spread to a very large area by means of two or mor tiers or rolled steel joints, each layer being laid at right angle to the layer bellow it. Both the tiers of the joists are then embeden in cement concrete to keep the joists in position and to prevent their corrosion.
The detailed method of construction has benn explained in 3.6 Grillage foundation is also constructed of timber beams and planks (Fig. 3.12 and 3.13)

TYPES OF FOUNDATIONS: Shallow foundation, Deep Foundations.

Foundations may be broadly classified under two heads:

(a) Shallow Foundations
(b) Deep Foundations.

According to Terzaghi, a foundation is shallow if its depth is equal to or less than its width. In case of deep foundaüons. the depth is equal to or greater than its width.

(A) SHALLOW FOUNDATIONS

From the point of view of design, shallow foundations may be of ttic following typcs:

1. Spread footings.
2. Combined footings
3. Strap footings.
4. Mat foundation.


Various types of shallow foundations are shown in Fig. 2.1. A brief description of these is given below.

FIG 2.1 VARIOUS TYPES OF SHALLOW FOUNDATIONS.

(B) DEEP FOUNDATIONS

Deep foundations may be of the following types:

1. Deep strip, rectangular or square footings.
2. Pile foundation.
3. Pier foundation or drilled caisson foundation.
4. Well foundation or caissons. 

As stated earlier, the usual strip, rectangular or square footings come under the category of deep foundations, when the depth of the foundation as more than the width of the footing. Well foundations are generally adopted for bridge piers etc. and not for building foundations.

Essential Requirements of a Good Foundation.

Foundations should be constructed to satisfy the following requirements

1. The foundations shall be constructed to sustain the dead and imposed loads and to transmit these to the sub-soil in such a way that pressure on it will not cause settlement which would impair the stability of the building or adjoining structures.

2. Foundation base should be rigid so that differential settlements are minimised, specially for the case when super-imposed loads are not evenly distributed.


3. Faundations should be taken suficient1y deep to guard the building against damage or distress caused by sweelling or shrinkage of the sub-soil.


4. Foundations should be so located that its performaced may not be affected due to any unexpected  future influence.

Functions of Foundations.

Foundations serve the following purposes:

1. Reduction of load intensity. Foundations distribute the loads of the super-structure, to a larger area so that the intensity of the load at its base (i.e. total load divided by the total area) does not exceed the safe bearing capacity of the sub-soil. In the case of deep foundations, it transmits the super-imposed loads to the sub-soil both through side friction as well as through end bearing.

2. Even distribution of load. Foundations distribute the nonuniform load of the super-structure evenly to be sub-soil. For example, two columns carrying unequal loads can have a combined footing which may transmit the load to sub-soil evenly with uniform soil pressure. Due lo this, unequal or differential settlements are minimised.

3. Provision of level surface. Foundations provide levelled and hard surface over which the super-structure can be built.

4. Lateral stability. It anchors the super-structure to the ground, thus imparting lateral stability to the super-structure. The stability of the buildlng, against sliding and overturning, due to horizontal forces (such as wind, earthquake etc.) is increased due to foundations.

5. Safety against undermining. It provides the structural safety against undermining or scouring due to burrowing animals and flood water.   

6. Protection against soil movements. Special foundation measures prevents or minimises the distress (or cracks) in the super-structure, due to expansion or contraction of the sub-soil because of moisture movement in sorne problematic soils.

Foundations of Buildings and Structures: Introduction.

Every building consists of two basic components : the superstructure and the substructure or foundations. The super-structure is usually that part of the building which is above ground, and which serves purpose of its intended use. The substructure or foundations is the lower portion of the building, usually localed below ground level, which transmits the load of the super-structure to the sub-soil. A foundalion is therefore that part of the structure which is in direct contact with the ground to which the loads are transmirred. The soil which is located immediately below the base of the foundation is called the sub-soil or foundation soil , while rhe lowermost portion of the foundation which is in direct contact with the sub-soil is called the footing.

The basic function of a foundation is to transmit the dead loads, super-imposed loads (or, live loads) and wind loads from a building to the soil on which the building rests, in such a way that (a) settlements are within permissible limits, without causing cracks in the super-structure, and (b) the soil does not fail. When loads are transmilted to the sub-soil, it settles. If this settlement is sligh and uniform throughout, no damage will be caused to the building. But if the settlement is exccessive or unequal, serious damage may result in the form of cracked walls, distorted doors and window openings, cracked linteis, walls thrown out of plumb etc., and somctimes the complete collapse of the building.

The foundation is thus the most important partof a buildíng. Since it remains below the ground level, the signs of failure of foundation are not noticeable tull it has already affected the building. A foundation should be sufficiently strong to preveni excessive settlement as well as unequal settlement. Unequal settlement or differenrial settlemen may be caused by (i) weak sub-soils, such as made up ground (ii) shrinkable and expansive soils (such as clay), (iii) frost action. (iv) movement of ground water, and uplift pressure, (v) excessive vibrations, due to traffic, machinery etc., (vi) slow consolidation of saturated clays, and (vii) slipping of strata on sloping sites. When designing the foundations, therefore, the above factors must be taken into account.

WIND LOADS IN A BUILDING.

Wind is the air in motion relative to the surface of the earth. Since vertical components of atmospheric motion are relatively small, specially near the surface of the earth, the term ‘wind’ denotes almost exclusively the horizontal wind. Wind pressure, therefore, acis horizontally on the exposed vertical surfaces of walls, columns etc, and inclined roof of the structure. Wind pressure or wind loading is expressed in terms of a basic pressure p which is an equivalcnt static pressure in the direction of flow of wind. The basic wind pressure p should be decided on the basis of local meteorological data and local conditions, such as characteristics and location of structure and characteristics and duration of wind flow. In absence of any other data, the following relationship may be obtained by the expression.

For structures of various plan shapes other than rectangular plan shape, the external pressures acting on the projected area in the plane perpendicular to the wind, should be the product of the basic pressure (p) given above and thc shape factors given in Table 1.4.

TABLE 1.4. PLAN SHAPE FACTORS

thereof, having due regard to the level of mean retarding surface and variation in wind pressure with height. In making calculations, due regard should be given to shape factor and internal air pressure.
 

The stability calculations of the building as a whole shall be done considering the combined effect, as well as separate effects of imposed loads and wind loads on vertical surfaces, roofs and any other part of the building above the general roof level.
 

Calculations shall be made for the effect of wind on the design of individual components of the building. Where, however, adequate stiffering is provided by walls, or floors and walls, calculations for the effecas of wind, except in regard to wind panels, roofs amd foundations, need not be made on:
 

(a) A building or part of a building of which the heigh h does not exceed twice the effective widths,

(b) a section adjoining two parts of an adequately stiffened building, if height of the section exceeds twice its width but the length of the section does not exeed four times its width, and

(c) a wing of such a building if it does not project more than twice its own width. 

In the case of sloped roofs, the wind pressure varies with the degree of slope. The slope of the roof towards the wind direction is known as the windward slope, while the slope to the other side of the wind direction is 

known as the leeward slope. For flat and pitched roofs, the wind pressures normal to the surface, due to wind blowing at right angles to the ridge, in terms of basic wind pressure(p) are given in Table 1.5.

 TABLE 1.5. WIND PRESSURE ON ROOFS (WIND NORMAL TO RIDGE)

LIVE LOADS IN A BUILDING: on floors, on roofs.

Live loads, also called as super-imposed loads, consisi of moving or variable loads, due to people or occupants, their furniture, temporary stores, machinery etc.

LIVE LOADS ON FLOORS

Live loads on floors shall comprise of all loads other than dead loads. The minimum live loads on different floors for different uses shall be as given in Table 1.2. The loads specified in Table 1.2 are uniformly distributed static loads in kg/m2 ( kN/m2) on the plane area and provide for normal effects of impact and acceleration, but do not take into consideration special concentrated loads, snow loads and other loads.

In the case of multistoreyed buildings, the following reductions in live loads may be made in designing column, walls, piers and foundations

No reduction shall be made in ihe case of warehouses, garages, and other buildings used for storage purposes, and for factories and workshops, designed for 500 kg/m2 (5 kN/m2). However, for bulidings, such as factories and workshops, designed for a live load of more than 500 kg/m2 (5 kN/m2), the reduction given above may be made, provided that the loading assumed for any column, walt etc, is not less than that which would have been if ah floors had been designed for 500 kg/m(5 kN/m2) with no reduction.

TABLE 1.2  LIVE LOADS ON FLOORS

*the lower value of 250 kg/m2 (2.5 k N/mZ) should be taken where separate storage facilities are provided and me higher value of 400 kg/m2 (4 kN/m2) should be taken where such provisions are laking.

Note 1. In the above table, a reference to a floor includes a reference to any part of that floor, and a reference to ‘slabs’ includes boarding and beams or ribs, spaced not farther apart than one metre between centres, and a reference to ‘beams’ means all other beams and ribs. 

Note 2. Under loading class No. 250, the reference to ‘light work rooms’ envisages roorns in which sorne light machines (for example, sewing machines, used for by milliners or tailors) are operated without a central power-driven unit, that is, the machines are independently operated either by hand or by srnall motors.

Under loading dass No. 400, the reference to ‘workrooms’ generally envisages the installation of machines operated with central power driven unit with individual machines being belt driven.

Note 3. ‘Fixed seating’ implies that the removal of the seating and the use of space for other purpose is improbable. The maximum likely load in this case is, therefore, closely controlled.

Note 4. The loading in workshops, warehouses and factories varies considerably and so these loadings under the terms ‘light’ ‘medium’ and ‘heavy’ are introduced in order to allow fon more economical designs but the terms have no special meaning in themselves other than the live load fon which the relevent floor is designed. It ts, however, important, panticularly in the case of heavy weight loads, lo assess the actual loads lo ensure that they are not in excess of 1000 kg/m2 (10 kN/m2) in case where they are in exccss, the design shall be based on the actual loading.

Note 5. The load classification fon stairs, corridors, balconies and landings provide fon the fact that these often serve several occupancies and are used fon transporting the furniture and goods.

LIVE LOADS ON ROOFS

Table 1.3 gives lives loads on flat roofs, sloping roofs and curved roofs.

Roofs of buildings used for promenade or incidental assembly purposes shall be designed for a minimum load of 400 kg/m2 (4 kN/m2) or heavier, it required.

Snow Load: It the roof is subjected to snow load, it should be designed for the actual load due to snow or lot the live loads specified in Table 1.3, whichever is more severe. Actual load due to snow will depend upon the shape of the roof and its capacity to retain the snow; and each case shall be treated on its own merits. In the absence of any specific information, the loading due to colletion of snow may be assumed to be 2.5 kg/m2 (25 kN/m2) per centimetre depth of snow.

Loads due to rain: On surfaces whose positioning, shape and drainage system are such as to make accumulation of rainwater possible, loads due lo such accumulation of water and the live loads for he roofs as given in Table 1.3 shall be considered separately and the more critical of the two shall be adopted in the design.

TABLE 1.3  LIVE LOADS ON ROOFS.

DEAD LOADS IN A BUILDING.

The dead load in a building shall comprise of the weight of all walls, partitions, floors and roofs and shall include the weights of all other permanent construction in the building.

The unit weights of sorne common materials, as per IS: 1911—1967 are given in Table 1.1

 TABLE 1.1  UNIT WEIGHTS OF SOME COMMON MATERIALS

DESIGN LOADS IN A BUILDING.

The basic requirement of any structural component of a building is that it should be strong enough to carry or support all possible types of loads to which it is likely lo be subjected. Loads coming on a structure may be of three types:

⇒ 1. Dead loads,
⇒ 2. Live loads, and
⇒ 3. Wind Loads.

Building Finishes.

Building finishes are used to give protective covering lo various building components, and at the same time, they provide decorative effects. Building fmishes consists of the following items:

(i) Plastenng
(ii) Pointing
(iii) Painting
(iv) Varnishing and polishing
(v) White washing
(vi) Distempering
(vii) Colour washing or colounng.

Plostenng consisis of providing a ihin ctwering of plastic materials such as ement mortar, lime mortar etc. on walls, columns and other surfaces. Poüumg is the process of finishing of mortar joinis in brick or stone rnasoniy. Painting vamishing and polishing is normally done on doors, windows and other timher and steel componenis. White washin& distemperingandcoIczurwahingetc. are doneon plastered surfaces, lo safegaurd them against weathering effects and to improve the appearance.

Building Components: Vertical Transportation Structures.

These nsists of stairs, ramps, ladders, lifts and escalators etc, to afford acces between various floors. Out of these, stairs are the most ammon. A srair may be defined as series of steps suitably arranged for the purpose of connecting different floors of a building. Alternatively, a stair may be defined as an arrangement of treads, risers, stringers, newel posts, hand rails and balustrades so designed and constructed as lo provide an easy, safe and quick access to the users of diffcrent floors. Stairs may be constructed of different materlais such as timber, stone, reinforced concrete or steel.

Doors, Windows and other Openings

A door is a movable barner provided in the opcning of a walt, to provide access to various spaccs of a buílding. A door is a frame work of wood, steel etc. secured in the walt openingg for the purpose of providing access to the users of the building. Simítarly, a window may be defined as an opening made in (he wall br ¡he purpose of providing day light, vision and ventílation. Windows are also made of frame work of wood, steel, aluminium etc., provided with shuters.

Since doors and windows are provided in the openings in the walls, a discontinuity is formed in the walt, in the vertical direction. Lintel. are therefore essential. A lintel is a horizontal structural member provided ove’r the doors, windows or other openings, lo span the gap, so as to support the super-imposed load carried by the walt aboye the opening. Linteis may be made of timber, stone, steel or reinforced cement concrete (R.C.C.).

Sometimes, an arch may be providcd to span the opening, in the place of a lintel. An arch is a structure consisting of a number of small wedge-shaped units and jointed together with moflar, which is construcied to bridge across any opening in the walt. The arch may also be constructed in R.C.C.