1
Introduction
1.1 GENERAL
Residential, educational, office and commercial buildings are the common examples of civil engineering structures. These structure consists of various elements like slabs, beams, columns, footings and staircases. Apart from the above buildings the civil engineers are associated with the design and constructions of retaining walls, water tanks, bridges, dams, towers, cooling towers. For the construction of all the above structures very commonly used material is reinforced cement concrete (R.C.C.), which is a composite material consisting of concrete and steel. When water is added to an intimate mixture of cement, sand (Fine Aggregate) and jelly (Coarse Aggregate), it forms a plastic mass, popularly known as concrete. This mass can be easily moulded to desired shape and size using formworks. The concrete gradually hardens and achieves the shape and size permanently. Apart from the main ingredients cement, sand, jelly and water, small quantities of admixtures like air entraining agents, water proofing agents, workability agents may also be added to impart special properties to the concrete. Concrete is good in resisting compressive stress but is very weak in resisting tensile stress. Hence it is to be reinforced with suitable material wherever tension develops. The best reinforcement is the steel, because the tensile strength of steel is quite high and the bond between steel and concrete is very good. Reinforcements are usually in the form of mild steel or high strength deformed steel bars of diameters 6 to 32 mm. A cage of reinforcement is prepared as per the design requirement, kept in the formwork and concrete in the plastic form is poured. After concrete hardens the form work is removed. The composite material of steel and concrete (R.C.C.) is now capable of resisting compressive as well as tensile stresses. The science of proportioning the structural elements to resist the applied loads and determining the numbers and sizes of reinforcing bars is called design of R.C.C. structures. In this chapter important properties of concrete and steel are presented and a brief introduction is given to the various loads to be considered for the analysis. Various methods of analysis and design are very briefly discussed.

1.2

IMPORTANT PROPERTIES OF CONCRETE

The following important properties of concrete are to be noted by designer:

Design of R.C.C. Structural Elements (i) Though it consists of different materials like cement, sand and jelly the intimate mixture is so good that for all practical purposes it may be assumed as homogeneous. (ii) For concrete, characteristic strength is defined as compressive strength of 150mm cube at 28 days in N/mm2, below which not more than 5 percent cubes give the result. Based on the characteristic strength (fck), concrete is graded as given below:
Table 1.1
Group Ordinary Concrete Standard Concrete

Grades of Concrete
Characteristic strength N/mm2 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

Grade Designation M10 M15 M20 M25 M30 M35 M40 M45 M50 M55 M60 M65 M70 M75 M80

High Strength Concrete

Note: Now a days ultra high strength of grade M500 are also produced in the laboratories and M250 concrete has been used for the construction of some bridges. IS 456 - 2000 recommends minimum grade of concrete to be used for various weather conditions as shown in table 1.2.

Table 1.2 Minimum Grade of Concrete for Different Exposure with Normal Weight Aggregates of 20mm Nominal Maximum Size. (Table 5 IS 456–2000)
Table 1.2

Exposure Mild Moderate Severe Very Severe Extreme

Minimum Grade of Concrete M20 M25 M30 M35 M40

The meaning of the different exposure conditions are as given in Appendix A. (iii) Stress Strain Relationship: Stress strain curve depend on strength of concrete as well as on the rate of loading. The short term stress strain curve is to be obtained for a constant rate of straining of 0.01 percent per minute or for a constant rate of stress increase of 14 N/mm2 per minute. Figure 1.1 shows a typical stress-strain curve for different mixes for constant stress and constant strain conditions.

Introduction
60 ¶s = Constant ¶t

!

¶e = Constant ¶t

45
Stress

30

15 Strain controlled Stress controlled 0 2 4 6 ´ 10
3

Fig 1.1. Stress Strain curve for Different Mixes of Concrete. (iv) Tensile Strength : A designer may use the following expression for flexceral tensile strength of concrete: Fcr = 0.7 f ck N/mm2. Where fck is the characteristic compressive strength of concrete. Modulus of Elasticity: The short term static modulus of elastically for concrete may be taken as Ec = 5000 f ck as per IS 456 – 2000. Poisson’s Ratio : It may be taken as 0.1 for high strength concrete and 0.2 for weak concrete. Usually it is taken as 0.15 for strength and 0.2 for serviceability calculations. Shrinkage : Total amount of shrinkage in concrete depends on the various factors including the amount of water present at the time of casting. In the absence of data the approximate value of the total shrinkage strain may be taken as 0.0003. Creep: It depends on various factors including the age of loading, duration of loading and stress level. The creep coefficient which is defined as ratio of ultimate creep strain to elastic strain at the age of loading may be taken as shown in table 1.3. (Clause 6.5.5.1 in IS 456 - 2000)
Table 1.3 Creep Coefficient

Strain

(v)

(vi) (vii)

(viii)

Age at loading 7 days 28 days 1 Year

Creep Coefficient 2.2 1.6 1.1

The ultimate creep strain shown above does not including elastic strain.

1.3

IMPORTANT PROPERTIES OF STEEL

The following important properties of steel are to be noted by a designer

" Design of R.C.C. Structural Elements
(i) It is treated as homogeneous material and it is really so to a very large extent. (ii) The characteristic strength of steel (fy) is its tensile strength determined on standard specimen below which not more than 5 percent specimen give the results. However code permits use of minimum yield stress or 0.2 percent proof stress as characteristic strength of steel. Types of steel reinforcements available in the market are shown in table 1.4.
Table 1.4
Type Mild Steel (Plain Bars) High Yield Strength Deformed Bars (HYSD) Hard Drawn Steel Wire Fabric

Grades of Steel
Yield Stress / 0.2% Proof Stress 26 kg / mm2 = 255 N/mm2 (i) 415 N/mm2 (ii) 500 N/mm2 49 kg / mm2 = 480 N/mm2

Conforming to I.S. Code IS: 432 – 1966 IS: 1786 – 1979 IS: 1566 – 1967

Taking the above values into consideration, Bureau of Indian Standards has brought up design aid to IS 456 (SP-16) for three grades of steel having characteristic strength fy equal to 250 N/mm2, 415 N/mm2 and 500 N/mm2. Hence the designer usually grades the available steels as Fe–250, Fe–415 and Fe–500. (iii) Stress Strain Curve. The typical stress strain curve for the above grades of steel is shown in Fig 1.2. It may
Fe - 500 Fe - 415 400
Stress is N/mm
2

500

300 200 100

Fe - 250

0 0.2 2

4

6

8 10 12 14 16 18 20 22 24 % Strain

Fig 1.2. Stress strain curve for Fe – 250, Fe -415 and Fe – 500 steel be noted that for mild steel (Fe–250) the yield point is clearly visible whereas there is no yield point for Fe–415 and Fe–500 steel. For these two steels, 0.2 percent proof stress is taken as characteristic strength fy. (iv) The characteristic strength fy in tension and in compression is taken as the same.

Introduction

#

(v) the young's modulus (E) for all grades of steel is taken as 200kN/mm2 = 2 ´ 105 N/mm2. The reinforcing bars are generally available in the market in the following sizes: Diameter of bars in mm. Mild steel 6, 10, 12, 16, 20, 25, and 32. Deformed steel bars 8, 10, 12, 16, 20, 22, 25, 28 and 32

1.4

CODE REQUIREMENT OF REINFORCEMENTS

(i) Mild steel conforming to IS 432, high strength deformed bars conforming to IS 1786 and hard-drawn steel fabrics conforming to IS 1566 may be used as reinforcements. Structural steel conforming to grade A of IS 2002 are also permitted. (ii) All bars should be free from loose mill scales, loose rust, mud, coats of paint or any other material which destroy or reduce the bond. (iii) If required, in exceptional cases and for rehabilitation of structures, special chemical coatings may be provided to reinforcements. (iv) For main bars steel of same grade should be used as main reinforcement. Simultaneous use of two different grades of steel for main and secondary reinforcement is permitted. (v) Bars may be arranged singly or in pairs. Use of 3 or 4 bundled bars is also permitted. Bundled bars are to be tied together to ensure that they remain together. Bars larger than 32 mm diameter are not to be bundled. Other detailing requirements for reinforcements are given in latter chapters when the design of structural elements is taken up

1.5

LOADS

The various loads expected on a structure may be classified into the following groups: (i) Dead loads (ii) Imposed loads (iii) Wind loads (iv) Snow loads (v) Earthquake forces (vi) Shrinkage, creep and temperature effects, and (vii) Other forces and effects.

Dead Loads (DL)
Dead loads in a building includes the weight of all permanent constructions, like roofs, floors, walls, partition walls, beams, columns, balcony’s, footing. These loads shall be assessed by estimating the quantity of each material and then multiplying it with the unit weight. The unit weights of various materials used in building constructions are given in the code IS 875 (part – 1) – 1987. It includes exhaustive list. For example, under the heading ‘brick masonry’, it has four types like common burnt clay bricks, engineering bricks, glazed bricks and pressed bricks. Under the heading plain concrete there are 10 groups. The commonly used values by the designers are listed in table 1.5.

$ Design of R.C.C. Structural Elements
Table 1.5 Unit weight of Important Building Materials Used by Designers
Sl. No. 1. 2. 3. 4. 5. Material Plain concrete Reinforced concrete Brick masonry, cement plaster Granite stone masonry Asbestos cement sheets Unit Weight 24 kN/m3 25 kN/m3 20 kN/m3 24 kN/m3 0.130 kN/m2

Imposed Loads (IL) The loads which keep on changing from time to time are called as imposed loads. Common examples of such loads in a building are the weight of the persons, weights of movable partition, dust loads and weight of furnitures. These loads were formerly known as live loads. These loads are to be suitably assumed by the designer. It is one of the major load in the design. The minimum values to be assumed are given in IS 875 (part 2)–1987. It depends upon the intended use of the building. These values are presented for square metre of floor area. The code gives the values of loads for the following occupancy classification: (i) Residential buildings–dwelling houses, hotels, hostels, boiler rooms and plant rooms, garages. (ii) Educational buildings (iii) Institutional buildings (iv) Assembly buildings (v) Business and office buildings (vi) Mercantine buildings (vii) Industrial buildings, and (viii) Storage rooms. The code gives uniformly distributed load as well as concentrated loads. The floors are to be investigated for both uniformly distributed and worst position of concentrated loads. The one which gives worst effect is to be considered for the design but both should not be considered to act simultaneously. In a particular building, imposed load may change from room to room. For example in a hotel or a hostel building the loads specified are, udl Concentrated load (a) Living rooms and bed rooms 2 kN/m2 1.8 kN (b) Kitchen 3 kN/m2 4.5 kN (c) Dining rooms 4 kN/m2 2.7 kN (d) Office rooms 2.5 kN/m2 2.7 kN (e) Store rooms 5 kN/m2 4.5 kN (f) Rooms for indoor games 3 kN/m2 1.8 kN (g) Bath rooms and toilets 2 kN/m2 – (h) Corridors, passages, 3 kN/m2 4.5 kN stair cases etc. and (i) Balconies 4 kN/m2 1.5 kN concentrated at outer edge. Some of the important values are presented in table 1.6, which are the minimum values and wherever necessary more than these values are to be assumed.

Introduction
Table 1.6 Sl. No. Minimum Imposed Load to be Considered UDL Load 2 kN/m2 2 kN/m2 2.5 kN/m2 3 2 3 3 kN/m2 kN/m2 kN/m2 kN/m2

%

Occupancy

Concentrated 1.8 kN 1.8 kN 2.7 kN 4.5 1.8 4.5 4.5 kN kN kN kN

1. Both rooms and toilets in all types of building 2. Living and bed rooms 3. Office rooms in (i) Hostels, hotels, hospitals and business building with separate store (ii) In assembly buildings 4. Kitchens in (i) Dwelling houses (ii) Hostels, hotels and hospitals 5. Banking halls, class rooms, x-ray rooms, operation rooms 6. Dining rooms in (i) educational buildings, institutional and mercantine buildings (ii) hostels and hotels 7. Corridors, passages, stair cases in (i) Dwelling houses, hostels and hotels (ii) Educational institutional and assembly buildings (iii) Marcantine buildings 8. Reading rooms in libraries (i) With separate storage (ii) Without separate storage 9. Assembly areas in assembly buildings (i) With fixed seats (ii) Without fixed seats 10. Store romms in educational buildings 11. Store room in libraries

3 kN/m2 4 kN/m2 3 kN/m2 4 kN/m2 5 kN/m2 3 kN/m2 4 kN/m2 5 kN/m2 5 kN/m2 5 kN/m2 6 kN/m2 for a height of 2.24 + 2 kN/m2 for every 1m additional height 5 kN/m2 7.5 kN/m2

2.7 kN 2.7 kN 4.5 kN 4.5 kN 4.5 kN 4.5 kN 4.5 kN .. 3.6 kN 4.5 kN 4.5 kN

12. Boiler rooms and plant rooms in (i) hostels, hotels, hospitals, mercantine and industrial buildings (ii) Assembly & storage buildings

4.5 kN 4.5 kN

Imposed loads to be considered on various roofs are presented in table 1.7

& Design of R.C.C. Structural Elements
Table 1.7 Imposed Loads on various Types of Roofs (Table 2 of National Building code - 1983) Sl.No. (i) Type of Roof Flat, sloping or curved roof with slopes up to and including 10 degrees (a) Access provided Imposed Load Measured on Plan Area Minimum Imposed Load Measured on Plan

1.5 kN/m2

(b) Access not provided except for maintenance

0.75 kN/m2

(ii)

(iii)

Sloping roof with For roof membrane sheets or slope greater than purlins – 0.75 kN/m2 less 0.02 10 degrees kN/m2 for every degree increase in slope over 10 degrees Curved roof with (0.75 - 0.52 a2) kN/m2 where slope of line obtained by joining springing point to the crown with the horizontal, greater than 10 degrees a = h/ l h = height of the highest point of the structure measured from its springing: and l = chord width of the roof if singly curved and shorter of the two sides if doubly curved. Alternatively, where structural analysis can be carried out for curved roofs of all slopes in a simple manner applying the laws of statistics, the curved roofs shall be divided into minimum 6 equal segments and for each segment imposed load shall be calculated appropriate of each segment as given in (i) and (ii)

3.75 kN uniformly distributed over any span of one metre width of the roof slab and 9 kN uniformly distributed over the span of any beam or truss or wall. 1.9 kN uniformly distributed over any span of one metre width of the roof slab and 4.5 kN uniformly distributed over the span of any beam of truss or wall. Subject to a minimum of 0.4 kN/m 2

Subject to a minimum of 0.4 kN/m 2

Introduction

'

Note : 1 - The loads given above do not include loads due to snow, rain, dust collection, etc. The roof shall be designed for imposed loads given above or snow / rain load, whichever is greater. Note : 2 - For special types of roofs with highly permeable and absorbent material, the contingency of roof material increasing in weight due to absorption of moisture shall be provided for. However in a multi-storeyed buildings chances of full imposed loads acting simultaneously on all floors is very rare. Hence the code makes provision for reduction of loads in designing columns, load bearing walls, their supports and foundations as shown in table 1.8.

Table 1.8 Reductions in Imposed Loads on Floors in Design of Supporting Structural Elements Number of Floors (including the roof) to be carried by Member Under Consideration 1 2 3 4 5 to 10 Over 10 Reduction in Total Distributed Imposed Load in Percent 0 10 20 30 40 50

Wind Loads The force exerted by the horizontal component of wind is to be considered in the design of buildings. It depends upon the velocity of wind and shape and size of the building. Complete details of calculating wind load on structures are given in IS-875 (Part 3) - 1987. Brief idea of these provisions are given below: (i) Using colour code, basic wind pressure 'Vb' is shown in a map of India. Designer can pickup the value of Vb depending upon the locality of the building. (ii) To get the design wind velocity Vz the following expression shall be used: Vz = k 1 k 2 k 3 Vb Where k1 = Risk coefficient k2 = Coefficient based on terrain, height and structure size. k3 = Topography factor (iii) The design wind pressure is given by pz = 0.6 V z2 2 where pz is in N/m at height Z and Vz is in m/sec. Up to a height of 30m, the wind pressure is considered to act uniformly. Above 30m height, the wind pressure increases.

Snow Loads
IS 875 (part 4) – 1987 deals with snow loads on roofs of the building. For the building to be located in the regions wherever snow is likely to fall, this load is to be considered. The snow load acts vertically and may be expressed in kN/m2 or N/m2. The minimum snow load on a roof area or any other area above ground which is subjected to snow accumulation is obtained by the expression S = m S0 Where S = Design snow load on plan area of roof.

 Design of R.C.C. Structural Elements m = Shape coefficient, and S0 = Ground snow load. Ground snow load at any place depends on the critical combination of the maximum depth of undisturbed aggregate cumulative snow fall and its average density. These values for different regions may be obtained from Snow and Avalanches Study Establishment Manali (HP) or Indian Meteorological Department Pune. The shape coefficient depends on the shape of roofs and for some of the common shapes the code gives these coefficients. When the slope of roof is more than 60° this load is not considered. It may be noted that roofs should be designed for the actual load due to snow or for the imposed load, whichever is more sever.

Earthquake Forces
Earthquake shocks cause movement of foundation of structures. Due to inertia additional forces develop on super structure. The total vibration caused by earthquake may be resolved into three mutually perpendicular directions, usually taken as vertical and two horizontal directions. The movement in vertical direction do not cause forces in superstructure to any significant extent. But movement in horizontal directions cause considerable forces. The intensity of vibration of ground expected at any location depends upon the magnitude of earthquake, the depth of focus, the distance from the epicenter and the strata on which the structure stands. The response of the structure to the ground vibration is a function of the nature of foundation soil, size and mode of construction and the duration and intensity of ground motion. IS:1983 – 1984 gives the details of such calculations for structures standing on soils which will not considerably settle or slide appreciably due to earthquake. The seismic accelerations for the design may be arrived at from seismic coefficients, which is defined as the ratio of acceleration due to earthquake and acceleration due to gravity. For the purpose of determining the seismic forces, India is divided into five zones. Depending on the problem, one of the following two methods may be used for computing the seismic forces: (a) Seismic coefficient method (b) Response spectrum method The details of these methods are presented in IS 1983 code and also in National Building Code of India. After the Gujarat earthquake (2000) Government of India has realized the importance of structural designs based on considering seismic forces and has initiated training of the teachers of technical institution on a large scale (NPEEE). In this book designs are based on normal situations and hence earthquake forces are not considered. There are large number of cases of less importance and relatively small structures for which no analysis be made for earthquake forces provided certain simple precautions are taken in the construction. For example (i) Providing bracings in the vertical panels of steel and R.C.C. frames. (ii) Avoiding mud and rubble masonry and going for light materials and well braced timber framed structures.

Other Forces and Effects
As per the clause 19.6 of IS 456 – 2000, in addition to above load discussed, account shall be taken of the following forces and effects if they are liable to affect materially the safety and serviceability of the structure:

Introduction  (a) (b) (c) (d) (e) (f) (g) (h) Foundation movement (See IS 1904) Elastic axial shortening Soil and fluid pressure (See IS 875, Part 5) Vibration Fatigue Impact (See IS 875, Part 5) Erection loads (See IS 875, Part 2) and Stress concentration effect due to point load and the like.

1.6

LOAD COMBINATIONS

A judicious combination of the loads is necessary to ensure the required safety and economy in the design keeping in view the probability of (a) their acting together (b) their disposition in relation to other loads and severity of stresses or deformations caused by the combination of various loads.
The recommended load combinations by national building are codes 1. 2. 3. 4. 5. 6. DL DL + IL DL + WL DL + El DL + TL DL + IL + WL 7. 8. 9. 10. 11. 12. DL + IL + EL DL + IL + TL DL + WL + TL DL + EL + TL DL + IL + WL + TL DL + IL + EL + Tl

where and

DL = dead load

IL = imposed load

WL = wind load EL = earthquake load TL = temperature load.

Note: When snow load is present on roofs, replace imposed load by snow load for the purpose of above load combinations.

1.7

STRUCTURAL ANALYSIS

Structural analysis is necessary to determine the stress resultants like bending moment, shear force, torsional moment, axial forces acting at various cross section of structural elements. IS 456 permits the analysis of all structures by linear elastic theory. For structural analysis one can use classical methods. Code also permit use of approximate methods like substitute frame method, use of coefficients for the continuous beams and slabs. Numerical methods also may be used for the analysis. Finite element method is becoming a popular method of analysis, since standard commercial packages are available. This method being versatile any complex structure may be analyzed with acceptable accuracy. Some of the popular packages are STAADPRO, GTSTRUDL, NISA - CIVIL, NASTRAN and ANSYIS.


1.8

Design of R.C.C. Structural Elements

METHODS OF RCC DESIGN

The aim of design is to decide the size of the member and provide appropriate reinforcement so that the structures being designed will perform satisfactorily during their intended life. With an appropriate degree of safety the structure should (i) sustain all loads (ii) sustain the deformations during and after construction (iii) should have adequate durability and (iv) should have adequate resistance to misuse and fire. Various methods of R.C.C. designs can be grouped into experimental and analytical methods. In civil engineering experimental methods of designs have superseeded the analytical methods. Whenever new materials have been developed, they have been tested and used on experimental basis. Once they have been found suitable theoretical methods have been developed to economize. If the structures are too complex model studies are carried out and then prototypes are built. However this method of design is time consuming and cannot be accepted for the design of each and every structure. Analytical methods are based on identifying failure criteria based on material properties. Expected loads quantified, analysis carried out and then based on failure criteria safe designs are found. Since there are limitations in estimating design loads, material behaviour and analytical methods, working condition is kept at a fraction of failure conditions. The design philosophies used in R.C.C. are listed below in the order of their evolutions and then they are briefly explained: (i) Working stress method (WSM) (ii) Load factor method (LFM) or Ultimate load method (ULM), and (iii) Limit state method (LSM)

Working Stress Method (WSM)
This method of design was evolved around the year 1900. This method was accepted by many national codes. India's code IS 456 –1953 was based on this method. It was revised in 1957 and 1964. This method is based on elastic theory of R.C.C. sections. The following assumptions are made in this method: (i) At any cross section, plane section before bending remains plane even after bending. (ii) All tensile stresses are taken by reinforcements and none by concrete. (iii) The stress strain relation, under working loads, is linear both for steel and concrete. (iv) The modular ratio between steel and concrete remains constant and is given by
Es 280 = Em 3 σ cbc Where scbc is permissible compressive stress in bending. Permissible stress in concrete is defined as ultimate stress divided by a factor of safety. In concrete a factor of safety upto 3 is used. In case of steel, the permissible stress is defined as yield stress or 0.2 per cent proof stress divided by factor of safety. Since steel is more reliable material, factor of safety used is 1.75 to 1.85 only. Based on elastic theory and assuming that the bond between steel and concrete is perfect stresses are estimated in reinforced cement concrete. The designer will aim at keeping these stresses as close to permissible stresses as possible but without exceeding them.

m=

Introduction !

The Limitation of this Method are
(i) It assumes stress strain relation for concrete is constant, which is not true. (ii) It gives the impression that factor of safety times the working load is the failure load, which is not true. This is particularly so in case of indeterminate structures. In these structures there will be redistribution of forces as plastic hinges are formed at critical sections. (iii) This method gives uneconomical sections.

The advantages of this method are
(i) It is simple (ii) Reasonably reliable and (iii) As the working stresses are low, the serviceability requirements are automatically satisfied and there is no need to check them. However this method has been deleted in IS 456 – 2000, but the concept of this method is retained for checking serviceability states of deflections and cracking. Hence knowledge of this method is essential and IS 456 – 2000 gives it in appendix. In this book, this method is briefly explained in Appendix B and few problems have been solved.

Load Factor Method (LFM) or Ultimate Load Method (ULM)
In this method ultimate load is used as design load and the collapse criteria used for the design. Ultimate load is defined as load factor times the working load. Hence it gives better concept of load carrying capacity of the structure. Its salient features are: 1. Uses actual stress strain curve and ultimate strain as failure criteria. 2. Redistribution of forces is accounted since it works in the plastic region. 3. It allows varied selection of load factors, a lower value for loads known with more certainty like dead load and higher value for a less certain loads like live load and wind loads. This method gives very economical sections. However it leads to excessive deformation and cracking. This phenomenon is more predominant when high strength deformed bars are used. Thus this method failed to satisfy the serviceability and durability criteria. To overcome these problems codes started adopting load factor method (LFM) in which load factors were modified. A load factor of 2 was used for design. Additional factor of safety of 1.5 was used for designed concrete mixes for calculating the permissible stresses to control serviceability requirement. As more and more research continued to investigate the problems related with ultimate load method it was felt instead of repairing this method there is need for comprehensive method to take care of design requirements from strength and serviceability criteria. This gave rise to limit state method and all codes replaced this method by limit state method. Hence there is no need to make a separate study of this method.

Limit State Method (LSM)
This is a comprehensive method which takes care of the structure not only for its safety but its fitness throughout the period of service of the structure. This method is thoroughly explained in chapters II and III and many structural elements are designed in the other chapters.