Objectives and Methods of Analysis and Design, and Properties of Concrete and
Steel
Reinforced concrete, as a composite material, has occupied a special place in
the modern construction of different types of structures due to its several
advantages. Due to its flexibility in form and superiority in performance, it
has replaced, to a large extent, the earlier materials like stone, timber and
steel. Further, architect's scope and imaginations have widened to a great
extent due to its mouldability and monolithicity. Thus, it has helped the
architects and engineers to build several attractive shell forms and other
curved structures. However, its role in several straight line structural forms
like multistoried frames, bridges, foundations etc. is enormous.
The design of these modem reinforced concrete structures may appear to be highly
complex. However, most of these structures are the assembly of several basic
structural elements such as beams, columns, slabs, walls and foundations (Anim.
1.1.1). Accordingly, the designer has to learn the design of these basic
reinforced concrete elements. The joints and connections are then carefully
developed.
Design of reinforced concrete structures started in the beginning of last
century following purely empirical approach. Thereafter came the so called
rigorous elastic theory where the levels of stresses in concrete and steel are
limited so that stress—deformations are taken to be linear. However, the limit
state method, though semi-empirical approach, has been found to be the best for
the design of reinforced concrete structures (see sec. 1.1.3.1 also). The
constraints and applicabilities of both the methods will be discussed later.
1.1.2 Objectives of the Design of Reinforced Concrete Structures
Every
structure has got its form, function and aesthetics. Normally, we consider that
the architects will take care of them and the structural engineers will be
solely responsible for the strength and safety of the structure. However, the
roles of architects and structural engineers are very much interactive and a
unified approach of both will only result in an ''Integrated'' structure, where
every material of the total structure takes part effectively for form, function,
aesthetics, strength as well as safety and durability. This is possible when
architects have some basic understanding of structural design and the structural
engineers also have the basic knowledge of architectural requirements. Both the
engineer and the architect should realize that the skeletal structure without
architecture is barren and mere architecture without the structural strength and
safety is disastrous. Safety, here, includes consideration of reserve strength,
limited deformation and durability. However, some basic knowledge of
architectural and structural requirements would facilitate to appreciate the
possibilities and limitations of exploiting the reinforced concrete material for
the design of innovative structures.
Before proceeding to the design, one
should know the objectives of the design of concrete structures. The objectives
of the design are as follows:
1.1.2.1 The structures so designed should have an acceptable probability of
performing satisfactorily during their intended life. This objective does not
include a guarantee that every structure must perform satisfactorily during its
intended life. There are uncertainties in the design process both in the
estimation of the loads likely to be applied on the structure and in the
strength of the material. Moreover, full guarantee would only involve more cost.
Thus, there is an acceptable probability of performance of structures as given
in standard codes of practices of different countries.
1.1.2.2 The designed structure should sustain all loads and deform within limits
for construction and use. | Anim.1.1.2 IAnim.1.1.3I Anim. 1.1.4 |
Adequate strengths and limited deformations are the two requirements of the
designed structure. The structure should have sufficient strength and the
deformations must be within prescribed limits due to all loads during
construction and use as seen in Anim. 1.1.2. Animation 1.1.3 shows the structure
having insufficient strength of concrete which falls in bending compression with
the increase of load, though the deformation of the structure is not alarming.

On the other hand, Anim. 1.1.4 shows another situation where the structure,
having sufficient strength, deforms excessively. Both are undesirable during
normal construction and use.
However, sometimes structures are heavily loaded beyond control. The
structural engineer is not responsible to ensure the strength and deformation
within limit under such situation. The staircases in residential buildings
during festival like marriage etc., roof of the structures during flood in the
adjoining area or for buildings hear some stadium during cricket or football
matches are some of the examples when structures get overloaded. Though, the
structural designer is not responsible for the strength and deformations under
these situations, he, however, has to ensure that the failure of the structures
should give sufficient time for the occupants to vacate. The structures, thus,
should give sufficient warning to the occupants and must not fail suddenly.
1.1.2.3 The designed structures should be durable. The materials of reinforced
concrete structures get affected by the environmental conditions. Thus,
structures having sufficient strength and permissible deformations may have
lower strength and exhibit excessive deformations in the long run. The designed
structures, therefore, must be checked for durability. Separate checks for
durability are needed for the steel reinforcement and concrete. This will avoid
problems of frequent repairing of the structure.
1.1.2.4 The designed structures should adequately resist to the effects of
misuse and fire. Structures may be misused to prepare fire works, store fire
works, gas and other highly inflammable and/or explosive chemicals. Fire may
also take place as accidents or as secondary effects during earthquake by
overturning kerosene stoves or lantern, electrical short circuiting etc.
Properly designed structures should allow sufficient time and safe route for the
persons inside to vacate the structures before they actually collapse.
All the above objectives can be fulfilled by understanding the
strength and deformation characteristics of the materials used in the design as
also their deterioration under hostile exposure. Out of the two basic materials
concrete and steel, the steel is produced in industries. Further, it is
available in form of standard bars and rods of specific diameters. However,
sample testing and checking are important to ensure the quality of these steel
bars or rods. The concrete, on the other hand, is prepared from several
materials (cement, sand, coarse aggregate, water and admixtures, if any).
Therefore, it is important to know the characteristic properties of each of the
materials used to prepare concrete. These materials and the concrete after its
preparation are also to be tested and checked to ensure the quality. The
necessary information regarding the properties and characteristic strength of
these materials are available in the standard codes of practices of different
countries. It is necessary to follow these clearly defined standards for
materials, production, workmanship and maintenance, and the performance of
structures in service.
1.1.3 Method of Design: Three methods of design are accepted in cl. 18.2 of IS
456:2000 (Indian Standard Plain and Reinforced Concrete — Code of Practice,
published by the Bureau of Indian Standards, New Delhi). They are as follows:
1.1.3.1 Limit state method: The term “Limit states” is of continental origin
where there are three limit states — serviceability / crack opening / collapse.
As mentioned in sec. 1.1.1, the semi-empirical limit state method of design has
been found to be the best for the design of reinforced concrete members. More
details of this method are explained in Module 3 (Lesson 4). However, because of
its superiority to other two methods (see sections 2.3.2 and 2.3.3 of Lesson 3),
IS 456:2000 has been thoroughly updated in its fourth revision in 2000 taking
into consideration the rapid development in the field of concrete technology and
incorporating important aspects like durability etc. This standard has put
greater emphasis to limit state method of design by presenting it in a full
section (section 5), while the working stress method has been given in Annex B
of the same standard. Accordingly, structures or structural elements shall
normally be designed by limit state method.

1.1.3.2 Working stress method This method of design, considered as the method of
earlier times, has several limitations. However, in situations where limit state
method cannot be conveniently applied, working stress method can be employed as
an alternative. It is expected that in the near future the working stress method
will be completely replaced by the limit state method. Presently, this method is
put in Annex B of IS 456:2000.
1.1.3.3 Method based on experimental approach
The designer may perform experimental investigations on models or full size
structures or elements and accordingly design the structures or elements.
However, the four objectives of the structural design (sec. 1.1.2) must be
satisfied when designed by employing this approach. Moreover, the engineer—in—
charge has to approve the experimental details and the analysis connected
therewith.
Though the choice of the method of design is still left to the designer as
per cl. 18.2 of IS 456:2000, the superiority of the limit state method is
evident from the emphasis given to this method by presenting it in a full
section (Section 5), while accommodating the working stress method in Annex B of
IS 456:2000, from its earlier place of section 6 in IS 456:1978. It is expected
that a gradual change over to the limit state method of design will take place
in the near future after overcoming the inconveniences of adopting this method
in some situations.
1.1.4 Analysis of Structures
Structures when subjected to external loads (actions) have internal reactions in
the form of bending moment, shear force, axial thrust and torsion in individual
members. As a result, the structures develop internal stresses and undergo
deformations. Essentially, we analyse a structure elastically replacing each
member by a line (with El values) and then design the section using concepts of
limit state of collapse. Figure 1.1.1 explains the internal and external
reactions of a simply supported beam under external loads. The external loads to
be applied on the structures are the design loads and the analyses of structures
are based on linear elastic theory (vide cl. 22 of IS 456:2000).
1.1.5 Design Loads: The design loads are determined separately for the two
methods of design after determining the combination of different loads.
1.1.5.1 In the limit state method, the design load is the characteristic
load with appropriate partial safety factor (vide sec. 2.3.2.3 for partial
safety factors). Fig.1.1.1: Simply supported beam
1.1.5.2 In the working stress method, the design load is the characteristic
load only. Characteristic load (CI. 36.2 of IS 456:2000) is that load which has
a ninety—five per cent probability of not being exceeded during the life of the
structure. The various loads acting on structures consist of dead loads, live
loads, wind or earthquake loads etc. The researches made so far fail to estimate
the actual loads on the structure. Accordingly, the loads are predicted based on
statistical approach, where it is assumed that the variation of the loads acting
on structures follows the normal distribution (Fig. 1.1.2).
Characteristic
load should be more than the average/mean load. Accordingly, Fig.1.1.2. Normal
distribution eunre
Characteristic load = Average/mean load + K(standard deviation for load) The
value of K is assumed such that the actual load does not exceed the
characteristic load during the life of the structure in 95 per cent of the
cases.
1.1.6Loads and Forces: The following are the different types of loads and forces
acting on the structure. As mentioned in sec. 1.1.5, their values have been
assumed based on earlier data and experiences. It is worth mentioning that their
assumed values as stipulated in IS 875 have been used successfully.
1.1.6.1 Dead loads: These are the self weight of the structure to be
designed. Needless to mention that the dimensions of the cross section are to be
assumed initially which enable to estimate the dead loads from the known unit

weights of the materials of the structure. The accuracy of the estimation thus
depends on the assumed values of the initial dimensions of the cross section.
The values of unit weights of the materials are specified in Part 1 of IS 875.
1.1.6.2 Imposed loads: They are also known as live loads (Anim. 1.1.5a) and
consist of all loads other than the dead loads of the structure. The values of
the imposed loads depend on the functional requirement of the structure.
Residential buildings will have comparatively lower values of the imposed loads
than those of school or office buildings. The standard values are stipulated in
Part 2 of IS 875.
1.1.6.3 Wind loads: These loads (Anim. 1.1.5a) depend on the velocity of the
wind at the location of the structure, permeability of the structure, height of
the structure etc. They may be horizontal or inclined forces depending on the
angle of inclination of the roof for pitched roof structures. They can even be
suction type of forces depending on the angle of inclination of the roof or
geometry of the buildings
(Anim. 1.1.5b). Wind loads are specified in Part 3 of IS 875.
1.1.6.4 Snow loads
These are important loads for structures located in areas
having snow fall, which gets accumulated in different parts of the structure
depending on projections, height, slope etc. of the structure (Anim. 1.1.6). The
standard values of snow loads are specified in Part 4 of IS 875.
Anim. 1.1.6
1.1.6.5 Earthquake forces
Earthquake generates waves which move from the
origin of its location (epicenter) with velocities depending on the intensity
and magnitude of the earthquake. The impact of earthquake on structures depends
on the stiffness of the structure, stiffness of the soil media, height and
location of the structure etc. (Anim. 1.1.7). Accordingly, the country has been
divided into several zones depending on the magnitude of the earthquake. The
earthquake forces are prescribed in IS 1893. Designers have adopted equivalent
static load approach or spectral method.
1.1.6.6 Shrinkage, creep and temperature effects Shrinkage, creep and
temperature (high or low) may produce stresses and cause deformations like other
loads and forces (Anim. 1.1.8, 9 and 10). Hence, these are also considered as
loads which are time dependent. The safety and serviceability of structures are
to be checked following the stipulations of cls. 6.2.4, 5 and 6 of IS 456:2000
and Part 5 of IS 875.
1.1.6.7 Other forces and effects: It is difficult to prepare an exhaustive
list of loads, forces and effects coming onto the structures and affecting the
safety and serviceability of them. However, IS 456:2000 stipulates the following
forces and effects to be taken into account in case they are liable to affect
materially the safety and serviceability of the structures. The relevant codes
as mentioned therein are also indicated below:
FIg.1.1.3: Foundation movement
Fig. 1.1.4: Erection loads
0 Foundation
movement (IS 1904) (Fig. 1.1.3)
o Elastic axial shortening Erection loads
(Please refer to IS 875 — Part 2) (Fig. 1.1.4) Stress concentration effect due
to point of application of load and the like.
1.1.6.8 Combination of loads: Design of structures would have become highly
expensive in order to maintain their serviceability and safety if all types of
forces would have acted on all structures at all times. Accordingly, the concept
of characteristic loads has been accepted to ensure that in at least 95 per cent
of the cases, the characteristic loads considered will be higher than the actual
loads on the structure. However, the characteristic loads are to be calculated
on the basis of average/mean load of some logical combinations of all the loads
mentioned in sec. 1.1.6.1 to 7. These logical combinations are based on (I) the
natural phenomena like wind and earthquake do not occur simultaneously, (ii)
live loads on roof should not be present when wind loads are considered; to name
a few. IS 875 Part 5 stipulates the combination of loads to be considered in the
design of structures.