DESIGN OF RCC ELEMENTS SESSION 5 PROF. YADUNANDAN

DESIGN OF RCC
ELEMENTS
SLABS, BEAMS, COLUMN, FOOTING, STAIRCASES, WATERTANKS
PROF C N YADUNANDAN
STRUCTURAL CONSULTANT &
RETD PROFESSOR OF SJCE,
MYSORE

INTRODUCTION – WHAT IS DESIGN
 Reinforced cement concrete has a history of over 120 years
 Today, it has seen a phenomenal development and it is the most widely used
construction material
 Over these years, there has been a tremendous change in design
understanding, that it has transformed the very meaning of design.
SO, WHAT IS DESIGN?
2

INTRODUCTION – WHAT IS DESIGN
 Does it mean sizing the structure, that it does not fail or collapse under
calculated load?
YES and NO
But it is still not the same.
 Firstly, we engineers came to realize that loads and material strengths are
not deterministic quantities but variables.
 Therefore safety/failure is also a probabilistic condition. We therefore have
to say that the objective of design is to size the structure such that there is a
very low probability of collapse occurring.
3

INTRODUCTION (CONTINUED...)
 Further, we cannot base our design on hypothetical concepts like stress but on
‘realistic’, ‘measurable’ / ’observable’ parameters like strain, displacement,
crack width etc
 And the design has to take care of other performance requirements such as
serviceability (limiting deflection and crack widths during service/working
loads), ductility, fire resistance, durability nature of failure, ductility, fire
resistance, water tightness etc.
 These conditions of performance are called ’limit states’ and the design has to
satisfy those conditions.
 Thus, in this limit state method, the important changes in approach are-
 From deterministic to probabilistic
 From hypothetical to real
4

 In this light, design is the process of fixing the shape, size and characteristics of
the structure to see that there is low probability of all these ‘limit states’ being
crossed.
 That way, for the strength limit state, we model a failure situation corresponding
to a ductile failure and consider highest expected actions (scaled up) have a low
probability of exceeding the lowest possible resistance (scaled down) of the
structure.
5

PROBABILISTIC DESIGN APPROACH
6

7
Total
105%
Formwork
60%
Material
Failure
15%
Overload
5%
Lack of
maintenance
10%
Foundation
Failure
10%
Accidents
5%
Probabilistic
approach is an
EYE OPENER
But, STATISTICS
can be used to
MISLEAD!
Failure Causes
OUR DESIGNS NEVER
FAIL!

 The section is designed i.e., the material strengths and quantities are provided
to take care of the limit state of collapse and checked for the deformations
during service i.e., the limit state of serviceability.
 Thus the designer has to have these major concerns while designing -
 Strength and Ductility : a very realistic model of the failure situation has
to be built up such that a ductile behavior is considered, so that there is
sufficient yield range giving warning of an impending failure. In a sense,
the design has to be controlled to react to forces in a specified manner.
 Redundancy : a robust rigid framing of components is aimed so that load
effect is resisted by a large number of sections and failure ‘mechanisms’
do not occur. 8

 Durability : In the structure it is necessary to protect the materials of the
structure from environmental conditions and see that they perform well over the
design life.
Protection to reinforcement - Corrosion of reinforcement is a serious concern
in RCC structures. Water is a strength giving elixir in green concrete and in the
hardening stages, whereas it is a poison for the concrete during service as it is
the agent of corrosion of rebar and carbonation. Concrete has to be made
dense and watertight to protect the reinforcement.
 Economy : Our objective is to do more with less material. Factors can be changed
with better model/better materials so that we economize.
 Constructability: the details of design must be suitable to construct it as desired 9

CODES
 The object of structural design is to fix the size and section details of the
structural components to make the structure take the specified loads
economically and safely over an acceptable time (design life).
 The design process involves the use of various thumb rules and design
constraints to have satisfactory performance. (i.e., control guidelines).
 This information is provided to the designer by the design codes which specify all
the design data required, thumb rules, control guidelines etc.
 Basically, the code is a compendium of all information needed for the analysis
and design. Its objective is to assist the designer by providing tabulated standard
values for easy computation and practical guidelines.
10

CODES
 Most countries have their own design code so that all designs conform to the
same standard guidelines over the region.
 In addition to the code, design aids and explanatory handbooks are most often
prepared to simplify and reduce repetitive calculations and reduce design time
 Here is a comprehensive list of such codes and handbooks-
11

CODES
 IS 456:2000 – Design of Plain and RCC Structures
 IS 875:1987 – Design Loads for Buildings and Structures
 IS 1343:2012 – Design of Prestressed Concrete
 IS 1893:2016 – (Part 1) – Earthquake Resistant Design of Structures
 IS 13920:2016 – Ductile Detailing of Reinforced Concrete Structures subjected
to seismic forces
 SP 16 : 1980 – Design Aids for Reinforced Concrete to IS 456
 SP 24: 1983 - Explanatory Handbook on Indian Standard Code of Practice for
Plain and Reinforced Concrete IS 456
 SP 34 : 1987 - Handbook on Concrete Reinforcement and Detailing 12

CODES
 The structural designer can also prepare his own excel sheet or design app (or
use available ones) in reducing the design effort.
 Present day standard design softwares are capable of analyzing, designing,
detailing and drafting structural details for a stated structural geometry,
material specification and load combination.
 Still a first hand experience of designing (at least using tables and Handbook)
manually for sometime will go a long way is making designer get the feel of
design and help in checking computer design outputs and other design
drawings.
13

RCC FRAMED STRUCTURE- MODES OF ACTION
 An RCC framed structure has 2 major modes of action
 Action under Gravity Loads
 Action under Lateral Loads
14

GRAVITY LOADS –
 Structure behaves like a multi legged chair with the slabs as the seating plank
transferring the loads to the frame->main beam, secondary beams-> Columns.
 Joint rigidities/flexibilities specified and adjusted so as to build maximum
redundancy/ continuity/ load resisted by a large number of cross sections
trying to provide as much rigidity as possible releasing or providing to avoid
undesirable or complex stress distributions and deformations.
 Loads specified and calculated based on the use of structure with distribution
are then stated
15

16
Moment is transferred from Beam to Column

LATERAL LOADS -
 For lateral loads (like wind and
earthquake), the loads act on the
sides and the building can be
considered to behave like a vertical
cantilever swaying sideways under
these loads. (mainly wind and seismic)
 OMRF, SMRF : Earthquake loads –
 Response spectrum
 Equivalent static load
17
Moment is transferred
from Column to Beams

RCC FRAME STRUCTURE
 Taking the loads from the slabs, beam transfer loads to columns wherein
they jointly resist the load by forming a rigidly jointed 2D/3D framework.
Typical RC floor systems are shown below -
1. One way Slab and Beam Framework
18

RCC FRAME STRUCTURE
2. Beam and girder framework (One way/Two way)
19

RCC FRAME STRUCTURE
3. Beamless (Flat slabs and Flat plates)
20
4. Grid Floor and Void Slabs

RCC FRAME STRUCTURE
 The columns transfer the load to the
foundation which in turn transfers to the
ground.
 We will now consider the design
approach for the design of the elements
of an RCC framed multi-storeyed
building
21
SLAB
BEAM
COLUMN
FOOTING
SOIL
LOAD PATH OF THE BUILDING

GENERAL PROCEDURE OF DESIGN– EXAMPLE
22
Considering a typical apartment
structure, the design process
starts with locating column
positions and beam lines.

23
GENERAL PROCEDURE OF DESIGN– COLUMN POSITION

GENERAL PROCEDURE OF DESIGN– BEAM LAYOUT
24

GENERAL PROCEDURE OF DESIGN– SLAB LOAD DISTRIBUTION
25
 Once column positioning
and beam lines are
acceptable, we first have
to assume concrete sizes
for columns, beams and
slab.
 These are done on the
basis of deformation limit
and other thumb rule
guidelines.

26
SLABS
 The vertical deflection limits may generally be assumed to be satisfied
provided that the span to depth ratios are not greater than the values
obtained as below:
(a) Basic values of span to effective depth ratios for spans up to 10m:
 Cantilever - 7
 Simply supported - 20
 Continuous – 26
(b) For spans above 10m,the values in (a) may be multiplied by 10/span in meters,
except for Cantilever in which case deflection calculations should be made.
(c) Depending on the area and the stress of steel for tension reinforcement, the
values in (a) or (b) shall be modified by multiplying with the modification factor
obtained.
PRELIMINARY SIZING OF STRUCTURAL COMPONENTS

27
(d) Depending on the area of compression reinforcement, the value of span to
depth ratio be further modified by multiplying with the modification factor
obtained.
 For slabs spanning in two directions, the shorter of the two spans should be
used for calculating the span to effective depth ratios.
 For two-way spanning slabs of shorter spans (up to 3.5m) with mild steel
reinforcement, the span to overall depth ratios given below may generally be
assumed to satisfy vertical deflection limits for loading class up to 3 kN/m2.
 Simply supported slabs – 35
 Continuous slabs – 40
For high strength deformed bars of grade Fe415, the values given above
should be multiplied by 0.8.

28

29
For two way slabs, as per ACI code,
Where, h is the thickness of the slab,
ln is the clear span,
h=
ln(0.8+
𝑓𝑦
20000
)
𝐹

NOTE: The guidelines for slab thickness in IS 456:2000 are being carried forward
from previous versions without much change. One can observe that in these
specifications
 One way ratios are based on effective depth.
 Two way ratios are based on overall depth.
 The specifications are for Fe250 steel with suggested modifications for
higher grade steels. In today’s practice Fe500 TMT steel is the most
common and Fe 250 is seldom used.
 Two way slab depth ratios are not mentioned for different boundary
conditions.
 It will be helpful to the designer if changes are made to suit present practice
conditions. Presently, it best way is to assume thicknesses equal to or near to
the specified ones and check for deflection after section determination.
30

31
BEAMS:
 Width equal to column/wall thickness.
 Limits as per Clause 23.2 of code, unlike for slabs, there are the
minimum most values for depth to limit deflection. These are the least
values.
 Based on span and functional/constructional requirements, span to
effective depth ratios - span/10 to span/15 is considered.

32
COLUMNS:
 Orientation deeper along framing/axis widths equal to wall thickness or
rebar placing convenience.
 Try to limit the column dimensions to a few standard sizes.

GENERAL PROCEDURE OF DESIGN – MATERIAL PROPERTIES
Material strengths for concrete and
steel are specified.
33
Grade of
Steel
Fe250
Fe415
Fe500
Fe550
Ordinary
Concrete
M10
M15
M20
High
Strength
Concrete
M60
M65
M70
M75
M80
Standard
Concrete
M25
M30
M35
M40
M45
M50
M55

GENERAL PROCEDURE OF DESIGN– 3D MODEL
A 3-dimensional structural model is
then generated.
 Joint coordinates
 Member nomenclature/end
coordinates
 Member incidences
 Section properties
 Material specifications
 Support and joint restraint
specifications
These will define the structure
34

GENERAL PROCEDURE – LOADS & LOAD COMBINATIONS
 On the basis of occupancy and
location, the loads are then
specified in load data (IS 875) and
self weight is obtained by the
knowledge of sizes.
 Then various critical load
combinations are considered and
the structure is analyzed for the
combinations
 For the member forces obtained,
the reinforcement content for
beams and columns are obtained
by member design
35
FOR STRENGTH
1) 1.5 (DL+LL)
2) 1.2 (DL+LL+EQX/WX)
3) 1.2 (DL+LL-EQX/WX)
4) 1.2 (DL+LL+EQY/WY)
5) 1.2 (DL+LL-EQY/WY)
6) 1.5 (DL+EQX/WX)
7) 1.5 (DL-EQX/WX)
8) 1.5 (DL+EQY/WY)
9) 1.5 (DL-EQY/WY)
10) 0.9 DL+1.5EQX/WX
11) 0.9 DL-1.5EQX/WX
12) 0.9 DL+1.5EQY/WY
13) 0.9 DL-1.5EQY/WY
FOR SERVICIBILITY
1) DL+LL
2) DL+LL+EQX/WX
3) DL+LL-EQX/WX
4) DL+LL+EQY/WY
5) DL+LL-EQY/WY
6) DL+EQX/WX
7) DL-EQX/WX
8) DL+EQY/WY
9) DL-EQY/WY

GENERAL PROCEDURE OF DESIGN - MEMBER FORCES
36
 The frame is then analyzed to
obtain joint and member forces,
displacements and reactions.
 Equilibrium and displacement
checks can then be performed to
ascertain the correctness of the
results.

GENERAL PROCEDURE OF DESIGN - MEMBER FORCES
37
ETABS DESIGN RESULTS SHOWING BEAM REBAR PERCENTAGE (PLAN)

38
COLUMNS – EXAMPLE BUILDING REBAR DETAILS
ETABS DESIGN RESULTS SHOWING BEAM AND COLUMN REBAR PERCENTAGE (ELEVATION)

SLABS
 Slabs are highly redundant continuous surfaces which behave like plates but
designed on a simplified basis like a wide beam.
 They resist loads mainly as flexural elements. They transfer load to the beams
 Types
 One-way
 Simply Supported
 Continuous Slab
 Two-way
 Simply supported
 Continuous
 Cantilever/Overhang
For manual design of one way and two way slabs, moments can be obtained using
design moment tables of IS 456:2000 41

SLABS - TYPES
42
ONE WAY SIMPLY SUPPORTED SLAB

SLABS - TYPES
43
ONE WAY CONTINUOUS SLAB

SLABS - TYPES
44
ONE WAY CONTINUOUS SLAB

SLABS - TYPES
45
TWO WAY SIMPLY SUPPORTED SLAB

SLABS - TYPES
46
TWO WAY SIMPLY SUPPORTED SLAB

SLABS - TYPES
47
TWO WAY CONTINUOUS SLAB

SLABS - TYPES
48
TWO WAY CONTINUOUS SLAB

SLABS - TYPES
49
CANTILEVER/OVERHANG SLAB

SLABS- REINFORCEMENT DETAILS
50
ONE-WAY SIMPLY
SUPPORTED SLAB

51
TWO-WAY SIMPLY
SUPPORTED SLAB

SLABS- TWO WAY SLAB CORNER REINFORCEMENT
52
 The two directional bending curls up the
corners creating torsion.
 The corners will have to be held-down to
prevent curling up which creates torsional
forces. These effect is to create bending at the
fold line at the bottom and perpendicular to the
fold line at the top reinforcement has to be
provided to prevent cracking.
 Otherwise the slab has to be stiffened
providing higher thickness and positive
reinforcement so that the curling deformations
are small.

53
TWO-WAY CONTINUOUS SLAB

SLABS- ADVANTAGE OF CURTAILED BARS
54
 The main bars are used to resist bending of concrete beam due to bending
moments (BM) & magnitude & direction of BM changes throughout the span of
the beam & therefore required quantity of main steel also varies accordingly.
 The BM of beam is positive near the center (bottom) & hence more steel is
required at the bottom & theoretically no steel at the top center is required.
 Similarly BM is negative near the supports (top) & more steel is required at the
top near the supports & no steel at bottom. Therefore to minimize the wastage
of steel, at these zones of BMs only hanger bars (corner bars) are provided.
 By providing curtailed bars, the reinforcement for bottom steel at mid-span
and top steel at supports may have different spacing.

55
CANTILEVER SLAB

SLABS- REINFORCEMENT DETAILS (EXAMPLE)
56
Advantage to be
taken of both
two way action
and continuity.
For Detailing most
important point to
remember – Maximum
Rebar length is 12m

SLABS – CONTROL SPECIFICATION
57
 Minimum Percentage of Steel for Slabs -
 0.15% for Mild Steel Reinforcement
 0.12% for High Strength deformed bars
 The diameter of reinforcing bars shall not exceed 1/8th of the total thickness
of the slab.
 Nominal cover to meet Durability Requirements-
Exposure
Cover (mm),
not less than
Mild 20
Moderate 30
Severe 45
Very Severe 50
Extreme 75

SLABS – CONTROL SPECIFICATION
58
 Minimum spacing between bars –shall usually be not less than the greater of
the following -
1) The diameter of the bar if the diameters are equal
2) The diameter of the larger bar if the diameters are unequal
3) 5mm more than the nominal maximum size of coarse aggregate
 Maximum spacing between bars -
(a) shall not be more than 3d or 300mm whichever is smaller
(b) shall not be more than 5d or 450 mm whichever is smaller, against
shrinkage and temperature
 Ductile detailing like additional anchoring for slab bars, higher lap lengths etc
can be adopted when the entire design follows special earthquake resistant
provisions

SLABS - PARTITION WALL LOADS AND OTHER POINT LOADS
59
 Partition wall loads and
other point loads on slabs
can be specified in the
program itself.
 If we have to design them
manually, codal provisions
specify how these loads
can be distributed by using
Pigeaud curves.
 Also, for concentrated
loads on slabs, Clause
24.3.2 shall be followed.

SLABS – VOIDED/FILLER SLABS
60

SLABS – NON RECTANGULAR SLABS
61
 Non rectangular slabs like
circular slabs, triangular
slabs can be designed
using Yield Line Method of
Analysis

SLABS – STRIP METHOD OF ANALYSIS
 For varied boundary conditions
like three edge supported, slabs
supported at corners, slab with
cut outs/openings etc, the strip
method is a very useful design
method.
 It is advantageous for designers
to apply it for such conditions
62

SLABS - SHUTTERING FAILURES
67
 The main causes of formwork failure are given under:
 Inappropriate stripping and shore exclusion,
 Lack of awareness to formwork deep details,
 Uneven soil under a muddy area,
 Shaking or vibration,
 Insufficient bracing,
 Inadequate arrangement of concrete placement
WORD OF CAUTION – TAKE CARE OF FORMWORK, MOST SLAB FAILURES ARE DUE
TO FORMWORK FAILURE

SLABS
68
 CANTILEVER ISSUES
 OVERHANG LIMITS
 ENSURING TOP BARS TO STAY AT TOP
 CONSIDERING MINIMUM LINE LOAD AT FREE END
 ELECTRICAL LINES : NOT TO AFFECT CONCRETE CONTINUITY
 FAN BOXES IN ROOF: POTENTIAL LEAKAGE POINTS

BEAMS
 Slabs transfer the load to beams. Beams resist these forces by flexural action.
 They are rigidly connected to the columns with hogging moments at support
and sagging moments at span.
 Because of smaller cross section beam sections should also resist
considerable shear and are provided with shear reinforcement.
 Types of beams-
 Simply supported Beams,
 Framed Beams
 Continuous Beams
 Cantilever Beams
 Main Beams
 Secondary Beams 70

BEAMS – MAIN BEAM AND SECONDARY BEAM
73
In a frame, moments are
developed at the joints are shared
between the beams and columns
meeting at that point based on
their relative stiffnesses.

BEAMS - SECTIONS
 DESIGN CRITERIA: section designed on strength basis for flexure and
shear. Design means determination of the cross section of concrete and
steel to have a ductile flexural failure and providing shear reinforcement to
resist shear and to have deflection at service loads within limits.
74

BEAMS- STRESS BLOCK PARAMETERS
75

BEAMS - SINGLY REINFORCED AND DOUBLY REINFORCED SECTION
76

BEAMS – T-SECTION ACTION IN SPAN
77
 In RCC construction, slabs and beams are cast monolithically. The portion of the
slab which acts integrally with the beam to resist loads could be called
as Flange of the T-beam. The portion of the beam below the flange acts as
Web of the T- beam.
 The flange of the beam (part of the slab) contributes in resisting compression by
adding more area of concrete in compression zone. This results in increasing
moment of resistance of the beam section.
 However, if the flange is located in tension zone, the concrete of the flange is to
be neglected (cracked) and beam is treated as a rectangular beam.

BEAMS – CRITICAL SECTION FOR MOMENT
78
The moments computed at the
face of supports shall be used
for design of members at
those supports.

BEAMS – CRITICAL SECTIONS FOR SHEAR
79
COLUMN BEAM
JUNCTION SUBJECTED
TO UDL
COLUMN BEAM
JUNCTION SUBJECTED
TO POINT LOAD ON ONE
SIDE
BEAM-BEAM JUNCTION

BEAMS – CRITICAL SECTIONS FOR SHEAR
80
BEAM FRAMING INTO
SUPPORTING MEMBER IN
TENSION
BEAMS LOADED NEAR
THE BOTTOM (AS IN CASE
OF INVERTED BEAM)
FOR BRACKETS AND
CORBELS

BEAMS – CONTROL SPECIFICATIONS
81
 MINIMUM TENSION REINFORCEMENT– shall be not less than 0.85bd/fy
 MAXIMUM TENSILE REINFORCEMENT – shall not exceed 0.04bD
 MAXIMUM COMPRESSION REINFORCEMENT – shall not exceed 0.04D
 SIDE FACE REINFORCEMENT – When depth of beam exceeds 750mm, side face
reinforcement along two faces shall be provided. Total area of side
reinforcement shall not be less than 0.1% of web area and be distributed
equally on two faces with spacing not exceeding 300mm or web thickness
whichever is less.

82
 MAXIMUM SPACING OF SHEAR REINFORCEMENT – shall not exceed 0.75d for
vertical stirrups and d for inclined stirrups. In no case it should exceed 450mm
 MINIMUM SHEAR REINFORCEMENT –
𝐴𝑠𝑣
𝑏𝑠𝑣
≥
0.4
0.87 𝑓𝑦
When nominal shear stress in beams (ζv) is less than the nominal shear stress of
concrete (ζc), minimum shear reinforcement should be given. When ζv> ζc, shear
reinforcement is calculated based on the below formula,
Vus =
0.87 fy Asv d
Sv
- for vertical stirrups
where Vus=Vu-Vc and Vc is the function of longitudinal rebar percentage and
Grade of concrete.

BEAMS – TORSION
83
 Two types of torsion
a)Equilibrium Torsion
b)Compatibility Torsion
 EQUIVALENT SHEAR - 𝑉𝑒 = 𝑉𝑢 + 1.6
𝑇𝑢
𝑏
 EQUIVALENT BENDING MOMENT - 𝑀𝑒1 = 𝑀𝑢 + 𝑀𝑡,
Torsional Moment, 𝑀𝑡 = 𝑇𝑢(
1+
𝐷
𝑏
1.7
)
 If the numerical value of Mt exceeds Mu, longitudinal reinforcement shall be
provided on flexural compression face, such that the beam can withstand Me2,
where 𝑀𝑒2 = 𝑀𝑢 − 𝑀𝑡
 Transverse reinforcement area shall be given by - 𝐴𝑠𝑣 =
𝑇𝑢𝑆𝑣
𝑏1𝑑1(0.87𝑓𝑦)
+
𝑉𝑢𝑆𝑣
2.5𝑑1(0.87𝑓𝑦
 MINIMUM TOTAL TRANSVERSE REINFORCEMENT –
𝐴𝑠𝑣 ζ𝑣𝑒−ζ𝑐 𝑏𝑆𝑣
𝑏𝑠𝑣
≥
0.4
0.87 𝑓𝑦

84
Exposure Cover (mm), not
less than
Mild 20
Moderate 30
Severe 45
Very Severe 50
Extreme 75
 NOMINAL COVER to meet Durability requirements :
 MINIMUM SPACING BETWEEN BARS - shall usually be not less than the greater
of the following -
1) The diameter of the bar if the diameters are equal
2) The diameter of the larger bar if the diameters are unequal
3) 5mm more than the nominal maximum size of coarse aggregate

85
 MAXIMUM DISTANCE BETWEEN BARS - The distance between parallel
reinforcement bars, or groups, near the tension face of a beam shall not be
greater than the value given below, depending on the amount of
redistribution carried out in analysis and the characteristic strength of the
reinforcement.
Clear distance between Bars (Table 15 of IS 456:2000)
fy Percentage Redistribution to or from Section Considered
(N/mm2
) -30 -15 0 .+15 .+30
Clear Distance between Bars (mm)
250 215 260 300 300 300
415 125 155 180 210 235
500 105 130 150 175 195

BEAMS – REINFORCEMENT DETAILING
CURTAILMENT RULES FOR CONTINUOUS BEAMS
86

87
CURTAILMENT RULES FOR SIMPLY SUPPORTED BEAMS

88
CURTAILMENT RULES FOR CANTILEVER BEAMS

89
 DEVELOPMENT LENGTH – The calculated tension or compression in any bar at
any section shall be developed on each side of the section by an appropriate
development length or end anchorage or by a combination
𝐿𝑑 =
ɸσ𝑠
ζ𝑏𝑑
 The development length is calculated based on diameter of the bar, stress in
the bar at section considered for design and design bond stress.
 The development length includes anchorage values of hooks in tension
reinforcement.
 The development length should be sufficient to develop the stress in the
bar beyond bond.
 The development length of each bar of bundled bars shall that for the
individual bar, increased by 10% for two bars, 20% for three bars in contact
and 33% for four bars in contact.

90
 SPLICING – the splicing shall be done as far as possible from the sections of
maximum stress and be staggered.
 It is recommended that splices on flexural members should not be at
sections where the bending moment is more than 50% of the moment of
resistance.
 Not more than half the bars shall be spliced. The straight length of lap shall
be greater than 15ɸ or 200mm
 Lap length with anchorage hooks in flexural tension shall be Ld or 30ɸ,
whichever is greater.
 Lap length for direct tension be 2Ld or 30ɸ, whichever is greater.
 Lap length in compression shall be Ld , but not less than 24ɸ.

91

BEAMS – JUNCTION DETAILS
COLUMN-BEAM JUNCTION
92

MAIN BEAM - SECONDARY BEAM
93
BEAMS – JUNCTION DETAILS

BEAMS – OPENING DUCTS IN BEAMS
94

BEAMS - DUCTILE DESIGN CONSIDERATIONS
DIMENSIONING -
 Minimum b/d - 0.3
 Minimum width - 200mm
 Maximum depth=1/4th clear span
 b of beam ≤ b of supporting
member plus distance on either
side of member.
 Minimum steel=#2-12ɸ
 Minimum Longitudinal Steel Ratio,
𝑚𝑖𝑛 = 0.24
𝑓𝑐𝑘
𝑓𝑦
 max - 0.025 95

LONGITUDINAL REINFORCEMENT -
 Steel at bottom face of a beam shall
be at least half the steel at top face.
 Steel at top and bottom shall be at
least 1/4th of steel provided at top
face of beam.
 At exterior joint, top and bottom bars
of beams shall be provided with
anchorage length beyond inner face
of column.
96

TRANSVERSE REINFORCEMENT -
 Only vertical links shall be used in beams.
 Minimum diameter of link shall be 8mm.
 Shear force capacity of the beam shall be
more than the larger of
 Factored shear force as per linear
structural analysis.
 Factored shear force, plus
equilibrium shear force when plastic
hinges are formed at both ends of
the beam. 97

 HEAD ROOM ISSUES
 WIDE BEAM
 MYTH OF CONCEALED BEAMS
 LONG SPAN BEAMS ; REBAR SPLICING ISSUES
98
BEAMS

 Columns are the most crucial components in a building safety and stability as
column failure initiates building collapse.
 All the loads are accumulated and transferred to the foundation through the
columns.
 Apart from this, they are the main elements resisting lateral loads and lateral
displacements should be kept to a small minimum for a satisfactory
occupancy.
 Important Consideration: Substantial reduction in load carrying capacity occurs
due to -
 Eccentric load transfer
 Slenderness
 Moment magnification due to lack of straightness.
 Columns are kept short to maximize their compressive load capacity. 100
COLUMNS

 Maintaining concentricity of load transfer being difficult with the construction
procedure, minimum eccentricity must always be considered for calculating
axial load capacity.
 Minimum Eccentricity, e is calculated as unsupported length/500 + lateral
dimensions/30, subject to a minimum of 20mm.
 The eccentricity factor should be incorporated in the strength assessment
equation.
 That way, the ultimate axial strength equation of an RCC column is factored
down to -
Pu=0.4 fck Ac + 0.67 fy Asc,
Wherein only 40% of concrete capacity and 67% of steel capacity are
considered.
101
COLUMNS

102
COLUMNS – MOMENT INTERACTION CURVES
FOR COLUMN SUBJECTED
TO COMBINED AXIAL AND
UNIAXIAL BENDING

103
COLUMNS – MOMENT INTERACTION CURVES
FOR COLUMN SUBJECTED
TO COMBINED AXIAL AND
BIAXIAL BENDING

COLUMNS
 Most buildings with one way slabs consist of mainframes spaced at regular
intervals so that significant moment comes from the framing beams and the
moment in the other direction (minor axis) is quite small.
 As the moment increases, the axial load capacity reduces. Considering this,
the model of ultimate condition utilizes the plastic centroid concept and it is
possible to express the load and moment factors as Pu/fckbd and Mu/fckbd2,
which determine the columns resistance.
 For easy computation, in the manual design level we had interaction charts to
assist us find the steel quantity to take the loads (SP 25 or Torsteel design
handbook).
 Later, spreadsheets or excel sheets were used in the computer domain with
the total design software they are built into the program. 104

 Similarly, two way slab systems transfer comparable moments in both
directions then a similar procedure for biaxial design is used.
 It is a good practice to keep the columns short (le/D<12). Though it is possible to
design slender columns, they are provided only when essential, as the capacity
reduction will be substantial.
 The second performance criteria for design is the lateral drift (both maximum
and story wise). This has to be kept at very small values for satisfactory
occupancy and stability purposes.
 The storey-drift in any storey due to the minimum specified design lateral
force, with partial load factor of 1.0. shall not exceed 0.004 times the storey
height.
 Dimension and Area controls for performance assurance. A small width like
105
COLUMNS

 The unsupported length between end restraints shall not exceed 60 times the
least lateral dimension of the column.
 If, in any given plane, one end of a column is unrestrained, its unsupported
length shall not exceed 100b²/D.
 To determine slenderness ratios, effective length factors (K) should be
considered (of course, slender columns if needed can be designed considering
P-delta effects with substantial concrete and steel sectional areas)
 In the absence of more exact analysis for K factor calculation, Annexure E shall
be used.
 The column effective length is decided based on the relative lateral
displacement of the ends of the column (sway or non-sway) 106
COLUMNS – SLENDERNESS AND EFFECTIVE LENGTH

COLUMNS - SLENDERNESS AND EFFECTIVE LENGTH
 To determine whether a column is a sway and no sway column, stability index
Q shall be calculated.
𝑄 =
𝑃𝑢 ∆𝑢
𝐻𝑢 ℎ𝑠
 It is calculated based on the axial loads on all columns, first order deflection
(elastically computed), total lateral force acting within storey and height if the
storey.
 If Q≤ 0.04, the column in the frame may be taken as no sway column, if not
then as sway column.
 For idealised conditions, Table 28 of IS 456: 2000 (Annexure E) can be used.
107

EFFECTIVE LENGTH FACTORS
FOR A COLUMN WITH NO SWAY
108
EFFECTIVE LENGTH FACTORS
FOR A COLUMN WITH SWAY

109

110
COLUMNS – EXAMPLE BUILDING REBAR DETAILS
ETABS RESULTS SHOWING BEAM AND COLUMN REBAR PERCENTAGE

COLUMNS – CONTROL SPECIFICATIONS
111
 LONGITUDINAL REINFORCEMENT –
 Minimum - 0.8% -for crack width control, to resist direct stress based on
area of concrete
 Maximum – 6% . The use of 6% may involve practical difficulties in
placing and compacting concrete. Hence a lower percentage of 4% is
considered. Wherever it is not possible to reduce reinforcement below
4%, self flowing concrete can be used
 MINIMUM NUMBER OF BARS – 4 FOR RECTANGULAR COLUMNS AND 6 FOR
CIRCULAR COLUMNS.
 MINIMUM BAR SIZE – Φ12mm
 Spacing between the longitudinal bars shall not exceed 300mm.

112
 TRANSVERSE REINFORCEMENT –
 Pitch – the pitch shall be not more than the
least of the following
 The least lateral dimension
 16times the smaller diameter of
longitudinal rebar used
 300mm
 Diameter – the diameter of the polygonal
links or lateral ties shall be not less than
1/4th of the largest longitudinal rebar
diameter and in no case less than 16mm.
 Stirrups are provided in 2L, 4L etc to avoid
longitudinal bar buckling.

113
 NOMINAL COVER –
 Minimum cover of 40mm shall be provided for column reinforcement
for both durable and fire resistance condition
 In case of columns with dimension less than 200mm or under, whose
rebars do not exceed Φ12mm, minimum cover of 25mm shall be
provided.
Note: in case of pedestals (length not exceeding 3b or 3D), 0.15% Ag of
longitudinal steel shall be provided.

COLUMNS – SPLICING
114
 Splicing shall be staggered
and should be avoid joints
and junctions.
 Mechanical couplers are
used to join lengths of
rebar together.

COLUMNS - DUCTILE DESIGN CONSIDERATIONS
DIMENSIONING -
 Minimum dimension od the column
shall not be less than –
 15 to 20 times the diameter of
largest reinforcement diameter
passing through the column.
 300mm
 The cross section aspect ratio shall
not be less than 0.45
115

 STRONG COLUMN WEAK BEAM – At
each column-beam junction of the
frame, nominal design strength of
columns at each principle plane,
shall not be at least 1.4 times the
combined nominal design strength
of all beams connecting at the joint
in the same plane.
 The longitudinal and transverse
reinforcement conforming IS
456:2000 specification
116
COLUMNS - DUCTILE DESIGN CONSIDERATIONS

 STILT FLOOR / SOFT STOREY
 COLUMNS – MINIMUM DIMENSION ISSUES
 CASTING RETAINING WALL WITH COLUMNS FOR CELLARS
 THICK AND THIN COLUMNS (6” DEVELOPMENT)
 SUPPORT CONDITION ;HINGED / FIXED, Better to have pedestal + plinth beam
preventing transfer of moments to footings.
117
COLUMNS

ISOLATED FOOTING
 Isolated footings are like pads/shoes where load is distributed and transferred
to the soil causing settlement within very small limits.
 The soil pressure bends the footing up causing flexure at the bottom. As the
magnitude of the soil pressure is high, it causes higher shear stresses.
 The concentrated load from the column punches the footing causing punching
shear stresses.
 The footing shall be designed to sustain the loads that are induced and to
ensure that any settlement which may occur shall be nearly as uniform as
possible and the SBC of soil is not exceeded.
 CONRETE PLACING – Casting shall be done with side shuttering.
 Step footing is favored than Tapered top footing.
 Top reinforcement in footing slab shall be considered when vibration is
involved.
119

ISOLATED FOOTING – CONTROL SPECIFICATIONS
 MINIMUM THICKNESS OF FOOTING – 300mm
 THICKNESS AT THE EDGE OF FOOTING – in RCC
and plain concrete footings, the thickness at
the edge shall be 150mm for footing on soils
 CRITICAL SECTION FOR MOMENT –
 Critical section for footings that will
support a column or a wall at the face of
the column or wall.
 Critical section for footings, which
supports a masonry wall is found at a
distance of (b/4). Here, the term b is wall
width.
120

 CRITICAL SECTION FOR ONE WAY AND TWO
WAY SHEAR –
 Footing essentially acts as wide beam,
with diagonal crack (one way shear)
potential across the width. Critical
section for this condition is taken as d
from the column face
 Two way action of footing, with potential
diagonal cracking along surface of
pyramid around the point load. The
critical section for this action is
considered at d/2 from the column face 121

TENSILE REINFORCEMENT -
 In one-way reinforced footing and two
way reinforced square footing, the
reinforcement extending in each
direction shall be distributed uniformly
across the full width of the footing.
 For two-way rectangular footing,
reinforcement in long direction shall be
distributed uniformly across full width.
For rebars in short direction, a central
band width of the footing shall be marked
for which reinforcement is calculated
based on the β 122

 MINIMUM REINFORCEMENT-
 Minimum reinforcement and spacing shall be as per the requirements of solid
slab
 The nominal reinforcement for concrete sections of thickness greater than
1m shall be 360mm2 per metre length in each direction on each face.
PERMISSIBLE SHEAR STRESS -
 When shear stress is not provided, the calculated shear stress at the critical
section shall not exceed ks ζc,
where ks=(0.5+βc), but not greater than 1, βc being the ratio of short side to
long side of the column.
ζc= 0.25 𝑓𝑐𝑘 in limit state and ζc= 0.16 𝑓𝑐𝑘 in working state method
123

124
ISOLATED FOOTING – EXAMPLE BUILDING FOOTING

125
ISOLATED FOOTING – EXAMPLE BUILDING FOOTING

STAIRCASE
 Stairs with waist slab
 Stairs with stringer beams
 Cantilever from beam or wall.
 tread-riser/waist-less/saw tooth
 Free standing stairs
127
 L-shaped stairs
 Dog legged stairs
 Open well stairs
 Spiral/Helical stairs

STAIRCASE
 Over time, certain standard types of stairs depending on convenience are
used in buildings. Usually, they are analyzed independently of the main
structure.
 Stairs with waist slab are analyzed as inclined slabs.
 Various approximate analysis methods and design charts based on it
(Reynold’s handbook) were available earlier.
 Now, difficult forms like saw tooth, helical stairs, free standing etc are
analyzed using computer modelling.
 DEPTH OF SECTION – The depth of section shall be taken as the minimum
thickness perpendicular to the soffit of the staircase 129

STAIRCASE – STAIRCASE HANBOOK DESIGN DATA
130
 FREE STANDING STAIRCASE -

STAIRCASE - STAIRCASE HANBOOK DESIGN DATA
131
 SAW TOOTH STAIRCASE-

STAIRCASE - STAIRCASE HANBOOK DESIGN DATA
132
 HELICAL STAIRCASE-

STAIRCASE – CONTROL SPECIFICATIONS
133
EFFECTIVE SPAN OF STAIRS –
 Where supported at top and bottom risers by
beams spanning parallel with the risers, the
distance between centre to centre of beams.
 Where spanning on to the edge of a landing slab
which spans parallel, with the risers, a distance
equal to the going of the stairs plus at each end
either half the width of the landing or one metre,
whichever is smaller.
 Where the landing slab spans in the same direction as the stairs, they shall be
considered as acting together to form a single slab and the span determined as
the distance centre-centre of the supporting beams or walls, the going being
measured horizontally.

STAIRCASE – CONTROL SPECIFICATIONS
134
DISTRIBUTION OF LOADING ON STAIRS –
 In the case of stairs with open wells, where
spans partly crossing at right angles occur, the
load on areas common to any two such spans
may be taken as one-half in each direction.
 Where flights or landings are embedded into
walls for a length of nor less than 110 mm and
are designed to span in the direction of the
flight, a 150mm strip may be deducted from
the loaded area and the effective breadth of
the section increased by 75 mm for purposes
of design

STAIRCASE – REINFORCEMENT DETAILS
135
STAIRCASE SUPPORTED AT END OF LANDING ; SHOWING THE POSITION OF MAIN
REINFORCEMENT -

136
STAIRCASE SUPPORTED AT END OF FLIGHTS ; SHOWING THE MAIN REINFORCEMENT -

137
CROSS SECTION DETAILS OF A SINGLE SPAN STRAIGHT FLIGHT SUPPORTED ON BRICK
WALLS-

WATER TANKS
 Structurally, RCC structures work out to be very economical for making
containers for water. Depending on the level of placement of the container,
we can classify RCC water tanks as –
 Water Tanks below Ground Level - underground tanks or sumps - fully
below GL or partly.
 Water Tanks placed on ground
 Overhead water heads (OHT) where tanks are placed over a staging.
 Rectangular, square and circular plan shapes are used. Based on the need,
the tanks would be open or closed.
139

WATER TANKS
 For relatively shallow heights, the wall resist the water load as a cantilever.
For small plan sizes- the cantilever walls will act as closed boxes -
rectangular/circular.
 Circular walls taking only hoop tension are most efficient and economical.
 Hoop tension is easily computed for circular tanks, moment distribution with
edge fixity for rectangular tanks can be computed using tables of IS 3370.
140

WATER TANKS
 The predominant design load will be ground earth pressure with the tank empty
for tanks below GL i.e., tension in the outer fiber.
 The maximum load for tanks above GL will be tank full case with water pressure
from inside.
 When fixed at base, the bottom 1m height can be considered as a cantilever. Such
horizontal spanning tank walls can be fixed to base or hinged to base.
 Floor is normally reinforced to take care of continuity and takes forces of uniform
pressure from continuity and loads transferred from walls.
 Due to the affectability of reinforcement with compression and the requirement
of holding the stored liquid without leakage and changes in exposure condition
due to changing levels of storage, there will be higher need to make concrete
impervious and also to make provisions to protect reinforcement.
144

WATER TANKS
 Overhead water tanks contain a braced staging and cylindrical wall with flat or
domed slabs for base and roof.
 Certain Standard geometrics of OHTs : funnel shaped tank with full vertical
cylindrical shell shaft staging is also one structural system commonly used.
 Lateral load analysis for wind and earthquake loads become very important in
seismically active regions for OHTs with higher staging heights.
 The staging will have to be made stiffer in such cases. The staging being
exposed to weather, the same considerations as to the vessel will have to be
used for design of the staging members also.
145

WATER TANKS
 With various types of liquid storage requirements arising there has been a
high use of RCC for such uses in industry.
 Further with the increased necessity of STP’s and ETP’s more severe
exposure conditions are encountered.
 Over the past 50 years, a vast majority of water retaining structures have
become unserviceable and many severely damaged due to exposure and
corrosion.
 Of course many of them were built before introducing present day strength
provisions.
146

WATER TANKS
 But still, the method of construction and design approach need a relook as it
is difficult to ensure good performance with the construction methods mainly
due to
 Inevitable joint in walls
 Difficulty in working and inspection at heights.
 Junction with inlets and outlets.
 Complexity of formwork.
147

WATER TANKS
 It is necessary to shift the focus on improving waterproofing specifications
and make financial provisions in estimate for that. I personally consider that
porosity of concrete due to improper compaction and reinforcement
obstruction as well as the crack sizes appearing in joints between lifts are
main causes of non-performance and deterioration. It is impossible to
contain crack widths by design as they are a function of construction.
 Further, no tension design needs highly uneconomical quantities of concrete
(large thicknesses adding to self weight also) and steel also as stresses in
steel are kept low. STPs and ETPs need better protection as they contain a lot
of deleterious material.
148

WATER TANKS
 Best practice suggestions are -
 Design on working stress basis.
 Use higher concrete strengths (above M30) and MS/TMT steel with lower
steel tensile stresses than in buildings. Use stringent waterproofing
specifications using present day chemicals. (making provisions for that
cost in the estimates)
 Construct walls in one stage using SFC without joints.
 Make more detailed waterproof joints for inlet, outlet pipes etc
 Consider pre-stressing options: It is possible to develop easy pre
tensioning measures in construction with a small increase in cost.
Tensioning will close the crack.
149

WATER TANKS
 It will therefore be necessary to see that the crack width in concrete are very
small. This is achieved by keeping lower tensile stress levels in concrete.
Higher covers are needed to protect reinforcement. Higher minimum steel
percentages are needed. Higher grades of concrete are preferred. Rebar
spacing are controlled. These factors have decided the control clauses in IS
3370.
150

WATER TANKS – CONTROL SPECIFICATIONS
MINIMUM REINFORCEMENT –
 For walls, floors and roofs, in each of two directions at right angles, within each
surface zone-
 0.35% for high strength deformed bars, can be reduced to 0.24% for span
not more than 15m
 0.64% mild steel reinforcement, can be reduced to 0.40% for span not more
than 15m
 In wall slabs less than 200 mm in thickness, the calculated amount of
reinforcement may all be placed in one face .
 For ground slabs less than 100 mm thick, the calculated reinforcement should be
placed in one face as near as possible as the upper surface consistent with the
nominal cover.
 Bar spacing should generally not exceed 300mm or the thickness of the section,
whichever is less.
151

WATER TANKS – TYPICAL DETAILING
152

ANOTHER STUDY EXAMPLE
BUDDHA STATUE:
We are providing structural
details for a medium sized
Buddha Statue in Karnataka.
In structural engineering
terms, it is an RCC tower
skeleton which will be covered
by the envelop of the statue
body.
153

ANOTHER STUDY EXAMPLE
 For stiffening, internal floor diaphragms and vertical walls are provided. The
statue is 113ft height.
 As it is an exposed structure of importance, it is considered to be in a
moderately exposed environment
 Specifications -
 Higher covers are used - Slab - 25mm,Beam - 40mm, Column - 50mm
 Strengths : Fe500 grade corrosion resistant reinforcement. M30 and
above concrete strength.
 It is situated in earthquake zone II. It is analyzed in detail for earthquake
and wind loads.
 Non metallic (Brass/Stainless Steel) are fastening projections.
Protective polymer cement waterproof finish on exterior surface. 154

THE FUTURE
All said and done, concrete and steel are denuding our planet. My friend Dr Ajit
Sabnis and fellow professionals are active in advocating sustainable and green
construction. If available, they will grab on a more sustainable material and stop
the use of RCC.
But, at present
THE ALTERNATIVE IS NOT IN SIGHT
EVEN IF WE FIND ONE, THE PRINCIPLES OF DESIGN WILL BE THE SAME.
OUR KNOWLEDGE WILL NOT BE A WASTE
155

156
THANK YOU!
WISH ALL YOUNG STRUCTURAL
ENGINEERS A BRIGHT FUTURE

DESIGN OF RCC ELEMENTS SESSION 5 PROF. YADUNANDAN

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