E2-E3 CIVIL

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E2-E3

CIVIL TECHNICAL

Design of RCC Bldg Components

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• This is a presentation for the E2-E3 Civil Technical

Module for the Topic: Design of RCC Bldg Components

• Eligibility: Those officers of civil wing who have got the

Upgradation from E2 to E3.

• This presentation is last updated on 21-3-2011.

• You can also visit the Digital library of BSNL to see this

topic.



AGENDA

Basic Codes for Design.

General Design Consideration of IS: 456-2000.

Steps for design of a multi-storied building.

Calculation of horizontal loads on buildings.

Vertical load analysis.

Horizontal load analysis.

Design of Various Structural Components

Column Design

INTRODUCTION

Analysis & design of building depends on type of building, its

complexity, number of stories etc.

Structural system is finalized after thorough Study of

architectural drawings.

Choice of an appropriate structural system is important for its

economy and safety. There are two type of building

systems:-

(a) Load Bearing Masonry Buildings.

(b) RCC Framed Buildings.

Structural frame is finalized & sizes of structural members

are conveyed to the concerned architect.


INTRODUCTION

Load Bearing Masonry Buildings

• Low rise buildings with small spans generally constructed as

load bearing brick walls with RCC slab & beams.

• Suitable for building up to four or less stories.

• Adequate for vertical loads & also serves to resists

horizontal loads like wind & earthquake by box action.

• Provisions of IS: 4326 e.g. providing horizontal RCC Bands

& vertical reinforcement in brick wall etc. need to be followed

to ensure safety against earthquake

• Design to be done as per BIS code IS:1905


INTRODUCTION

RCC Framed Structures

• RCC frames are provided in both principal directions

and

• Loads are transmitted to ground through vertical framing

system i.e Beams, Columns and Foundations.

• Effective in resisting both vertical & horizontal loads.

• Brick walls are non load bearing filler walls only.

• Suitable for multi-storied building as it is very effective

in resisting horizontal loads due to earthquake / wind.


INTRODUCTION

RCC Framed Structures

• Before starting structural design of a RCC building, the

following information/ data are required:

(i) Set of architectural drawings;

(ii) Soil Investigation report

(iii) Location of the place or city in order to decide on

wind and seismic loadings

(iv) Data for lifts, water tank capacities on top, special

roof features or loadings, etc.


BASIC CODES OF DESIGN

Useful Codes/Hand Books For Structural Design of RCC

Structures

(i) IS 456 : 2000 – Plain and reinforced concrete – code of practice

(ii) Loading Standards:

IS 875 (Part 1-5) – Code of practice for design loads (other than

earthquake) for buildings and structures

Part 1 : Dead loads

Part 2 : Imposed (live) loads

Part 3 : Wind loads

Part 4 : Snow loads

Part 5 : Special loads and load combinations


BASIC CODES OF DESIGN

Earthquake Resistant Design

• IS 1893 : 2002 – Criteria for earthquake resistant design of

structure.

• IS 13920: 1993 – Ductile detailing of reinforced concrete

structure subject to seismic forces – Provisions of IS 13920-

1993 shall be adopted in all reinforced concrete structures

located in seismic zone III, IV or V

Design Handbooks (Bureau of Indian standards) -

• SP 16 : 1980 – Design Aids to IS 456 : 1978 (Based on

previous version of code but still useful)

• SP 34 : 1987 – Handbooks on Reinforced Concrete Detailing


BASIS OF DESIGN

Aim Of Design

• To design structures with appropriate degree of safety to –

• Perform satisfactorily during its intended life.

• Sustain all loads/ deformations of normal construction & use

• Have adequate durability & resistance to fire.

Method of Design

• Structure and structural elements to be normally designed by

Limit State Method.

• Working Stress Method may be used where Limit State

Method can not be conveniently adopted


LOADS

DESIGN LOAD –

Design load to be taken in appropriate method of design is –

• Characteristic load with appropriate partial safety factors for

limit state design

• Characteristic load in case of working stress method


LOAD COMBINATIONS

BASIC LOAD CASES USED FOR ANALYSIS

No. Load case Directions

1 DL Downwards

2 IL(Imposed/Live load) Downwards

3 EXTP (+Torsion) +X; Clockwise torsion due to EQ

4 EXTN (-Torsion) +X; Anti-Clockwise torsion due to EQ

5 EZTP (+Torsion) +Z; Clockwise torsion due to EQ

6 EZTN (-Torsion) +Z; Anti-Clockwise torsion due to EQ


LOAD COMBINATIONS

LOAD COMBINATIONS

• 1.5 (DL + IL)

• 1.2 (DL + IL ± EL)

• 1.5 (DL ± EL)

• 0.9 DL ± 1.5 EL

• EQ load must be considered for +X, -X, +Z and –Z directions.

Moreover, accidental eccentricity can be such that it causes

clockwise or anticlockwise moments.

• Thus, ±EL above implies 8 cases, and in all, 25 cases must be

considered. It is possible to reduce the load combinations to

13 instead of 25 by not using negative torsion considering

symmetry of the building. For internal circulation of BSNL only

Grade of Concrete

Minimum Grade of Concrete


Steps of Design

Steps for Design of Multi-Storeyed

RCC Framed Buildings

Step1: Study of architectural Drawings

Step2: Finalization of Structural Configuration.

Step3: Preliminary Sizes of Structural members.

Step4: Load Calculation and

Step5: Analysis for various load combinations.

Step6: Design of various structural components for

most critical load combination.


Preliminary Sizes

Finalising Preliminary Sizes

• Slab: Slab thickness is decided based on span/depth ratio.

• Beam: Width of beam to be at least equal to width of wall

(230 or 300 mm). Larger beam width is helpful in placement

of reinforcement in one layer & for resisting shear & torsion.

- Depth of beam generally taken as 1/12th (for Heavy Loads)

to 1/15th (for Lighter Loads) of span.

• Column: Size of column depends upon axial load &

moments from both directions and is finalized after

approximate calculations.


Loads

Types of Loads –

Vertical Loads –

• Dead Load (Self Weight) – Dl – As per IS-875(part-1)

• Imposed Load (Live Load) – LL Or IL – As per IS-875 (Part-2)

• Snow Load

Horizontal Loads –

• Earthquake Load (Seismic) – EQX & EQZ (As per IS-1893)

• Wind Load – WL –As Per IS-875 (Part-3)

• Special Loads & Load Combinations


Dead Loads – Unit Wt of Bldg Materials (IS 875 Pt-1)


MATERIAL

UNIT WEIGHT

kN/m3 kN/m2

PLAIN CONCRETE 24

REINFORCED CONCRETE 25

BRICK MASONRY 19-20

STONE MASONRY 21-27

TIMBER 6-10

CEMENT-PLASTER 21

LIME -PLASTER 18

STEEL 78.5

AC SHEET -ROOFING 0.16

GI SHEET -ROOFING 0.15

MANGLORE TILES 0.65

STEEL WORK -ROOFING 0.16-0.23

Live Loads on Floors of T.E. Bldgs


TYPE OF FLOOR USAGE

LIVE LOAD

(kN/m2)

SWITCH ROOM(NEW TECHNOLOGY) 6.0

OMC ROOM,DDF ROOM,POWER PLANT,

BATTERY ROOM

6.0

MDF ROOM 10.0

WEATHER MAKER 12.0

LIVE LOADS ON ROOFS

ROOF WITH ACCESS 1.5

ROOF WITHOUT ACCESS 0.75

Loads

Procedure for Vertical load calculation on Columns–

Step(i): Transfer slab floor load (both LL & DL) to beams

using formulae for equivalent UDL as :-

Equivalent UDL on short span beam = w B/4

Equivalent UDL on long span beam = w B/4 x [2-(B/L)]

where w is the total load on slab panel in KN/Sqm &

L & B respectively are long span & short spans of slab

panel.


Loads

Procedure for Vertical load calculation on Columns–

Step(ii): Add weight of wall (if any), self weight of beam etc.

to obtain load on beam (in running meter). Calculate

similarly for each beam

Step(iii): Transfer loads from beams to columns.

Step(iv):Repeat Step (i) to Step (iii) for each floor.

Step(v): Add for each column for all floors to get total load on

each column at footing level for entire building.


Loads

Procedure for Horizontal (Seismic) Load Calculation–

• Load Calculations for Seismic Load case is carried out as

per IS:1893-2002 clause 7.5.3.

• The Seismic Shear at various floor levels is calculated for

the whole Building using the values from IS 1893-2002.

• Design Seismic base shear is –

Vb = Ah W

Where W= Seismic weight as per clause 7.4.2 (Full dead load

+ appropriate percentage of imposed load of building as

given in Table 8)


Procedure for Horizontal (Seismic) Load Calculation :

Design Imposed Load for eq. Force Calculation

Table 8 (IS 1893)

Percentage of Imposed Load to be Considered

in Seismic Weight Calculation (Clause 7.3.1 )

Imposed Uniformity Percentage of

Distributed Floor Load Imposed Loads

( kN/ m2 )

(1) (2)

Upto and including 3.0 25

Above 3.0 50


Load Calculation

Loads

Procedure for Horizontal (Seismic) Load Calculation –

Ah = Design Horizontal acceleration spectrum value (cl. 6.4.2)

= (Z/2) (I/R) (Sa/g)

Where Z = Zone factor as per table 2 of IS 1893

I= Importance factor as per table 6 of IS-1893

= 1.5 (If the bldg. is T.E. Bldg.)

R = Response reduction factor as per table 7 of IS 1893

= 3.0 for OMRF or 5.0 for SMRF

(Sa/g) = Average response acceleration coefficient based on

soil type & natural periods and damping of structure. (Refer

Fig. 2 page 16 of IS 1893)


• Procedure for Horizontal (Seismic) Load Calculation :

For calculating (Sa/g) value as above we have to calculate

T i.e. Fundamental Natural Time Period (in Seconds)

(Clause 7.6 of IS Code)

• T = 0.075 h0.75 (For RC Frame building)

where h = Height of building in Meter

• In case of RCC building with brick in fills walls.

T = 0.09 h / d ½ where h = height of building in meter

& d = Base dimension of the building at plinth level in

meter along the considered direction of lateral force.


Load Calculation

Procedure for Horizontal (Seismic) Load Calculation :

• Distribution of base shear (Clause 7.7 of IS 1893) –

Distribution of total design base shear to different floor

levels along height of building is done using formula –

Fi = w i h i2 / ∑(i=1 to n) w i h i

2 x Vb

Where Fi = Design lateral force at floor i

Wi = Seismic weight of floor i

hi = height of floor in m from base.

n = number of storyes in the building is equal to

number of levels at which masses are located.

Vb = Total Design base shear


Load Calculation

ANALYSIS OF STRUCTURE

VERTICAL LOAD ANALYSIS

a) GENERAL:

• It is presumed that all joints of the frame are monolithic.

• To simplify analysis, three dimensional multistoried R.C.C.

framed structure is considered as combination of planer

frames in two directions.

• It is assumed that each of these planer frames act

independently of other frames.



Vertical Load Analysis

• Procedure for Frame analysis for calculation of moments

in Columns & beams:

• Step(i): First, the load from slab is transferred to adjoining

beams using formula given below:-

• For computation of Bending Moments in beams, equivalent

uniformly distributed load of beam is taken as

Equivalent UDL on short beam of slab panel = w B/3.0

Equivalent UDL on long beam of slab panel = w B/6 x [ 3-(B/L)2 ]

where w is the total load on the slab panel in KN/Sqm &

L & B are long span & short spans of slab respectively.



Vertical Load Analysis

• Procedure for Frame analysis for calculation of

moments in Columns & beams:

Step(ii): Over this load, weight of wall, self weight of beam

etc. are added to get load on beam (in running metre).

Step(iii): The load (in running Metre) on each beam is

calculated as in Step (i) & Step (ii).

Step(iv): Step (i) to Step (iii) is repeated for each floor

Step(v): Then these loads are used as u.d.l on a particular

frame for analysis by moment distribution method.



METHOD OF ANALYSIS:

• Analysis of large framed structures is too Cumbersome

with classical methods of structure analysis such as –

– Clapeyron’s theorem of three moments,

– Castingiliano’s therefore of least work,

– Poison’s method of virtual work etc.

Therefore, simpler methods are mostly followed in 2-D

manual analysis of structures. These are –

• Hardy cross method of moment distribution.

• Kani’s method of iteration.



Horizontal Load Analysis

• Frame analysis for horizontal loads calculated in step 4

may be carried out by using Approximate Methods:-

(i) Cantilever method.

(ii) Portal method.

• Approximate methods are used for preliminary designs

only.

• For final design exact methods are used which are –

(i) Slope deflection or matrix methods

(ii) Factor method.


DESIGN OF RCC STRUCTURE

Design of Various Structural Components –

• After load calculation & analysis for vertical & horizontal

loads, design & of various structural components e.g. –

– Columns,

– Foundations,

– Beams,

– Slabs & staircase etc

are carried out as per various clauses of IS codes with

help from charts & tables given in BIS handbooks.


BIS 456 EXTRACT

26.4 Nominal Cover to Reinforcement

• Nominal cover is the design depth of concrete cover to

all steel reinforcements, including links.

• It shall be not less than the diameter of the bar.

• Minimum values for nominal cover of normal weight

aggregate concrete which should be provided to all

reinforcement, including links depending on the condition

of exposure shall be as given in Table 16 of IS 456.


BIS 456 EXTRACT

26.4 Nominal Cover to Reinforcement (Table 16 )

Exposure Nominal Concrete Cover

in mm not Less Than

Mild 20

Moderate 30

Severe 45

Very Severe 50

Extreme 75

• For longitudinal reinforcing bar in column nominal cover shall

in any case not be less than 40 mm, or less than bar dia.

• For footings minimum cover shall be 50 mm.


Design of Columns

Design of Columns

• After obtaining (i) Vertical load, (ii) Moments due to

horizontal loads on either axis & (iii) Moments due to

vertical loads on either axis, acting on each column, at all

floor levels of the building,

• Columns are designed by charts of SP-16(Design Aids).

• Design of each column is carried out from the top of

foundation to the roof, varying the amount of steel

reinforcement for suitable groups for ease in design.

Slenderness effects in each storey are also considered

for each column group.


Design of Columns

Column

A compression member, the effective length > three times

the least lateral dimension.

Short and Slender Compression Members

When both slenderness ratios lex/D and ley/b are <12

• Column is a short column

• If more than 12, then it is long or slender column.

Effective height of column:-

• For effective column height refer table 28 (Annexure E)

of IS:456-2000.


Design of Columns

Design Of Columns – Important Considerations

(ii) Unsupported Length –

In beam-slab construction, it is the clear distance between the floor &

under side of shallower beam framing into columns in each direction at

next higher floor level.

(iii) Slenderness limits for columns –

The unsupported length between end restraints shall not exceed 60

times the least lateral dimension of a column.

(iv) Minimum Eccentricity – All columns shall be designed for

emin ≥ l/500+ D/30 ≥ 20 mm

Where l= Unsupported length of column in mm. D= Lateral dimension

of column in the direction under consideration in mm.


Design of Columns

Design Of Columns – Design Approach

• The design of column is complex as it is subjected to axial

loads & moments which may very independently.

Column design requires –

– Determination of the cross sectional dimension.

– The area of longitudinal steel & its distribution.

– Transverse steel.

• The maximum axial load & moments acting along the length

of column are considered for design of the column section.

• The transverse reinforcement is provided to impart effective

lateral support against buckling to every longitudinal bar.


Design of Columns

Design Of Columns – Reinforcement Provisions as per

IS:456-

A. Longitudinal reinforcement

• Area of longitudinal reinforcement shall be not less than

0.8% nor more than 6% of cross sectional area of the

column.

• However maximum area of steel should not exceed 4% to

avoid practical difficulties in placing & compacting concrete.

• In pedestals, in which the longitudinal reinf. is not taken into

account in strength calculations, nominal reinforcement

should be not be less than 0.15% of cross sectional area.

• Minimum dia of longitudinal bar should be 12 mm


Design of Columns

Design Of Columns – Reinforcement Provisions as per

IS:456

A. Longitudinal reinforcement

• Spacing between bars < 300mm along periphery of column

• The minimum number of bars shall be four in rectangular

columns & six in circular columns.

B. Transverse reinforcement (STIRRUPS)

• Diameter of lateral ties should not be less than 1/4th of dia of the

largest longitudinal bar & in no case should be less than 6 mm.

• Spacing of lateral ties should not > least of the following:-

–Least lateral dimension of the column.

–16 times the smallest diameter of longitudinal bars to be tied.

–300 mm.For internal circulation of BSNL only

SLAB DESIGN

TYPES OF SLABS

Based on Ratio of long span to short span –

• One way slab – Long span (ly)/Short span (lx ) > 2

• Two way slab – Long span (ly)/Short span (lx ) < 2

Based on Edge Conditions

• Simply supported

• Restrained – Edge Conditions of supporting edge

• Cantilever


SLAB DESIGN

• The design of floor slab is carried out as per –

Clause 24.4 &

Clause 37.1.2 & Annexure D of IS:456-2000 .

The Bending moment coefficients are taken from

Table- 26 or

Table – 27 of BIS code

• depending on support conditions

• Bending moment is calculated & reinforcement

steel is obtained from charts given in SP-16.


BIS 456 EXTRACT

Clause 22.2 Effective Span –


S.

No.

Support condition Effective span

1 Simply supported not built integrally

with its supports

Lesser of (i) clear

span + effective

depth of slab, &

(ii) centre to

centre of

supports

2 Continuous when the width of the

support is < 1/12th of clear span

Do

BIS 456 EXTRACT


S.

No.


3 Continuous when the width of

the support is > lesser of 1/12th

of clear span or 600 mm

(i) for end span with one end

fixed and the other end

continuous or for intermediate

spans,

(ii) for end span with one end

free and the other end

continuous,

(iii) spans with roller or rocker

bearings.

(i) Clear span between the

supports

(ii) Lesser of (a) clear span +

half the effective depth of slab,

and (b) clear span + half the

width of the discontinuous

support

(iii) The distance between

centers of bearings

BIS 456 EXTRACT


S.

No.


4 Cantilever slab at the end of

a continuous slab

Length up to the centre

of support

5 Cantilever span Length up to the face of

the support + half the

effective depth

6 Frames Centre to centre

distance

BIS 456 EXTRACT

EFFECTIVE DEPTH Clause 23.0

• Effective depth of beam or slab =

distance between centroid

of area of tension reinf.

& maximum comp. fiber,

• Excluding thickness of finishing material not placed

monolithically with member and the thickness of any

concrete provided to allow for wear.


BIS 456 EXTRACT

Clause 23.2 CONTROL OF DEFLECTION

The deflection shall generally be limited to following:

• Final deflection < span/250

(Due to all loads & measured from as-cast level of

supports of floors, roofs and all other horizontal

members.)

• Final deflection < span/350 or 20mm whichever

is less

(Including effects of temperature, creep & shrinkage

occurring after erection of partitions & application of

finishes.).


BIS 456 EXTRACT


• For beams, vertical deflection limits may

generally be assumed to be satisfied provided

that span/depth ratio are not greater than the

value obtained as below –

(a) Basic values of span/effective depth ratios for

spans up to 10m:

Cantilever 7

Simply supported 20

Continuous 26


BIS 456 EXTRACT


• For spans >10m, values in (a) may be multiplied by

10/span in meters,

Modification Factors are applied –

• Based on area & type of steel for tension

reinforcement (As per Fig. 4 of IS456)

• Based on area of compression reinforcement (As per

Fig. 5 of IS456)

• For flanged beams (As per Fig. 6 of IS456)


BIS 456 EXTRACT

Clause 24.1 SLABS –Control of Deflection

• The provisions of 23.2 for beams apply to slabs also.

• For slabs spanning in two directions shorter of the two

spans to be used for span/effective depth ratios.

• For two-way slabs of shorter spans (≤3.5 m) with mild

steel reinf., span/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 HYSD bars grade Fe 415 & Fe500, values given

above to be multiplied by 0.8.


BIS 456 EXTRACT

26.5.2 Requirement of Reinforcement – SLABS

26.5.2.1 Minimum reinforcement

• Mild steel reinf. in either direction in slabs ≥ 0.15 %

of total cross sectional area.

• For high strength deformed bars ≥ 0.12 percent of

total (Fe415/Fe500 bars) cross sectional area.

26.5.2.2 Maximum diameter

• The dia of reinforcing bars < 1/8th of total thickness

of slab


BIS 456 EXTRACT

Requirement of Reinforcement – SLABS

26.3.3 Maximum distance between bars

The horizontal distance between parallel main

reinforcement bars ≤ 3d or 300 mm

The horizontal distance between parallel

reinforcement bars provided against shrinkage

and temperature ≤ 5d or 300 mm whichever is

smaller.


SLAB DESIGN

Steps for Design of Slabs –

• Step 1: Selection of preliminary depth of slab

• Step 2: Calculate design loads, bending moments

• Step 3: Determination/checking of the effective and

total depths of slabs

• Step 4: Determination of areas of steel

• Step 5: Selection of diameter & spacing of

reinforcing bars


BIS 456 EXTRACT

• Torsion reinforcement is provided at any corner

where the slab is simply supported on both edges

meeting at that corner.

• It consist of top and bottom reinforcement, each with

layers of bars placed parallel to sides of slab &

extending from edges a minimum distance of one-

fifth of the shorter span.

• Area of reinf. in each of these four layers is three-

quarters of the area required for maximum mid-span

moment in slab


BEAM DESIGN

26.5.1.1 Tension reinforcement

a) Minimum reinforcement -

As = 0.85

bd fy

where

AS =minimum area of tension reinforcement

b =breadth of beam or the breadth of the web

d =effective depth of T-beam

fy =characteristic strength of reinforcement in N/mm2 &

b) Maximum reinforcement - The maximum area of

tension reinforcement not to exceed 0.04 bD.


BEAM DESIGN

• Compression reinforcement

• The maximum area of compression reinforcement

not to exceed 0.04 bD

• Side face reinforcement

•Where depth of web in a beam >750 mm, side face

reinf is to be provided along the two faces. The total

area of such reinf. should not < 0.1 percent of web area

and shall be

• distributed equally on two faces at a spacing not >

300 mm or web thickness whichever is less.


BEAM DESIGN

Minimum shear reinforcement (Clause 26.5.1.6)

• Minimum shear reinforcement in the form of stirrups shall

be provided such that:

Asv = 0.4

bsv 0.87fy

Maximum spacing of shear reinforcement (Clause 26.5.1.5)

• The maximum spacing of shear reinforcement measured

along axis of member shall be < 0.75 d for vertical

stirrups and d for inclined stirrups at 45 degrees.

• In no case shall the spacing to be >300 mm.


FOUNDATION DESIGN

Design of Foundations – Important Considerations

• Foundations transfer loads from the building or individual

columns to earth. Foundations must be designed to

prevent –

• Structural Failure

• Shear failure of soil

• Excessive settlement &

• To minimize differential settlement

• Depth of footing is determined from the consideration of –

(a) Bending Moment

(b) One way shear

(c)Two way shear


FOUNDATION DESIGN


• To determine area required for proper transfer of total

load on the soil, the total load (the combination of dead,

live and any other load without multiplying it with any load

factor) need to be considered.

Total Load including Self Weight of footing

Plan Area of footing = -----------------------------------------------

Allowable bearing capacity of soil

Thickness of the edge of footing –

The thickness at the edge shall not be less than 15 cm for

footing on soils.


FOUNDATION DESIGN


Bending Moment (Reference Clauses- 34.2.3.1 & 34.2.3.2)

• The critical section for bending Moment is considered

at the face of column, Pedestal or wall.

Shear (Reference Clause 33.2.4.1)

• The critical section for one way shear is at the vertical

section located at a distance equal to the effective

depth (d) from the face of the column, pedestal or wall

of the footing in case of footings on soils.


FOUNDATION DESIGN


For one way action

For one way shear action, the nominal shear stress is

calculated as follows:-

Vu

τv = -------

b.d

Where

τv = Shear stress, Vu = Factored vertical shear force

b = Breadth of critical section, d = Effective depth

τv < τc ( τc = Design Shear Strength of concrete based on % of

longitudinal tensile reinforcement refer Table 61 of SP-16)


FOUNDATION DESIGN


For Two Way Action (Punching shear )

Critical section for punching shear is at d/2 from the face of

column or pedastal

For two way shear action, the nominal shear stress is calculated

in accordance with clause 31.6.2 of the code as follows:-

Vu

τv = ----------

b0.d

Where b0 = Periphery of the critical section


FOUNDATION DESIGN


Development Length (Reference Clause 34.2.4.3)

• The critical section for checking the development length in a

footing shall be assumed at the same planes as those

described for bending moment in clause 34.2.3 of code and

also at all other vertical planes where abrupt changes of

section occur.

Reinforcement –

Minimum % of steel in footing slab should be 0.12% &

Maximum spacing should not be more than 3 times

effective depth or 300mm which ever is less.


DETAILING

• Reinforcing steel of same type and grade shall be

used as main reinforcement in a structural member.

• Simultaneous use of two different types or grades of

steel for main and secondary reinforcement is

permissible.

• 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 thereof.


Development Length

Development Length of Bars

Ld = υσst /4τbd,

φ = nominal diameter of bar, τbd = design bond stress

σst = stress in bar at the section considered at design load

• Design bond stress in limit state method for plain bars in

tension is given in clause 26.2.1.1

• For deformed bars conforming to IS 1786 these values

are to be increased by 60 %.

• For bars in compression, the values of bond stress for

bars in tension is to be increased by 25 percent


Development Length



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