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“IMPLICATION OF IRC 112:2011 ON RCC BRIDGE DESIGN:

SYNOPSIS, APPLICATION AND COMPARISON WITH ITS PREDECESSOR

IRC 21:2000 AND MENTOR EUROCODE”

Krunal J Mehta*, Prof. Paresh Patel**, Devang Patel***

* PG Student, Civil Engineering Department, Institute of Technology, Nirma University, Ahmedabad;

** Head & Professor, Civil Engineering Department, Institute of Technology, Nirma University, Ahmedabad

*** Joint Principal Consultant, Spectrum Techno Consultants (P) Ltd., Ahmedabad

ABSTRACT

Past half century has seen tremendous growth of knowledge in the field of concrete as material and

its design process. Limit State philosophy a more realistic and comprehensive over Working Stress

philosophy has found its way to almost all countries’ design standards. Unlike western countries India has

separate codes and formation committee for concrete design as general (BIS) and bridge design (IRC).

Indian Road Congress is the latest committee to publish a code on basis of Limit State Design Philosophy

(IRC-112:2011).

Owing to wide scope of subject and limitation of content that can be justified in one paper, present

study has been concentrated around RCC segment of the code covering sections such as Basis of Design,

Materials and ULS of Flexure). Comparison of relevant clauses of IRC 112 has been made with IRC 21 and

EUROCODE (considered to be major source of influence). In the end an illustrative example of T-Beam is

used to compare the various code provisions of IRC 112 and IRC 21 quantitatively.

1. INTRODUCTION

IRC 112, published in year 2011 (November) is a unified code for Reinforced concrete and

Prestressed concrete superseding IRC 21:2000 and IRC 18:2000. In line with international practice

IRC 112 also divides limit state into two groups Ultimate Limit State (ULS) and Serviceability Limit

State (SLS). To mention some of major facets: section 4 & 5 of code provides a detailed explanation

of “Basis of Design” which provides a transparent view of codal recommendations, applicability and

limitations. Section 7 of “Analysis” covers classical methods of analysis, modern methods such as

non-linear analysis and specialized method for torsion. Preceding sections 8 to 11 covers “ULS” for

flexure, axial, shear, torsional and induced deformations. Section 12 covers “SLS” for cracking and

deflection. Section 14 covers “Durability” requirements. Next three sections 15 to 17 covers

detailing requirements as a general and for seismic resistance separately. Lastly section 18 covers the

requirement of Quality control and workmanship. Code allows design using working stress method

as an alternative for verification of ULS and accordingly annexure A-4 covers the same. In order to

make descriptions more manageable, relevant section/clauses of code are mentioned in bracket.

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2. SCOPE (SECTION 4)

Compared to IRC 21 which provides a general description stating “This code deals with the

structural use of PCC and RCC in road bridges”, IRC 112 gives a meticulous scope under section 4.

It covers purpose, aim, aspects covered alongside limitations and assumptions as shown in Table 1.

Table 1 Scope as per IRC 112:2011

1. Purpose: To establish common procedures for design and construction of concrete road bridges including foot

bridges in India.

2. Aim: To achieve construction of safe, serviceable, durable and economical bridges.

3. Aspects covered: Design principles, detailed designed criteria and practical rules, material specifications,

workmanship, quality control, all such aspects which affect characteristics/ability of bridge to meet the aims.

4. Limitations:

Applicable to normal weight concrete (Density: 24 +/- 4 kN/m3)

Not applicable for hybrid structural system

Not applicable to other types of concrete (LWC, HWC, concrete with specially modified properties)

5. Assumptions:

Choice of structural system and design carried out by competent personnel

Execution carried out by competent personnel

Adequate supervision and quality control

Construction material and products used are as per relevant standards

Intended properties considered for design are available

Use as intended & Adequate maintenance

3. BASIS OF DESIGN (SECTION 5)

Designing is similar to “walking a tight rope”, which requires balance between safety-

serviceability-durability on one hand and economy on other. Present section though being non-

operative to design, is a step to bridge the gap between codal approach and designer’s intuition to

achieve the same.

In-line with international practice, IRC 112 divides Limit State Philosophy into two parts.

Ultimate Limit State (ULS) covering equilibrium and strength of structure and Serviceability Limit

State (SLS) covering deflection, crack width, vibrations and other secondary effects such as creep,

shrinkage, relaxation of steel, fatigue etc. (ref. cl. 5.2 & 5.3)

For a structure designed as per LSM, has to be reliable enough to perform as desired under

given circumstances. To measure reliability (probability of failure) code has come up with

approximate methods based on combination of following aspects:

1. Known statistical parameters describing properties of materials and actions

2. Deterministic model of structural behaviour

3. International practices & past experience

4. Partial factors for actions (loading) and resistance models (materials)

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4. MATERIAL PROPERTIES AND THEIR DESIGN VALUES + QUALITY CONTROL

AND WORKMANSHIP (SECTION 6, ANNX.-A2 & SECTION 18)

Evidently Analysis & Design of a structure as a whole or its individual element require

knowledge of the properties of constituting materials (i.e. permissible stresses and strains, strength,

elongation etc.). Accordingly section 6 along with Annexure-2 provides the same. However

attainment of these properties is highly dependent on its manufacturing processes adopted, Quality of

workmanship and construction/work procedures followed. Accordingly section 18 provides

minimum acceptable standards to achieve the same.

4.1 UNTENSIONED STEEL (ref. cl. 6.2 & 18.2)

Code permits use of mild steel and carbon steel (hot rolled, TMT, de-coiled or cold worked)

of various grades as specified in Table 2. Actual and idealized Bilinear Stress v/s Strain diagram of

untensioned reinforcement is shown in figure 1 & 2 below. Table 2 also shows comparison of

properties among IRC 21, IRC 112 (WL/AS – Annex.4) and IRC 112 Limit State Method. Modulus

of Elasticity to be considered for design is 200 GPa. Code permits use of idealized or simplified

bilinear diagram for design purpose; after reducing the stresses by partial safety factor for material

γs. Design strain shall be limited to 0.9 times characteristic strain obtained from manufacturer of

reinf

orce

ment

.

Fig 1

& 2

Actu

al & Idealized Stress – Strain Diagram of Untensioned Reinforcement

4.2. CONCRETE (ref. cl. 6.4 & 18.4)

Presently Indian Construction industry is facing a severe scarcity of skilled labour. Since

majority of concrete, being casted at site/locally often faces quality related issues. Under these

circumstances performance of concrete becomes the weakest link in achieving the design standards

set earlier. Foreseeing these, IRC 112 has provided very detailed literature describing minimum

standards, production methodologies & guidelines for concrete. It covers individual ingredients of

concrete under clause 18.4, Mix proportions under clause 18.5, acceptance criteria under section

18.6, Quality control and Workmanship criteria (such as its production, transportation, placing,

falsework, Inspection and

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Table 2 Properties of Untensioned Steel (Comparison between IRC 21 & IRC 112)

Sr.

No. Description

Type of Steel Mild Steel High Yield Strength Deformed Steel

Code Grade-I Fe415 Fe415D Fe500 Fe500D Fe550 Fe550D Fe600

1 Grade of Steel

IRC 21 - - - - -

IRC 112 – WL/AS - - -

IRC 112 – LSM

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Characteristic

Strength / Min.

Yield Stress / 0.2%

proof stress (MPa)

IRC 21 240 415 - 500 - - - -

IRC 112 – WL/AS 240 415 415 500 500 - - -

IRC 112 – LSM bars ≤ 20

mm = 250

bars > 20

mm = 240 415 415 500 500 550 550 600

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Min. Tensile

Strength / as % of

actual 0.2% proof

stress / yield stress

(MPa)

IRC 21* 410 110%

( ≥ 485) -

108%

( ≥ 545) - - - -

IRC 112 – WL/AS 410 110%

( ≥ 485) -

108%

( ≥ 545) - - - -

IRC 112 – LSM 410 110%

( ≥ 485)

112%

( ≥ 500)

108%

( ≥ 545)

110%

( ≥ 565)

106%

( ≥ 585)

108%

( ≥ 600)

106%

( ≥ 600)

4 Min. % Elongation

IRC 21* 23% 23% 14.50% - 12% - - - -

IRC 112 – WL/AS 23% 23% 14.50% - 12% - - - -

IRC 112 – LSM 23% 23% 14.50% 18% 12% 16% 10% 14.50% 10%

5 Permissible Stress

for Tension in Shear

IRC 21 125 200 - 240 - - - -

IRC 112 – WL/AS 125 200 200 200 200 - - -

IRC 112 - LSM ** Same as minimum yield stress / 0.2% proof stress (Sr. No. 2 of the table)

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Permissible Stress

for Tension in

Flexure or combined

bending

IRC 21 125 200 - 240 - - - -

IRC 112 – WL/AS 125 200 200 240 240 - - -

IRC 112 - LSM ** Same as minimum yield stress / 0.2% proof stress (Sr. No. 2 of the table)

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Permissible Stress

for Direct

Compression

IRC 21 115 170 - 205 - - - -

IRC 112 – WL/AS 115 170 170 205 205 - - -

IRC 112 - LSM ** Same as minimum yield stress / 0.2% proof stress (Sr. No. 2 of the table)

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Permissible Stress

for Tension in

helical rein.

IRC 21 95 95 - 95 - - - -

IRC 112 – WL/AS 95 95 95 95 95 - - -

IRC 112 - LSM ** Same as minimum yield stress / 0.2% proof stress (Sr. No. 2 of the table)

* Cross reference from relevant Indian Standards

** Values to be divided by Partial safety factor for material (γs) = 1.15 for basic and seismic combination & 1.0 for accidental combination.

Note: For seismic zone III, IV & V; HYSD steel bars having minimum elongation of 14.5% and confirming to IS 1786 shall be used.

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testing etc.) Under clause 18.8. Mechanical properties of concrete are covered in section 6.4 and

Annexure A-2 of IRC 112. For brevity of the space only basic mechanical properties are covered

here. Grades of concrete are classified in three categories as follows:

1. Ordinary Concrete: M15 & M20 made on basis of nominal mixed proportioned by weight.

2. Standard Concrete: M15 to M50 (in multiples of 5) made on basis of Mix design which apart

from standard ingredients may also contain chemical admixtures.

3. High Performance Concrete: M30 to M90 (in multiples of 5) made on basis of Mix design

which is similar to standard concrete but may also contain one or more mineral admixtures for

property modifications.

Similar to majority of western countries EUROCODE has adopted concrete strength in terms of

standard cylinder strength. However as per Indian practice IRC follows a model based on cube

strength. Accordingly co-relation equations of relative mechanical properties are converted to

equivalent cube strength. Co-relation between cylinder and compressive strength is considered as: fck,

cyl = 0.8 x fck, cube accordingly equation fcm, cyl = fck, cyl + 8 MPa (ref. EC-2) is converted to fcm, cube =

fck, cube + 10 MPa.

Un-confined concrete

Design compressive stress for concrete is obtained by:

Where, α = 0.67, factor for effect of sustained loading and gain of strength with time [ref. 6.4.2.2(2)]

γm = Partial factor of safety for material = 1.5 for Basic & Seismic combination

1.2 for Accidental combination

IRC 112 provides three alternatives of Stress-Strain relationship for design of section,

a. Parabolic rectangular stress-strain relationship (fig. 3)

b. Bi-linear stress-strain relationship (fig. 4)

c. Simplified rectangular stress-strain relationship (fig. 5)

Fig 3 Parabolic rectangular stress-strain relationship Fig 4 Bilinear stress-strain relationship

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Where, λ = 0.8 for fck ≤ 60 MPa

λ = 0.8 – (fck – 60) / 500 for 60 ≤ fck ≤ 110 MPa

η = 1.0 for fck ≤ 60 MPa

η = 1.0 – (fck – 60) / 250 for 60 ≤ fck ≤ 110 MPa

Note: If the width of compression zone

decreases in the direction of the extreme

compression fiber, value of ηfcd should be reduced

Table 3 compares these three idealizations in terms of average stress (fav) over a rectangular

compression zone (from extreme compression fiber to neutral axis) and the distance from the

compression face of section to the center of compression, which can be used for flexure design

calculations.

Table 3 Comparison of Stress block idealization (α = 0.67 & γm=1.5)

Grade

Parabolic rectangular Bilinear Simplified rectangular

fav β fav β Fav β

Average Stress

(MPa)

ratio of depth to

n.a. depth

Average

Stress (MPa)

ratio of depth

to n.a. depth

Average Stress

(MPa)

ratio of depth to

n.a. depth

M15 5.424 0.416 5.025 0.389 5.360 0.400

M20 7.232 0.416 6.700 0.389 7.147 0.400

M25 9.040 0.416 8.375 0.389 8.933 0.400

M30 10.848 0.416 10.050 0.389 10.720 0.400

M35 12.656 0.416 11.725 0.389 12.507 0.400

M40 14.463 0.416 13.400 0.389 14.293 0.400

M45 16.271 0.416 15.075 0.389 16.080 0.400

M50 18.079 0.416 16.750 0.389 17.867 0.400

M55 19.887 0.416 18.425 0.389 19.653 0.400

M60 21.695 0.416 20.100 0.389 21.440 0.400

M65 22.624 0.405 21.284 0.383 22.478 0.395

M70 22.927 0.389 21.928 0.372 23.412 0.390

M75 23.235 0.377 22.536 0.363 24.247 0.385

M80 23.626 0.368 23.158 0.356 24.985 0.380

M85 24.165 0.362 23.828 0.351 25.628 0.375

M90 24.873 0.358 24.554 0.347 26.178 0.370

5. ULTIMATE LIMIT STATE FOR FLEXURE:

Capacity of a flexure member can be found by use of strain compatibity method as shown below:

1. Assume a neutral axis depth and calculate the strains in the tension and compression

reinforcement by assuming linear strain distribution and a strain of εcu2 (or εcu3 as the case may be) at

the extreme fiber of the concrete in compression.

2. From stress-strain idealization, calculate steel stresses appropriate to the calculated steel strains.

3. From stress-strain idealization, calculate the concrete stresses appropriate to the strains associated

with the assumed neutral axis depth.

4. Calculate the net tensile and compressive forces at the section. If they are not equal, adjust the

neutral axis depth and return to step-1.

Fig 5 Simplified rectangular stress-strain relationship

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5. When net tensile force are equal to net compressive force, take moment about a common point in

the section and determine moment of resistance.

This method, being iterative, is tedious for hand calculations however shall be used for non-

uniform section. Formulas for sections such as Rectangular and Flanged-Tee are given below.

However special care must be taken regarding strain level in steel so as to avoid brittle failure (when

strain in concrete reaches it limiting value prior to steel).

For rectangular section:

1. Singly under- reinforced:

2. Singly Balanced:

3. Doubly reinforced :

For compression steel to yield before concrete:

For Flanged Section:

1. Neutral Axis lies in Flange: Similar to Singly reinforced rectangular section

2. Neutral Axis lies in Web:

I. Depth of rectangular part of stress block is greater than the depth of flange

II. Depth of rectangular part of stress block is less than the depth of

flange

Considering Whitney stress block, replace

Df by,

In above equation

Limiting value x/d can directly be taken

from Table 4.

ILLUSTRATIVE EXAMPLE: An example of RCC T-Beam is used to compare Working Stress

(IRC 21) & Limit State (IRC 112) design philosophy. (Ref. Table 5 below)

For tension steel to yield

Steel

Concrete

fck ≤ 60 0.77 0.66 0.62 0.59 0.57

65 0.76 0.65 0.61 0.58 0.56

70 0.75 0.63 0.59 0.56 0.54

75 0.73 0.62 0.57 0.55 0.53

80 0.73 0.6 0.56 0.54 0.51

85 0.72 0.6 0.55 0.53 0.51

90 0.72 0.59 0.55 0.52 0.5

Limiting value of x/d for all three idealizations of strain block

MS-G-I Fe415 Fe500 Fe550 Fe600

Table 4 Limiting value of strain x/d

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Design Philosophy

Parameters

IRC 21 IRC 112

unit Working

Stress

Method

Limit State Method

Rectangular - Parabolic Bi-linear Simplified

Rectangular

Effective Flange Width 3000 3000 3000 3000 mm

Flange Thickness 240 240 240 240 mm

Web Thickness 300 300 300 300 mm

Overall Depth 1400 1400 1400 1400 mm

SLS - Moment 1865 1865 1865 1865 kN-m

ULS - Moment 2750 2750 2750 2750 kN-m

Clear cover 40 40 40 40 mm

Ast Assumed 8 # 32 6 # 32 + 2 # 20 6 # 32 + 2 # 20 6 # 32 + 2 # 20 Nos

Area of Steel (Ast) 6434 5454 5454 5454 mm2

Effective Depth (d) 1288 1302 1302 1302 mm

Concrete Grade (fck) 40 40 40 40 MPa

Steel Grade (fy) 500 500 500 500 MPa

fav - 14.463 13.4 14.293 MPa

β - 0.416 0.389 0.4 -

Ast,min 464 609 609 609 mm2

Permissible Comp. stress 4.69 < 13.33 - - - MPa

Permissible tensile stress 227.98 < 240 - - - MPa

Moment of Resistance - 3033 3032 3034 kN-m

Crack Width calculation

Actual Crack width - 0.13 0.13 0.00 mm

Limiting crack width - 0.30 0.30 0.30 mm

CONCLUSION

IRC 112, Code is based on design philosophy which gives fair importance to each aspects of

safety, serviceability, durability & economy. It also emphasizes on quality control and workmanship

to achieve the desired standards. The code seems less user friendly initially, in a manner, it requires a

thorough understanding of elementary concepts of engineering and design. However once

understood, it provides more freedom / choices to designers while restricting the violation of very

basic fundamentals of safety.

REFERENCES

1. N. Koshi, S G Joglekar, T. Viswanathan, A K Mullick, A K Mittal, Vinay Gupta, Alok

Bhowmick, Umesh Rajeshirke, V N Heggade. “Code of practice for Concrete Road Bridges

IRC 112:2011. Proceedings of Workshop by Indian Concrete Institute (New Delhi Centre),

New Delhi” May, 02-04 , 2013

2. IRC-21:2000. “Standard specifications and code of practice for road bridges -section-III -

Cement Concrete (plain and reinforced), third revision”.

3. IRC-112:2011. “Code of practice for concrete road bridges first publication.”

4. EUROCODE 2 (EN 1992-2). “Design of Concrete Structures Part 2: Concrete Bridges.”

5. C.R.Hendy, D.A.Smith. “Designers’ Guide to EN 1992-2, EUROCODE 2: Design of

Concrete Structures Part 2: Concrete Bridges.”

Table 5 Illustrative Example Results