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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 safetyserviceability-durability on one hand and economy on other. Present section though being nonoperative 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
4 Table 2 Properties of Untensioned Steel (Comparison between IRC 21 & IRC 112) Sr. No.
Description
1
Grade of Steel
2
Characteristic Strength / Min. Yield Stress / 0.2% proof stress (MPa)
3
Min. Tensile Strength / as % of actual 0.2% proof stress / yield stress (MPa)
4
Min. % Elongation
5
Permissible Stress for Tension in Shear
6
Permissible Stress for Tension in Flexure or combined bending
Type of Steel Code IRC 21 IRC 112 – WL/AS IRC 112 – LSM IRC 21 IRC 112 – WL/AS IRC 112 – LSM
Mild Steel Grade-I 240 240 bars ≤ 20 bars > 20 mm = 250 mm = 240
IRC 21*
410
IRC 112 – WL/AS
410
IRC 112 – LSM
410
IRC 21* IRC 112 – WL/AS IRC 112 – LSM IRC 21 IRC 112 – WL/AS IRC 112 - LSM ** IRC 21 IRC 112 – WL/AS IRC 112 - LSM **
23% 23% 23%
125 125
Fe415D 415
415
415
110% ( ≥ 485) 110% ( ≥ 485) 110% ( ≥ 485) 23% 23% 23%
125 125
Fe415 415 415
112% ( ≥ 500)
High Yield Strength Deformed Steel Fe500 Fe500D Fe550 500 500 500 500 108% ( ≥ 545) 108% ( ≥ 545) 108% ( ≥ 545)
Fe550D -
Fe600 -
500
550
550
600
-
-
-
-
-
-
-
-
110% ( ≥ 565)
106% ( ≥ 585)
108% ( ≥ 600)
106% ( ≥ 600)
14.50% 12% 14.50% 12% 14.50% 18% 12% 16% 10% 14.50% 200 240 200 200 200 200 Same as minimum yield stress / 0.2% proof stress (Sr. No. 2 of the table) 200 240 200 200 240 240 -
-
Same as minimum yield stress / 0.2% proof stress (Sr. No. 2 of the table)
IRC 21 115 170 205 IRC 112 – WL/AS 115 170 170 205 205 7 ** IRC 112 - LSM Same as minimum yield stress / 0.2% proof stress (Sr. No. 2 of the table) IRC 21 95 95 95 Permissible Stress 8 for Tension in IRC 112 – WL/AS 95 95 95 95 95 helical rein. 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. Permissible Stress for Direct Compression
10% -
-
-
-
-
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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 Fig 5 Simplified rectangular stress-strain relationship
compression
fiber,
value
of
decreases in the direction of the extreme 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.
Grade M15 M20 M25 M30 M35 M40 M45 M50 M55 M60 M65 M70 M75 M80 M85 M90
Table 3 Comparison of Stress block idealization (α = 0.67 & γm=1.5) Parabolic rectangular Bilinear Simplified rectangular fav β fav β Fav β Average Stress ratio of depth to Average ratio of depth Average Stress ratio of depth to (MPa) n.a. depth Stress (MPa) to n.a. depth (MPa) n.a. depth 5.424 0.416 5.025 0.389 5.360 0.400 7.232 0.416 6.700 0.389 7.147 0.400 9.040 0.416 8.375 0.389 8.933 0.400 10.848 0.416 10.050 0.389 10.720 0.400 12.656 0.416 11.725 0.389 12.507 0.400 14.463 0.416 13.400 0.389 14.293 0.400 16.271 0.416 15.075 0.389 16.080 0.400 18.079 0.416 16.750 0.389 17.867 0.400 19.887 0.416 18.425 0.389 19.653 0.400 21.695 0.416 20.100 0.389 21.440 0.400 22.624 0.405 21.284 0.383 22.478 0.395 22.927 0.389 21.928 0.372 23.412 0.390 23.235 0.377 22.536 0.363 24.247 0.385 23.626 0.368 23.158 0.356 24.985 0.380 24.165 0.362 23.828 0.351 25.628 0.375 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.
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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 nonuniform 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:
For tension steel to yield
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.
II.
Depth of rectangular part of stress block is greater than the depth of flange
Depth of rectangular part of stress block is less than the depth of flange Considering Whitney stress block, replace
Table value of strainofx/d Limiting value4ofLimiting x/d for all three idealizations strain block Steel
Df by, In above equation
Limiting value x/d can directly be taken
MS-G-I
Fe415
Fe500
Fe550
Fe600
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
Concrete
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)
8 Table 5 Illustrative Example Results
Design Philosophy Parameters Effective Flange Width Flange Thickness Web Thickness Overall Depth SLS - Moment ULS - Moment Clear cover Ast Assumed Area of Steel (Ast) Effective Depth (d) Concrete Grade (fck) Steel Grade (fy) fav β Ast,min Permissible Comp. stress Permissible tensile stress Moment of Resistance Crack Width calculation Actual Crack width Limiting crack width
IRC 21 Working Stress Method 3000 240 300 1400 1865 2750 40 8 # 32
IRC 112 Limit State Method
unit
3000 240 300 1400 1865 2750 40 6 # 32 + 2 # 20
Simplified Rectangular 3000 240 300 1400 1865 2750 40 6 # 32 + 2 # 20
mm mm mm mm kN-m kN-m mm Nos
5454 1302 40 500 14.463 0.416 609 3033
5454 1302 40 500 13.4 0.389 609 3032
5454 1302 40 500 14.293 0.4 609 3034
mm2 mm MPa MPa MPa mm2 MPa MPa kN-m
0.13 0.30
0.13 0.30
0.00 0.30
mm mm
Rectangular - Parabolic
Bi-linear
3000 240 300 1400 1865 2750 40 6 # 32 + 2 # 20
6434 1288 40 500 464 4.69 < 13.33 227.98 < 240 -
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.”
