Rationalised Codes for Concrete Structures – International Approach and Indian Standards

    The intent of this presentation is to highlight the urgency and necessity of developing the ‘New Generation Codes’ for structural concrete and to inform readers about the efforts being made internationally and in India in this respect. The discussion necessarily traces the history of code making, development of design philosophy, ideal contents of a code, etc. The need to revamp the codes arises from factors like unprecedented growth of knowledge, developments in design philosophy, and rapid advances in construction technology, need to frame codes which do not hinder development, transparency and others. The international and Indian response to this challenge is described. In the Indian context the required nature of the code and its contents are discussed. They are illustrated by example of the contents of the proposed ‘Limit State Concrete code for bridges’.

    S. G. Joglekar, Sr. Advisor, STUP Consultants P Ltd., Navi Mumbai

    Introduction

    Archaeologists distinguish different eons of human history by the main material developed and used by them – such as Stone Age, Bronze Age, Iron Age, Age of Painted pottery etc. Future archaeologist will surely call our period as ‘Concrete Age’. Concrete structures are an essential part of our life and a mainstay of our infrastructure in any developed and developing country. This infrastructure is built spending large sums of money and the structures are expected to remain in service for long period – 50 to 100 years or even more. The dams and aqueducts, ports and harbours, and the monumental structures have even longer life expectancy. No wonder then that these constructions are closely controlled in any country. This control is achieved by creating and enforcing guidelines in form of codes of practices. This control by codes, however, should not restrict the development of new improved technology and better ways of designing and constructing structures. Many new types of concrete and structural solutions are being evolved all over the world. In this sense development of concrete technology and structures is truly an international activity, which has spreads across countries, sharing knowledge and technology. It should be obvious that the practices in any country should remain more or less in line with the international developments, without any country lagging too much behind, or following a totally different path. Need for ‘Internationalisation’ is obvious.

    Not so obvious is the need for ‘Rationalisation’ of design practices or the need for revamping existing codes and developing ‘New Generation’ codes! (Note ‘codes’ in plural)

    The intent of this presentation is to highlight the urgency and necessity of developing the ‘New Generation Codes’ and to inform readers about the efforts being made internationally and in India in this respect. The discussion necessarily digresses into topics like history of code making, development of design philosophy, ideal contents of a code, etc., which makes the exercise more interesting. In order to keep the presentation short, - like proverbial ideal length of a skirt, - only the concrete related structural codes are discussed. The concepts remain valid for the full range of other structural codes.

    International Scenario

    In the last 5 to 10 years a number of countries have made major revisions to their codes relating to structural concrete. Bridge codes have also followed the general trend. The revisions are also taking place at closure intervals.
    • Canadians have adopted the Limit State Philosophy and Published new Bridge Code in 1995
    • In USA new LRFD Bridge Design Code is published by AASHTO in 2005(4th edition 2007).
    • ACI also has revised basic Concrete Code (318) in 2005 and 2008; also brought out metric version and a Spanish version (An example of attempt at internationalization).
    • Australians have revised Concrete Code AS: 3600 in 2001. Draft of revision 2005 released.
    • ‘Eurocodes’ is a comprehensive system of codes being prepared from 1989 and by now most of the important codes have been published. All common market countries are expected to adopt the same with minor variations based on the national practices, which have also been restricted to limited number of parameters.
    What had been the reasons behind this flurry of activities? Why such short spans of life for new revisions of codes as compared to relatively peaceful longer reign for the codes of olden days?

    Unprecedented Growth of Knowledge

    In the later half of 20th century developmental efforts picked up pace in the countries which had become newly independent. In the war affected developed countries, reconstruction of damaged economy, taken together with growth of industry, put new demands on engineering infrastructure. “Necessity is mother of invention”’ says an old saying, This increases demand led to search for better methods of construction, efficient use of existing materials, achievement of durability, economy and speed of construction and other such aspects. Rapid developments were made in theoretical and experimental research in universities, and new technologies developed - like prestressed concrete, precast concrete, high strength steels and high performance concretes. New structural forms like cable stayed bridges developed. Design and construction of seismic resistant structures demanded research in plastic behaviour of materials and structures prior to failure. Higher levels of analytical methods were needed. New powerful methods of computer based analysis came to the fore. Pressure of development of infrastructure and durability crisis of concrete structures, motivated the researchers and practicing engineers to understand their materials, and invent new technologies. The construction techniques, like cantilever construction, segmental construction, precasting of large segments of bridge and assembling them by use of heavy lifting equipment developed in this period.

    This process has been accelerated in last 15 years with China and India, - words largest and the second largest construction industries, - having tremendously increased investment in infrastructure.

    Long and short of it, the explosion of knowledge, advent of new technologies and demand from the market had changed everything in the construction industry so much that revamping and frequent updating of engineering codes has become unavoidable.

    Development of Design Philosophy

    It is useful to trace briefly the evolution of design philosophies in the order to put the whole issue of new generation of codes in its proper perspective.

    The Beginning:
    • The industrial revolution in Europe led to development of cast iron, and then steel as structural materials. The economy of construction and the problem of variability of strength of materials had opposite pulls in achieving optimal solutions. A series of problems faced in practical applications and consequential theoretical studies led to development of concepts of safety, - indicated by safety factors - measured by the ratio of minimum expected strength to maximum working load. This is an overall factor, - combining material factor and load factor in one, - as is presently understood.
    • In case of brittle fracture, or in case of sudden failure like buckling, the acceptable ‘safe’ margins were proposed and were improved from time to time on the basis of full scale tests. (Mathematical solutions did not work well enough !)
    • The theory of bending developed, which combined with the knowledge about ductility (or yielding) of materials led to another approach based on keeping enough margin between the yield stress and permissible stress at all sections of a member (allowable stress approach) Safety factor then is then measured by the ratio of Yield stress to Allowable stress.
    • Serviceability was checked by putting controls on deformations (i.e. deflection)
    • Methods for ensuring safety against dynamic effects of loads, were developed. Impact was handled by increasing the static load to an equivalent dynamic load by a suitable factor. Fatigue was controlled either by limiting stresses bellow endurance limit ( of stress) of the material, or by limiting the number load cycles well bellow the cycles causing failure. These approaches worked well for steel.
    • When reinforced concrete and prestressed concrete developed, both allowable (working) stress methods and overall safety factors on members (i.e. strength factor) were followed by engineers, and as a result a mixed approach has entered in the design of concrete structures.
    • The understanding of the behaviour of concrete structures and its many applications in practice had developed side by side. This led to frequent changes in not only the detailed design rules, but also in the basic approach, or design philosophy depending on the approach of the developer of the concept. These basic philosophies also differed from country to country.
    • The American Practice followed semi-empirical approach. In this approach theories are developed based on the understanding of behaviour to the extent possible, but for the practical use experimentally establishing parameters for prediction of strength are defined. This approach has practical limitations of applicability and built in disincentive for promoting growth of knowledge.
    • The British and European approach was theoretically more ‘pure’, but it had to accept artificial compromises to accommodate observed (or experimental) behavior in its theoretical methods. The example of design of rectangular footings is well known. Another example is the design of reinforced concrete column by working stress method in which the real decision making is based on the observed sharing of load in steel and concrete at failure (after creep and shrinkage takes place), but the strength formulae in working-load codes are made to look live ‘allowable stress’ method.
    • The Russians developed the Ultimate Load Factor methods.

    Semi-Probabilistic, Limit-State Philosophy

    • In last few decades, borrowing the philosophical approach from the field of aircraft design, concepts of deciding the acceptable risk of failure and targeting the design strength (just) bellow that was taken as the aim to be achieved.
    • The reliability of design-choices or the risk of failure is ideally assessed by using probabilistic methods. The risk management based on the ‘reliability’ methods was made feasible by greater understanding of the statistical nature of material properties and nature of loads. The earlier established safety approaches are re-formatted in light of the new understanding.
    • In place of theoretically pure probabilistic estimation of risk, semi-probabilistic methods of partial factors was adopted. This was combined with the concept of ‘States of Structure’ described by various ‘Limit States’. This philosophy is a practical compromise between the old methods and the fully probabilistic risk management methods.
    • This semi-probabilistic, limit-state philosophy, at present, forms the basis of many of the national codes.

    International Response

    Awareness of the rapid developments in for past 40 years had led CEB and FIP in Europe to frame Model Code -1978 (revised in 1990) as a guide for code makers. This is presently under updation for its 2010 version. The formation of European Common Market made it necessary to unify and standardise codes of practices across Europe. A political directive was given for the same, under which, in 1989, the programme for synthesising new Eurocodes from codes of advanced member countries like U.K., France, Germany etc. was taken up. CEB-FIP model codes provided the basic scientific background for incorporating the most up-to-date research.

    Other advanced countries like Japan, U.S.A., Canada etc. followed similar approaches as suited their own established organisations.

    Thew commercial advantages of having common, - or at least similar – codes became obvious to many countries from the example of Eurocodes. Now some organisations from the Asian countries are attempting to evolve ‘Asian Model Code’ suitable for their region, in spite of many practical difficulties in implementing a common code.

    The international organisations like IABSE, CEB & FIP (now merged a FIB), ACI/PCI and others provided common international platforms for exchange of views at regular interval. This has greatly helped development of structural Concrete across the world.

    Indian Scenario

    Indian codes cannot lag far behind. How they can catch-up with the Progress needs to be discussed.

    Code committees of Bureau of Indian Standards (BIS), Indian Roads Congress (IRC), and Indian Railways (IR) are active, but are their efforts need to be speeded up if they have to catch up with the international codes. For this to happen they need to be centrally coordinated and become futuristic in their approach

    Presently Indian Standards are written mostly by comparing and updating their knowledge contents borrowing from foreign codes – viz. British, American, Australian, etc. Some of the practical difficulties are:
    • Basic theoretical work or practical research for confirming the design data (e.g. Load and material factors) for Indian conditions is not done in a pro-active way by codal committees.
    • The committee members are voluntary experts supported by their own organisations, who have practical limitations in putting large amount of man-months in these efforts.
    In spite of these difficulties the BIS, IR and IRC committees are revising the knowledge contents of the codes, but their efforts even within their own organisations lack central policy directive, and as such different structural codes have discrepancies both between and within the codes.

    The IRC has made significant progress in revamping and updating the Concrete Bridge Code based on the limit state concepts following a format of presentation different from the normal code formats. It is interesting to understand it in details. For this one has to go into the nature of code, its status, its value in achieving continued education of users and so on. This is briefly presented in section 9. The remaining article illustrates the general approach that needs to be adopted using the example of the IRC Bridge Code for illustration.

    New Strategy is Needed

    It should be realised that the ‘New Indian Bridge Codes’, will be used – and extended for newer applications - in early two or three decades of the 21st century. To develop the same, an appropriate common design philosophy and a standardised and purposeful format is essential. For this purpose it is necessary to understand the nature of codes and their place in practice. The remaining parts of this discussion suggests a suitable philosophy and the format, which is suitable for combining established theory, basic common principles and experience based rules’ The code should stipulate the acceptable modelling of the problems and their solutions ( for structural, hydraulic, and geotechnical issues). It should incorporate up-to-date knowledge of materials, and come out with a set of requirements and recommendations which are achievable based on the available technologies, accounting for their strengths and limitations. By being transparent and stating the limits of applicability the format of codes will become helpful to accommodate new developments and be suitable for incorporating changes as the art and science of engineering bridges develops.

    Design Philosophy - Limit State, Semi-Probabilistic Approach

    It is most appropriate to adopt what is commonly known as “Limit State Philosophy” as the basic philosophy of all bridge codes, with possible exception (at this stage) for the geotechnical aspects of design. The up-to-date information and suitable format of the same is available (for concrete structures) in CEB/FIP Model Code of 1978 and 1990. (The format adopted by ‘Eurocodes’ is based on it, but is a practical compromise between the model code, existing national codes and divergent approaches of member states. Thus, it deviates in details from the model code formats.)

    It is useful to know the development of the philosophical basis of “Limit State Design” in relation to the historical developments in structural engineering. Many excellent publications are available on this topic.

    The advantages of adopting limit state philosophy are briefly listed below without going in detailed discussion:
    • The basic aims of safety, serviceability, durability and economy are recognised. Other aspects such as fire resistance and environmental impacts can be easily added where applicable.
    • With better understanding of material behaviour in its elastic range, plastic range and failure mechanisms, appropriate mathematical models for the analysis are chosen. The behaviour (i.e. performance) of the structure being designed can be pre-determined as a design choice.
    • Appropriate values of material factors depending upon the variability in properties of different materials subjected to different types of loads can be incorporated.
    • Variability of loads arising from the natural events (e.g. wind, earthquake) and also of those loads from intended use (man-made loads) can be incorporated knowing their statistical distribution functions in time and spatial domains. The probability of combination of normal and extreme values can be estimated and allowed for in the design in an consistent way. This permits proper evaluation of “risk” in probabilistic terms and provides more rational approach to risk management.
    • Where knowledge is inadequate and full rationalisation is not possible, the existing practices and experiences can be incorporated by techniques of retrofitting. This is what was done when the first limit state codes were written and tried in practice concurrently with existing codes for a few years. This approach permits immediate change over to the new formats, and then changing the same in details when further studies / experiences justify the change, (i.e. values of partial factors)

    Nature of Code

    What ‘Code-is-Not’?

    In an engineering environment, it is easier to understand what the code is and what should be its coverage or contents, if one understands what code is not:
    • Code is not a Text Book: A textbook contains fundamental principles and basic knowledge of the subject matter, which is well established and widely accepted. A code refers to this knowledge base, but does not quote it in a comprehensive manner. The codal text is not a ‘stand-alone’ write-up covering theory.
    • Code is not a Hand-Book: A hand-book on any subject is a collection of theory, data-base, statement of practices, rules and information needed by users, and is basically a comprehensive reference document. The code may refer to some commonly known and acceptable data base, acceptable practices, approximations and thumb rules – but it is not a comprehensive collection of the same.
    • Code is not a Statutory Document: In most of the countries code contains recommendations which are not legally mandatory by themselves, - (they may become so by virtue of provisions of contract document between the purchaser and supplier), - nor does the use of code relieves user of his statutory, professional, or moral responsibilities in the social context.
    • Code is not a ‘Standard’: A ‘standard’ tries to formally define as accurately as practicable the properties, methods of measurements, tests and procedures which lend precise meaning to general concepts, and form basis of acceptance/ rejection tests of a product. This is the case of ASTM (American Standards of Testing Materials). In many a case the structural ‘code of practice’ has been raised to a status of national standard, but in reality it continue to be a ‘code’ in its essence. This happens when a statutorily established national body like BIS formulates codes of practices.

    Contents of Code

    These can be grouped in the following parts:
    • Principles: These are general statements or definitions for which there are no alternatives within the time-frame of validity of code. Principles by this definition also include fundamental knowledge (theory or text-book knowledge).
    • Application Rules: These are the statements of methods acceptable to code, which follow the principles and are consistent the principles in their requirements. Alternative methods may be permitted explicitly provided they comply with the spirit of the principles and satisfy the general requirements, such as strength, durability etc. Often, the use of alternative rules is left unstated (or uncommented in commentary also, where one is provided) which leads to various interpretations and debate about validity and/or acceptability of the new and/or unstated propositions.
    • Commentary: This supplies clarifications, background information, limits of applicability, further guidance and any such help that code writers care to bring to the attention of users.
    This division of contents is explicitly used by some codes like French codes in force before Eurocodes, (viz. BAEL and BPEL Codes dealing with RCC and Prestressed Concrete Bridges). This is also a preferred format of Model Code of CEB/FIP-1978 and 1990. Some other code makers publish separate Explanatory Guide. The rest leave it to the enterprising authors, or to the mercy of the owner- authority and users.

    Coverage of Code

    Whether explicitly stated or implied, the subject matter covered by codes covers the following:
    1. Statement of Philosophy, and Basis of Design: The code should clearly state its aims and its approach adopted to achieve the aims.
    2. Established Knowledge/Theory: Established knowledge/theory is reiterated in code to a minimum necessary extent in order to indicate the basis for application clauses.
    3. Analytical Models and simplifications: Modern codes explicitly cover this topic and distinguish between well established general models and simple, normally used methods (established by long use). More advanced methods of analysis and use of more exact accurate properties of materials are also given. Guidance for the use of latter in preference to ‘simple’ methods in special cases should be clearly indicated.
    4. Stated (or Unstated) Aims: Safety, serviceability, durability, and economy – Most often these aims being of qualitative nature, and not easily measurable, may or may not be stated explicitly, but they form a strong background for contents of the code. New aims are getting added to this list. Eco-friendliness, aesthetics, energy conservation, operation and maintenance requirements are being now added by modern codes directly or indirectly.
    5. Expected Minimum Requirements of Structures: The ‘requirements’ are the description of the characteristics that a structure (and/or its components) should possess in order to meet the “aims” stated in (b). This is achieved by setting up corresponding ‘design criteria’. The code presumes that by satisfying design criteria the requirements will be met.
    6. Rules and Practices: A set of rules, derived from theory or from practice, and acceptable analytical models used in predicting the characteristics/properties of the designed structure (element), which meets the ‘requirements’ stated in (c).
    7. Materials: Acceptable materials and their properties. The most commonly used materials are included covering the essential - but not exhaustive - properties.
    8. Workmanship: Recommended workmanship practices, which essentially are technical specifications, but are not complete by themselves and are not exhaustive like tender specifications are covered. Aspects of Quality Assurance and Reliability, Operation and Maintenance are being addressed to varying degree.
    The extent to which the above aspects are detailed decides the extent to which the document stands on its own, (or has to draw support from other similar codes). This choice lies with the code writing bodies.

    Proposed Contents of IRC Limit State Concrete Code

    The IRC Limit State Code for Concrete Bridges is a unified code being drafted under the following section headings. The headings and sub-headings are self explanatory.

    Section
    1.0 Contents
    2.0 Foreword and Introduction
    3.0 Definitions and Notations
    4.0 General
    5.0 Basis of Design
    6.0 Material Properties and their Design Values
    7.0 Analysis
    8.0 Ultimate Limit State of Linear Elements for Bending and Axial Forces
    9.0 Ultimate Limit State for two Dimensional Elements for out of Plane and in Plane Loading Effects
    10.0 Design for Shear, Punching Shear and Torsion.
    11.0 Ultimate Limit State of Induced Deformation
    12.0 Serviceability Limit State
    13.0 Prestressing Systems
    14.0 Durability
    15.0 Detailing - General Requirements
    16.0 Detailing Requirements of Structural Members
    17.0 Ductile Detailing for Seismic Resistance
    18.0 Material Specifications
    19.0 Workmanship and Quality Controls
    Normative Annexures
    A-1: Load Combinations
    A-2: Additional Information and Data about Properties of Concrete
    A-3: Testing of Ducts for Prestressing tendons
    Informative Annexures
    B-1: Analysis and Design for Impact
    B-2: Analysis and Design for Fatigue
    B-3: Recommended Practice for Prestressing Operations and Grouting.

    Recapitulation and Conclusions

    1. The topic of Developing New Generation Rationalised Codes of Practices for Bridges in India, has been presented, bringing about the necessity and urgency of the same. Every word of the topic has a deeper meaning attached to it,
    2. The international scenario has been described and the reasons behind the frequently carried out revisions have been explained. Briefly, it is the rapid growth in the scientific knowledge, triggered by the pressure of development of infrastructure and durability crisis of concrete structures. It motivated the researchers and practicing engineers to understand their materials, and invent new technologies.
    3. For proper spread of the knowledge and to assure proper application of the same, codes of practices have to remain up to date.
    4. New rational design philosophy which is based on concepts of reliability, safety, serviceability, durability, economy and is aware of the new issues of eco-friendliness, energy conservation etc. will have to be adopted in the new codes of practices.
    5. This can be done in India by initially borrowing from the work done in developed countries, but it is recognised that it is necessary to carry out research to establish needs and environment which are peculiar to India.
    6. The code makers in India are active and have made a good, although somewhat delayed, .start, Hard work, and centrally planned activity is needed to catch up with the up-to-date international codes.

    NBMCW October 2010

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