Sangeeta Pandey, Chief Manager (Civil) Power Grid Corporation of India Limited, Regional Headquarter, Shastri Nagar, Board Colony, Eastern Region – 1, Patna.
PVS Sudhakar, Sr. General Manager, Station (I/C), Power Grid Corporation of India Ltd, Maheshwaram 765 KV GIS Substation, Meerkhanpet, Kandukur Mandal Ranga Reddy District Telangana.
Achintya, Professor of Civil Engineering and Principal, Darbhanga College of Engineering, Mabbi, Darbhanga, Bihar.
Standard codes of practice for civil engineering enumerate prospects and necessity of the design and construction practices of the structures. They provide a set of rules and regulations that specify the standards for the design of all types of structures within the ambit of civil engineering. The purpose of codes is to provide minimum guiding standards for safety, health and general civil works (encompassing sanitation, sewerage, water supply, water drainage, rain water harvesting, lighting and ventilation), structural and mechanical integrity, means of egress, fire protection &control and energy conservation.The standard codes for civil structures become laws of having particular jurisdiction when formally enacted by the concerned, appropriate government, semi government or private authority.
Standard codes provide safeguards to the public. Though no code can eliminate all risks but it can reduce risk to an acceptable level. Standard codes for structures have been a primary source for guidance in design and construction of building structures for many decades. Earlier the main focus was curtailing loss of life and property. Currently the codes are focused to address a myriad of new technologies / design concepts. Contemporary provisions, requirements and stipulations have expanded beyond health and safety requirements to include other socio-economic values such as accessibility, energy efficiency, indoor air quality and sustainability. Hence, codes are developed, reviewed and revised from time to time for incorporating additions, alterations and modification of factors considered important with respect to criterion for design, construction practices and operation of buildings.
Every new code is a response to review of a prior failure. Natural and manmade disasters have revealed our underestimation of safety requirements and compelled us to formulate ways to improve. The addressal of experienced inadequacies, over the years, has led to evolution of codes. This has helped to prevent mass casualties arising out of structural failures, inadequate lighting, inadequate ventilation, fires, and flooding. As society moves forward, standard codes for civil structures shall continue to evolve too, reflecting the lessons learned about the materials used / practices followed and the way it is to be re-implemented.The code, being adopted as Standards by jurisdiction, may or may not be implemented with intent to serve as regulatory requirements.
Rationale of the Study
Energy waste is one of the most expensive consequences of older building codes. As energy conservation becomes more important for economical and environmental reasons, new building codes are being enforced specifically to address this factor. Energy codes are not just designed to protect natural resources; they also make buildings safer, more efficient and also ensure healthier environs. For example, public health improves when indoor air quality is more strongly regulated whereas sustainability incentives have given homeowners more control over their utility costs.
Standard codes are generally intended to be followed and applied by architects, engineers, interior designers, constructors, facility managers, and regulators but are also used for various purposes by safety regulators, environmental scientists, real estate developers, subcontractors, manufacturers of building products / materials, insurance companies, facility managers, owners and tenants. Codes, when adopted into law, regulate the design, construction, operation and maintenance of structures. The flexibility of the code towards acceptance of advanced materials and improved methods of construction is of utmost importance. Innovative materials and practices are demanding equal treatment as with older materials which have already proven their worth. Lack of clear and explicit requirements with regard to improved specifications and procedural requirements render the codes deficient and are one of the most pressing problems of structural regulations today.
Examination of the properties, features and failures related to current materials, disasters like cyclone, tsunamis, earthquakes, flood, landslides, study of requirements for modern buildings /blocks /roads/cities, design methods, encountered and suspected failures, information on latest innovative materials, reports of tests, proceedings of engineering societies, compilations of data of various kinds, quality of materials, methods of test, methods of assembly of materials and performance under conditions of use, improved uses of a particular material etc. should give a good idea of whether a few selected amendments will accomplish all that is needed for standard codes or the same needs to be reviewed and revised.
With the passage of time, rational stipulations in terms of provisions in codes of practice of civil engineering design and construction have gradually come up to the attention all over the world and the current practices across the world are based on either Limit State Method (LSM) or Load Resistance Factor Design (LFRD) method. The design carried out by LSM / LFRD method takes precise care of performance of structural components efficiently and economically in different structural system. Since LSM / LFRD method has been acquired as the design philosophy in most of the countries because of its rational approach, it is being widely perceived that Indian Standard Codes need to be modified in tune with Limit State Method of design while maintaining Working Stress Design (WSD) as a transition alternative. This, in turn, has helped the civil engineers to understand the nuances and spirit of design as well as construction practices.
The switching of design methodologies from WSD to LSM is to ensure flexibility of structure along with strength. As with WSD method, structures are supposed to work upto elastic limit but as per LSM, structures are supposed to perform beyond elastic limit. Importance is given to serviceability requirements in deciding structures stability in addition to the strength requirement. Change of seismic zones, considering dynamic loadings with static loadings, Erection load, mass irregularity, and vertical geometric considerations are some of the factors which cannot be avoided. This, in turn, would help the civil engineers to understand the nuances and spirit of design as well as construction, operation and maintenance practices.
Objective of the Study
The present study seeks to present a close perspective on the evolution of various Indian Standard Codes of civil engineering practices in Indian conditions and subsequent changes in such codes from the point of view of design practices. For the last 70 years, the Bureau of Indian Standards (BIS), New Delhi has published and regularly revised codes such as Code of Practice –Plain and Reinforced Concrete (IS:456), Criteria for Earthquake Resistant Design of Structures (IS: 1893),General Construction in Steel (IS: 800), and also several others, e.g. Code of practice for Design Load (Other than Earthquake Load) for Building and Structures (IS: 875), Ductile Design and Detailing of Reinforced Concrete Structures(IS: 13920), etc.
The Indian Standard codes for civil engineering practices are being revised by the BIS as per basic design and engineering requirements, new inventions, latest design concepts, improved materials and environmental happenings. Hence it is pertinent to spell out the major changes that are reflecting in the revised or latest codes with respect to the previous ones. The modifications, additions, eliminations and revisions in IS codes definitely reflect the modified requirements for existing structures and revisions to be admissible for designing new structures. The design and construction of all the structures must comply with the latest relevant codes. In the fitness of the context, a comparative study has been made between IS: 456 – 1978 and 2000 (Code of Practice – Plain and Reinforced Concrete), IS: 1893 – 1984 and 2002 and also IS: 1893 – 2002 and 2016 (Code of Practice – Criteria for Earthquake Resistant Design of Structures), and IS: 800 – 1984 and 2007 (Code of Practice–General Construction in Steel).This has been presented here:
Comparison of IS 456: 1978 & 2000
- Type of cement: Types of cement were duly mentioned in IS 456 – 1978 version including OPC, PSC and PPC in clause 4.1. Further types of cements were also included in the 1978 version of the standard through an Amendment No. 2 in August 1994 including all the three grades of OPC. It may be noted that the cement types were given even in 1964 version of the standard. In IS 456 – 2000, recognition of all three grades of OPC cements along with other types of cements has been given vide clause 5.1.
- Admixtures: In IS 456 – 1978, Fly ash as a pozzolana was covered in clause 4.5 and chemical admixture was covered in clause 4.4 whereas in 2000 version, enumeration of allowable mineral admixtures [clause 5.2] and approval of practice of chemical admixtures [clause 5.5] has been taken care of.
- Water testing: The provision of testing water was covered in 1978 version through the Amendment No. 2 issued to it. In IS 456 – 2000, method of testing water for concreting has been described [clause 5.4].
- Characteristic strength of steel: Even though the characteristic strength of steel was not specified in 1978 version under its clause 4.6, the same could be construed. In IS 456 – 2000, characteristic strength of steel has been defined as minimum yield or 0.2% proof stress [clause 5.6.3].
- Grade of concrete: In IS 456 – 1978, M15 is the minimum strength of concrete for structural purpose whereas in IS 456-2000, M20 is the minimum strength of concrete for structural purpose [clause 6.1,2].
- Modulus of elasticity of concrete: In IS 456-1978, modulus of elasticity is considered as Ec = √5700 (fck) in N / mm2 [clause 22.214.171.124] whereas in 2000 version, whereas modulus of elasticity of concrete is taken as Ec = √5000 (fck) in N/mm2 [clause 6.2.3].
- Quality assurance factor: In IS 456-1978 quality of concrete was proposed to be controlled through the provisions of workability, durability, mix proportioning, and production and control of concrete, etc. and a clause on transporting, placing, compaction, joints, curing and supervision covered therein. In 2000 version, while elaborating the provision Quality Assurance Factor, an additional clause on quality assurance measures was added as clause 10.1.
- Basis of design: Working stress method (WSM) primarily is the basis of design in IS 456-1978, however it covered structural design by Limit state method (LSM) in Section 5. In IS 456-2000, Limit state method (LSM) of design was further elaborated and the WSM was relegated to as an Annexure. Applications of these methods were clarified in Clauses 18.2.1 and 18.2.2.
- Bending moment coefficient: In 1978 version, it is l/24. In 2000 version, bending moment coefficient at midpoint of interior spans has been increased from 1/24 to 1/16 and brings its value to ¾ the value at support.
- Tolerance limit for cover: In IS 456-1978, the tolerance was specified in the last paragraph of clause 11.3. In IS 456-2000, tolerance limit for covers to steel fabrication have been specified [clause 12.3.2]. Of course, the provisions were made quite stringent in the 2000 version as compared to those in 1978 version.
- Cracking of concrete: Though this was specified in IS 456-1978 vide clause 34.3, but provisions were not detailed. An Annexure F was also included on crack width calculation. In IS 456-1978, limit state of cracking guidance regarding width of cracks allowed to different environment [clause 35.3.2]. As a matter of fact, provisions were detailed in the 2000 version as compared to those in 1978 version.
- Fire resistance: In 1978 version, it is not mentioned directly; instead fire resistance requirement was mentioned through reference to the Indian Standard, IS 1642. It is glaring to note that in IS 456-2000, minimum requirement of concrete cover, member dimensions for concrete members, to have required fire resistance has been mentioned in clause 21.
- Workability: In 1978 code, workability is related with the slump and compacting factor [clause 6.1]. However, it also recommends that, in the ‘very low’ category of workability where strict control is necessary, for example pavement quality concrete, measurement of workability by determination of compacting factor should be done. Similarly, in ‘very high’ category of workability, the measurement of workability by determination of flow will be appropriate. In IS 456-2000, workability has been simplified in terms of slump only [clause7.1].
- Reinforcement: For steel reinforcement, IS 456-1978 deals with hot rolled deformed bars conforming to IS: 1139-1966 along with Rolled steel made from structural steel conforming to IS: 226-1975 [clause 4.6] through Amendment no. 2 whereas in IS 456-2000, structural steel conforming to Grade A of IS 2062 [clause 5.6] is mentioned.
- Grades of concrete: Specified grades from M10 to M40 are mentioned in clause 5.1 of II 456-1978 whereas from M10 to M80 are in IS 456-2000 vide clause 6.1. Subsequently grades up to M 100 were included through Amendment No. 4 to the 2000 version of the standard code of practice.
- Stripping Time: In IS 456-1978, period for the vertical frame removal is 24 to 48 hours [clause 10.3] whereas in IS 456-2000, vertical formwork to columns, walls and beams can be removed after 16 to 24 hours [clause 11.3]. There is a substantial change brought in this clause through Amendment No. 5 to the standard with respect to stripping time specified for concrete based on OPC and those fly ash or slag based, as well as stripping of formwork based on gain of strength.
- Seismic zones: In IS 1893-1984, seismic zones are divided into five zones whereas in IS 1893-2002, there are four seismic zones as Zone I and Zone II are merged together as shown in Figs. 1 and 2.
- Seismic zone factor: In IS 1893 – 1984, values of seismic zone factor Z are different which do not reflect realistic values of effective peak ground acceleration whereas in IS 1893-2002 (Part 1), values of seismic zone factor Z have been changed to reflect more realistic values of effective peak ground acceleration.
- Response spectra: In 1984 version, response spectra are same for all types of soil conditions. Though it had BETA factor (A factor to modify the basic seismic coefficient and seismic zone factor, depending upon the soil foundation system) for Types I, II & III of soil for various foundation systems. In the 2002 version, soil conditions for response spectra are specified for three types of soil items named rock-hard, medium and soft soil.
- Fundamental natural period: In the former version, empirical expression for estimating fundamental natural period is different for all types of structures. It provided T for 2 types of buildings. It also provided Ta for dams (as provisions for Dams still continue to remain part of 1984 version). In the latter version, empirical expression for estimating fundamental natural period is the same for all types of structures. It provided Ta for 3 types of buildings.
- Load condition: In 1984 version, at the time of designing, only static load condition is considered whereas in 2002 version, at the time of designing, static and dynamic load conditions are considered.
- Soft storey: It is not detailed in IS 1893 – 1984. Its clause 126.96.36.199 mentioned use of MODAL ANALYSIS for buildings having irregular mass/ stiffness distribution; (including in Fig. 3). In IS 1893 – 2002, soft storey clause was EXPLICITLY MENTIONED (after the failures observed in Bhuj earthquake). Soft Storey is one in which the lateral stiffness is less than 70 percent of that in the storey above or less than 80 percent of the average lateral stiffness of the three-storey above [clause 4.20].
- Performance of building: It is not mentioned in 1984 edition. It cautioned with respect to irregular shape; and irregular distribution of MASS & STIFFNESS [clause 4.2.1]. In 2002 edition, it is mentioned that buildings do perform well in an earthquake; a building should possess four main attributes, namely simple and regular configuration, and adequate lateral strength, stiffness and ductility [clause 7.1].
- Response reduction to earthquake: Only Performance factor is considered in 1984 version. Response reduction factor was known as PERFORMANCE FACTOR (K), and its specified values based on structural framing system. (See also Foreword, 0.7 for ductility related requirements). Ductility requirements were available in 1984 version; for which a reference was made to IS 4326 (see notes under 3.3.2). In 2002 version of the code, concept of response reduction due to ductile deformation or frictional energy dissipation in the cracks has been given by Response reduction factor.
- Load combinations: In IS 1893-1984, load combination is UL = 1.4 (DL+LL+EL) whereas in 2002 version, load combinations for plastic design of structures are as follows: (a) 1.7 ( DL + IL ), (b) 1.7( DL ± EL) and (c) 1.3( DL + IL ± EL).
- Partial safety factors: Partial safety factors for limit states of serviceability and collapse and the procedure as given in relevant Indian Standards (IS : 456 – 1978 and IS : 1343 – 1980) are considered in 1984 code. In IS 1893 -2002 (Part 1), Partial safety factors for limit state design of reinforced concrete and prestressed concrete structures are as follows:
(a) 1.5( DL + IL) (b) 1.2 ( DL+IL ± EL) (c)) 1.5 ( DL ± EL) (d) 0.9DL± 1.5EL.
These values in are in line with those given in IS 456:2000.
- Unreinforced masonry infill (URM): In the case of URM walls modelling, IS 1893-2002 code is silent about modelling of masonry infill walls. Only equation for Ta= 0.09h/√d for Buildings with masonry infill walls is given in its clause 7.6.1. Hence, in analysis, Ta is taken considering masonry infill whereas stiffness of infill is not considered in analysis. In IS 1893-2016, EQ loads when infills exist have been considered for RC framed building with URM. A detail procedure for URM infill by Equivalent Diagonal Strut method has been given in its clause 188.8.131.52.
- Soft storey: As per 2002 version, soft storey is defined as the storey in which lateral stiffness is less than 70 % of that in the storey above [Clause 4.20] whereas as per 2016 version, soft storey is defined as the storey in which lateral stiffness is less than that in the storey above [clause 4.20.1].
- Weak storey: As per clause 4.25 of 2002 version, weak storey is the storey in which the lateral strength is less than 80 % of that in the storey above whereas as per clause 4.20.2 of 2016 version, weak storey is the storey in which the lateral strength [cumulative design shear strength of all structural members other than that of unreinforced masonry infill (URM)] less than that in the storey above [clause 4.20.2].
- Dynamic analysis: In IS 1893-2002 (Part 1), requirement of dynamic analysis is given in clause 7.8.1 for regular buildings (Zone – IV, V... height <40m and Zone – II, III ... height > 90m) as well as for irregular buildings (Zone – IV, V... height <12m and Zone – II, III ... height <40m). In IS 1893-2016, as per clause 7.6 and 7.7.1, equivalent static analysis shall be applicable for regular buildings with Height < 15m in seismic Zone II as per clause 6.4.3, equivalent static method should be used for regular building structure with approximate natural periods is less than 0.4 second.
- Re-entrant corners: According to IS 1893-2002 code vide clause 7.1, condition for re-entrant corner is A/L > 0.15 – 0.20. As per IS 1893-2016 for re-entrant corner vide clause 7.1, A/L > 0.15 in buildings with re-entrant corners, three – dimensional dynamic analysis shall be performed.
- Mass irregularity: In the former version, mass irregularity is considered to exist when the seismic weight of any floor is more than 200 % of that of the floor below or above vide clause 7.1 whereas in the latter, vide clause 7.1, mass irregularity is considered to exist when the seismic weight of any floor is more than 150% of that of the floor below such that Wi > 1.5 W – 1 and Wi > 1.5 Wi+1 where Wi as shown in figure in the code. Further it reads that in buildings with mass irregularity and located in seismic zones III, IV and V dynamic analysis shall be performed.
- Vertical geometric irregularity: As per 2002 version vide its clause 7.1, Vertical Geometric Irregularity exists when the horizontal dimension of the lateral force resisting system in any storey is more than 150 % of the storey below or above such that A/L<0.15L and L2/L1<1.5 where A, L, L1 and L2 are as shown in figures in the code. As per 2016 version of the code, vide clause 7.1, Vertical Geometric Irregularity exists when the horizontal dimension of the lateral force resisting system in any storey is more than 125 % of the storey below A/L<0.125L and L2/L1<1.25.
- Increase in soil pressure: As per IS 1893-2002, when earthquake forces are considered, increase in allowable pressure in soils for different types of soils (Type I, II, III) and different types of foundations, namely piles, raft, well foundations, etc., was from 25 %to 50 % as given in clause 184.108.40.206. In percentage increase in net bearing pressure and skin bearing pressure and skin foundations for soil types A, B and C as 50%, 25%, and 0% respectively, as per 2016 version of ISI 1883. For soft soil, no increase in bearing pressure shall be applied because settlements cannot be restricted by increasing bearing pressure.
- Liquefaction potential: IS 1893-2002 speaks that Specialist literature was to be referred whereas in IS 1983-2016, Annexure F has been introduced.
- Open storey: IS 1893-2002 mentions the requirement including 2.5 times storey shear [Clause in 7.10.3] whereas clause 7.10 of IS 1983-2016 deals with more details.
- Design spectra: In IS 1893-2002, deign spectra is available only up to 4s, for 5% damping and multiplying factors for varying values of damping while in IS 1893-2016, it is provided up to 6s; and is same for all buildings (irrespective of material of construction).
Comparison of IS 800: 1984 & 2007
- Basis of design: IS 800-1984 deals with working stress design concept in which, permissible stress value is never let to reach yield stress whereas IS 800-2007 primarily focuses on Limit State Design Concept which means steel is used beyond its yield value. Working stress design concept is still given in separate chapter of IS 800:2007.
- Material characteristic: In 1984 version, non-linear behaviour of material was not considered while in 2007 version, the same has been taken into account.
- Load combinations: In 1984 version of the code, the load combinations are given in clause in 3.4.2 which are (1) DL + IL, (2) DL+IL+WL or EL and (3) DL + WL or EL where DL - dead load, IL - imposed load, WL - wind load and EL - Earthquake Load. It had provisions relating to consideration of stresses generated through secondary effects such as Erection, handling, temperatures effects, settlement of foundations, etc.(Clause 220.127.116.11). In 2007 version, besides the above three load combinations, one more load combination, i,e, DL + ERL has been added vide clause 3.5. Here, ERL stands for erection load.
- Permissible stress: As per clause: 18.104.22.168 of IS: 800–1984, shear stress shall not exceed 110 MPa whereas in IS 800-2007, shear stress shall not exceed 110MPa vide clause 10.5 nor as calculated using clause 10.5.7.
- Classification based on buckling and rotation before failure: No such classification has been made in IS 800-1984 whereas in IS 800-2007, sections are classified in clause 3.7, based on its local buckling strength and the ability to allow rotation before failing. These are (a) Class 1 (Plastic), (b) Class 2 (Compact), (c) Class 3 (Semi-compact) and (d) Class 4 (Slender).
- Restoring Moment: In 1984 version, Restoring moment > 1.2 times maximum overturning moment (due to DL) + 1.4 times maximum overturning moment (due to IL and WL/EL). In cases where DL provides the restoring moment, only 0.9 times DL shall be considered vide its clause 3.12. As per the design philosophy of IS 800-2007, structure should satisfy two limit states, viz. Limit state of strength and limit state of serviceability. As per this version, the structure as a whole or any part of it is to be designed to prevent instability due to overturning, uplift or sliding under factored load as given below:
6.1 The actions shall be divided into components aiding instability and components resisting instability.
6.2 The permanent and variable actions and their effects causing instability shall be combined using appropriate load factors as per the Limit State requirements, to obtain maximum destabilizing effect.
6.3 The permanent actions (loads) and effects contributing to resistance shall be multiplied with a partial safety factor 0.9 and added together with design resistance (after multiplying with appropriate partial safety factor). [Clause 5.5.1 of IS 800-2007].
- Deflection criteria: As per IS 800-1984, max. deflection for all applicable loads (Vertical / Horizontal) = l / 325 where l is the span [clause 3.13], whereas in IS 800-2007, deflection limits have been provided separately for Industrial buildings and other buildings and separate limits have been mentioned for different members in vide clause 5.6.1.
- More criteria for load combinations: In IS 800-1984, no additional criteria are given. In IS 800-2007, two more combinations of loads have to be considered (clause 12.2) which are 1.2 DL + 0.5 LL ± 2.5 EL and 0.9 DL ± 2.5 EL.
- Fire resistance criteria: No such criteria are given in IS 800-1984 whereas IS 800-2007, deals with the Fire Resistance Level Period of structural adequacy of the following:
9.1 Variation of mechanical properties of steel with temperature
9.2 Limiting steel temperature
9.3 Thermal increase with time in protected members
9.4 Determination of period of structural adequacy from a single test
9.5 Three-sided fire exposure condition.
Engineering principles and practices undergo constant experiments, innovations and improvements to suit the demands and required needs of the time. Engineering prudence demands Indian standard codes of civil engineering design and practices to be not left in a static state. Therefore, it requires upgradation and revisions with new perspective so as to give thrust in the direction of construction audits. It is to be ensured that constructions taken place shall be reviewed with the point of view of its auditing in terms of quality, strength and sustainability. Thus, it is most imperative that civil engineering design and practices in Indian conditions are given a prime focus in light of the methods and measures as pointed in the preceding paragraphs.
Continual execution and implementation is, therefore, a watch-word and the conception and creation of any structure is thus as cardinal as the implementation and practice of the modification introduced at intervals by the Bureau of Indian Standards.