EN1990 Eurocode Basis of structural design

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1 Proceedings of ICE Civil Engineering 144 November 2001 Pages 8 13 Paper Keywords codes of practice & standards; design methods & aids; strength and testing of materials Haig Gulvanessian is director of BRE s Construction Division EN1990 Eurocode Basis of structural design H. Gulvanessian EN1990 Eurocode Basis of structural design was finally approved in October 2001.As well as being one of the first structural European design standards to be published it is the world s first material-independent design code, a major achievement in its own right. EN1990 is also a highly strategic document, establishing for all nine other structural Eurocodes the principles and requirements for safety, serviceability and durability.this paper provides an introduction to the head Eurocode, its innovative approach to reliability and riskmanagement and its limit-state design philosophy. It also summarises the loading combinations for which all European structures will need to be assessed in the forseeable future. EN1990 Eurocode Basis of structural design 1 is the head Eurocode for the European harmonised set of structural design standards known as the structural Eurocodes, now in an advanced state of development in Europe. It provides comprehensive information and guidance on the principles and requirements for safety, serviceability and durability that are normally necessary to consider in the design of building and civil engineering structures including bridges, masts, towers, silos, tanks, chimneys and geotechnical structures. When completed as EN (EuroNorm) standards there will be ten Eurocodes for structural design, comprising as a whole a portfolio of more than 50 parts. 2 Eurocodes aim to harmonise widely differing design practices Structural design practice varies substantially across the countries of Europe. Different design loads, design methods, fabrication and construction techniques have evolved based on local tradition and circumstances. Variations in economic and sociological standards are reflected in local practice and industry together with differences arising from the local climate which ranges widely from the continental climate in central Europe to the maritime climates of the north and north west, to the warmer climates of the south. National codes of practice mirror the local national situation. Some countries have sophisticated national structural design codes whereas others have no codes for specific types of structure and use national codes from other countries. The preparation of the Eurocodes as a whole is being undertaken against this background, with the primary objective being to achieve convergence to a consistent structural design practice throughout Europe. An additional objective of the Eurocodes is to provide design rules for everyday use. For complete construction projects and for the manufacture of specific construction products, the aim is to facilitate innovative design and to ensure that the Eurocodes do not prevent the development of innovative products. EN1990 a late arrival in the Eurocode family A proposal to develop an international set of codes of practice for structural design was first agreed in 1974 by several technical-scientific organisations based largely in Europe. 3 Following preparatory work by these organisations, the Commission of the European Communities together with the European Free Trade Association took the initiative for developing the structural Eurocodes at the end of the 1970s by establishing a steering committee to oversee the work. In 1989 the responsibility for their development was transferred to the Comité Européen de Normalisation (CEN) the European committee for standardisation. Until the mid-1980s EN1990 was known as Eurocode 1 and consisted only of common safety requirements and common principles and rules. These common unified rules were published in They were not operational but provided a basis for preparing the operational Eurocodes 2 8. Eurocode 9 was not added to the Eurocode preparation programme until The initial work of Eurocode preparation did not therefore include development of rules for actions (loading). An outline for a comprehensive Eurocode for 8 C I V I L E N G I N E E R I N G

2 EN1990 EUROCODE BASIS OF STRUCTURAL DESIGN EN 1990 EN 1991 EN 1992 EN 1993 EN 1994 EN 1995 EN 1996 EN 1999 EN 1997 EN 1998 Fig. 1. Links between the Eurocodes actions was accepted by the steering committee in 1985, which is described in the following paper. 5 Until 1997, EN1990 was known as ENV (ENV stands for EuroNorm Vornorm, meaning pre-standard) and it formed the first of the ten parts of ENV1991 Basis of design and actions on structures. For conversion to EN standards, a decision was made in 1997 to divide ENV Eurocode 1: Basis of design and actions on structures into two separate documents EN1990 Eurocode Basis of structural design EN1991 Eurocode 1: Actions on structures. In addition it was also decided in 1997 that all material-independent clauses would be removed from the material Eurocodes 2 7 and 9 and only included in EN1990, thus making EN1990 the head operational Eurocode. A theoretical background to EN1990, which was originally based on ISO 2394 and CEB bulletins, is given by Gulvanessian and Holicky. 7 Objectives, use and assumptions of EN1990 EN1990 establishes for all the structural Eurocodes the principles and requirements for safety and serviceability. It also provides the basis and general principles for the structural design and verification of buildings and civil engineering works (including geotechnical aspects) and gives guidelines for related aspects of reliability and durability. Structural safety, serviceability and durability It is based on the limitstate concept used in Actions on conjunction with the partial safety factor method. structures The standard is to be used together with Design and EN1991 Eurocode 1: detailing Actions on structures and the design Eurocodes 2 9 (EN1992 EN1999) (see Geotechnical and seismic Fig. 1). design EN1990 also covers aspects of structural reliability relating to safety, serviceability and durability for design cases not covered by Eurocodes 1 9 (e.g. other actions, structures outside the scope of the Eurocodes, other materials). Due to the scope and objectives of EN1990, it is likely to have wider usage than other Eurocodes. Users will include code-drafting committees clients (e.g. for the formulation of their specific requirements on reliability level and durability) designers and contractors (as for the other Eurocodes) public authorities (e.g. to set safety criteria). The following assumptions are associated with the validity of the design principles of EN1990 and indeed for the whole Eurocode suite choice of structural system and design of structure made by appropriately qualified and experienced personnel execution carried out by personnel having appropriate skills and experience adequate supervision and quality control provided during execution of work (i.e. in design offices, factories, plants and on site) construction materials and products used as specified in EN1990 or in EN1991 EN1999 or in relevant supporting material or product specifications structure adequately maintained structure used in accordance with design assumptions. There may be cases when the above assumptions need to be supplemented. EN1990 [1] has an annex B entitled Management of structural reliability for construction works that provides additional guidance with regard to appropriately qualified and experienced personnel and appropriate skill and experience. In addition this annex gives guidance on adequate supervision and quality control, which is dependent upon the assumed consequences of failure and the exposure of the construction works to hazards. Calgaro and Gulvanessian have written a paper explaining the annex. 8 It is the intention for each of the material Eurocodes together with EN1990 and EN1991, and their supporting standards, to be grouped into appropriate packages and used together when the package is available. Fundamental requirements and reliability levels EN1990 states that structures shall be designed and executed in such a way that they meet fundamental requirements for serviceability, safety and robustness. Serviceability requirement the structure will, during its intended life with appropriate degrees of reliability and in an economic way, remain fit for the use for which it is required Safety requirement the structure will sustain all actions and influences likely to occur during execution and use. In the case of fire, the structural resistance shall be adequate for the required period of time. Robustness requirement the structure will not be damaged by events such as explosion, impact or consequences of human errors, to an extent disproportionate to the original cause. EN1990 gives ways of avoiding or limiting potential damage as follows avoiding, eliminating or reducing the hazards to which the structure can be subjected selecting a structural form which has low sensitivity to the hazards considered selecting a structural form and design that can survive adequately the acci- C I V I L E N G I N E E R I N G 9

3 GULVANESSIAN EN1990 and Eurocodes 1 9, together with appropriate quality control measures, should ensure an appropriate degree of reliability for the majority of structures dental removal of an individual member or a limited part of the structure, or the occurrence of acceptable localised damage avoiding as far as possible structural systems that can collapse without warning tying together the structural members. Design and execution according to EN1990 and Eurocodes 1 9, together with appropriate quality control measures, should ensure an appropriate degree of reliability for the majority of structures. EN1990 provides guidance for adopting a different level of reliability for structural safety or serviceability by considering the cause and mode of failure the possible consequences of failure in terms of risk to life, injury, potential economic losses and the level of social inconvenience the expense and procedures necessary to reduce the risk of failure the different degrees of reliability required at national, regional or local level. EN1990 states that the required reliability relating to structural safety and serviceability may be achieved by the suitable combination of measures relating to design which include serviceability requirements; the representative values of actions; the choice of partial factor; the consideration of durability; the consideration of the degree of robustness; the amount and quality of preliminary investigations of soils and possible environmental influences; the accuracy of the mechanical models used; and the stringency of the detailing rules measures relating to quality assurance and control to reduce the risk of hazards in gross human errors; design; and execution. Further guidance on reliability is contained in annex B. Design situations and durability EN1990 stipulates that a relevant design situation is selected taking account of the circumstances in which the structure may be required to fulfil its function (Fig. 2). It classifies design situations as permanent variable accidental seismic. The design working life is the assumed period for which a structure is to be used for its intended purpose with anticipated maintenance but without major repair. Table 1, taken from EN1990, gives a design working life classification. The code stipulates that the structure shall be designed such that deterioration over its design working life does not impair the performance of the structure. The durability of a structure is its ability to remain fit for use during the design working life given appropriate maintenance. The structure should be designed in such a way, and/or provided with protection so that no significant deterioration is likely to occur within the period between successive inspections. The need for critical parts of the structure to be available for inspection without complicated dismantling should be considered in the design. Other interrelated factors that shall be considered to ensure an adequately durable structure are intended and future use of the structure required performance criteria expected environmental influences composition, properties and performance of materials choice of a structural system shape of members and structural detailing, and buildability quality of workmanship and level of control particular protective measures maintenance during the intended life. EN1990 stipulates that appropriate quality assurance measures should be taken in order to provide a structure that corresponds to the requirements and to the assumptions made in the design. These measures should include organisational measures and controls at the stages of design, execution, use and maintenance. Ultimate and serviceability limit states All Eurocodes use the concept of limitstate design. Limit states are states beyond which the structure no longer satisfies the design performance requirements. EN1990 makes a distinction between ultimate limit states and serviceability limit states. Ultimate limit states are those associated with collapse or with other forms of structural failure. They concern the safety of the structure and its contents and the safety of people. Serviceability limit states correspond to conditions beyond which specified service requirements for a structure or structural element are no longer met. They concern the functioning of the construction works or parts of them, the comfort of people and the appearance. EN1990 recommends that the serviceability requirements should be determined in contracts and/or in the design. It distinguishes between reversible and irreversible serviceability limit states, and gives three expressions for serviceability design: characteristic, frequent and quasipermanent. The verification of serviceability limit states should be based on criteria considering the deformations that affect the appearance, the comfort of users or the functioning of the structure (including the functioning of machines or services), Fig. 2. Loads on a structure are categorised as permanent, variable, accidental or seismic. 10 C I V I L E N G I N E E R I N G

4 EN1990 EUROCODE BASIS OF STRUCTURAL DESIGN Table 1. Design working life classification Design working Indicative design Examples life category working life (years) 1 10 Temporary structures Replaceable structural parts (e.g. gantry girders, bearings) Agricultural and similar buildings 4 50 Buildings and other common structures Monumental buildings, bridges and other civil engineering structures Table 2: Actions classified as permanent, variable or accidental Permanent action Variable action Accidental action Self-weight of structures, fittings Imposed floor loads Explosions and fixed equipment Prestressing force Snow loads Fire Water and soil pressures Wind loads Impact from vehicles Indirect action Indirect action (e.g. settlement of supports) (e.g. temperature effects) Actions due to traffic Table 3. Reduction factors for variable actions on buildings Variable action Reduction factors Combined (ψ 0 ) Frequent (ψ 1 ) Quasipermanent (ψ 2 ) Imposed loads in buildings domestic, residential offices congregation areas shopping storage roof (including snow) Traffic loads in buildings vehicle weight < = 30 kn vehicle weight < = 160 kn roofs Wind loads on buildings Snow loads on buildings (see EN ) in Finland, Iceland, Norway, Sweden Remainder of CEN member states, for sites located at altitude H > 1000 masl Remainder of CEN member states, for sites located at altitude H > 1000 masl Temperature (non fire) in buildings or that cause damage to finishes or nonstructural members. Limit-state design is carried out by setting up structural and load models for relevant ultimate and serviceability limit states to be considered in the various design situations and load cases. Designers then verify that the limit states are not exceeded when the design values for actions, material properties and geometrical data are used in models. Design values are generally obtained by using the characteristic or representative values in combination with partial and other factors. EN1990 allows for designs directly based on probabilistic methods, and has an annex giving appropriate guidance. Defining actions on structures In EN1990 an action (F) is defined as direct a force or load applied to the structure indirect an imposed or constrained deformation or an imposed acceleration caused by temperature changes, etc. Actions are described by a model, and its magnitude is represented in the most common cases by one scaler. For example vehicle-axle spacing and magnitude is commonly represented by a single scalar. The scalar may adopt several representative values, for example a dominant or non-dominant action. Several scalars are used when the action is multi-component. More complex representations are required for fatigue and dynamic actions. Actions are classified as three types permanent, variable and accidental as shown in Table 2. The term single action is also used to define an action that is statistically independent in time and space from any other action acting on the structure. The self-weight of a structure can be represented by a single characteristic value (G k ), provided the variability of G is small, and it can be calculated on the basis of the nominal dimensions and the mean unit mass. If the variability of G is not small and the statistical distribution is known, two values are used, an upper value (G k,sup ) and a lower value (G k,inf ). More information on this subject has been given by Ostlund. 9 In EN1990 a variable action has four representative values (see Fig. 3). In decreasing order of magnitude, these are characteristic value (Q k ) combination value (ψ 0 Q k ) frequent value (ψ 1 Q k ) quasi-permanent value (ψ 2 Q k ). The combination value (ψ 0 Q k ) takes account of the reduced probability of simultaneous occurrence of the most unfavourable values of several independent variable actions. It is used for the verification of ultimate limit states and irreversible serviceability limit states. The frequent value (ψ 1 Q k ) is used for verification of ultimate limit states involving accidental actions and reversible limit states. The quasi-permanent value (ψ 2 Q k ) is also used for ultimate limit state verification involving accidental actions and for reversible serviceability limit states. The recommended values of ψ 0, ψ 1, ψ 2 for buildings are shown, reproduced from EN1990, in Table 3. In EN1990 properties of materials (including soil and rock) and products are represented by characteristic values that C I V I L E N G I N E E R I N G 11

5 GULVANESSIAN EN1990 provides principles that are common for structures of different type and material and the section on structural analysis provides guidance on the modelling of static actions, dynamic actions and fire actions correspond to the value of the property having a prescribed probability of not being attained in a hypothetical, unlimited test series. For a particular property they generally correspond to a specified fractile of the assumed statistical distribution of the property of the material in the structure. Geometric data are also represented by their characteristic value, or in the case of imperfections by their design value. EN1990 provides principles that are common for structures of different type and material and the section on structural analysis provides guidance on the modelling of static actions, dynamic actions and fire actions. Where calculation rules or material properties given in Eurocodes 1 9 are not sufficient or where economy may result from tests on prototypes, part of the design procedure may be performed on the basis of tests. EN1990 requires that tests are set up and evaluated in such a way that the structure has the same level of reliability to all possible limit states and design situations as achieved by design based on calculation procedures specified in Eurocodes 1 9. An annexe supports the rules. In the partial-factor method it has to be verified that, for all relevant design situations, the limit states are not exceeded when design values for actions, material properties and geometrical data are used in the design models. Procedures for verifying Arbitrary point-in-time value Q Charachteristic value Q k t Combination value ψ 0 Q k Frequent value ψ 1 Q k Quasi-permanent value ψ 2 Q k Time Fig. 3. EN1990 classifies variable actions into four frequencydependent values ultimate and serviceability limit states are set out in Appendices 1 and 2. Actions are combined so that they produce the most unfavourable effect on the t t Appendix 1: verifying ultimate limit states For the ultimate limit state verification, EN1990 stipulates that the effects of design actions do not exceed the design resistance of the structure at the ultimate limit state.the following four ultimate limit states need to be verified. EQU loss of static equilibrium of the structure or any part of it considered as a rigid body, where minor variations in the value or the spatial distribution of actions from a single source are significant and the strengths of construction materials or ground are generally not governing. For the limit state verification for static equilibrium, E d,dst E d,stb, where E d,dst is the design value of the effect of destabilising actions and E d,stb is the design value of the effect of stabilising actions. STR internal failure or excessive deformation of the structure or structural members, including footings, piles, basement walls, etc., where the strength of construction materials of the structure governs. GEO failure or excessive deformation of the ground where the strengths of soil or rock are significant in providing resistance.when considering a limit state of rupture or excessive deformation of a section, member or connection (STR and/or GEO), E d R d, where E d is the design value of the effect of actions such as internal force, moment or a vector representing several internal forces or moments and R d is the design value of the corresponding resistance. FAT fatigue failure of the structure or structural members. Verification of resistance of building structures (footings, piles, basement walls, etc.) involving geotechnical actions and resistance of the ground in permanent and transient situations may be done using one of the three alternative approaches, as chosen by the national annex and described in annex A1 of EN1990 and other references. 10, 11 Combination of actions for ultimate limit states For the ultimate limit state verification, three types of combination of actions must be investigated: fundamental, accidental and seismic. The fundamental (persistent and transient) situations for ultimate limit state verifications, other than those relating to fatigue, are symbolically represented as follows γ G,j G k,j + γ p P + γ q,1 Q k,1 + γ Q,i ψ 0,i Q k,i This is the combination of all permanent actions including self weight (G k,j ), the pre-stressing action P, dominant variable action (Q k1 ) and combination values of all other variable actions (ψ 0,i Q k,i ),.All actions are modified by appropriate partial safety factors γ. Alternatively EN1990 allows the use of the following equations together γ G,j G k,j + γ p P + γ Q,1 ψ 0,1 Q k,1 + γ Q,i ψ 0,i Q k,i ξ j γ G,j G k,j + γ p P + γ Q,1 Q k,1 + γ Q,i ψ 0,i Q k,i The upper equation uses a combination value of the dominant variable action (ψ 0,1 Q k,1 ) and the lower equation reduces the permanent action partial safety factors (γ G,j ) with a reduction factor ξ between 0 85 to 1.The more unfavourable of these alternate equations may be applied instead of the first equation, but only under conditions defined by the national annex. The accidental design situation is symbolically represented as follows G k,j + P + A d + (ψ 1,1 or ψ 2,1 )Q k,1 + ψ 2,i Q k,i This is the combination of all permanent actions (G k,j ), the pre-stressing action (P), the design accidental action (A d ), the frequent or quasi-permanent value of the dominant variable action (ψ 1,1 Q k,1 or ψ 2,1 Q k,1 ) and all quasi permanent values of other variable actions (ψ 2,i Q k,i ). Partial safety factors are not included.this combination considers that accidents are unintended events, such as explosions, fire or vehicular impact, which are of short duration and have a low probability of occurrence a degree of damage is generally acceptable in the event of an accident accidents generally occur when structures are in use. The seismic design situation is symbolically represented as follows G k,j + P + A Ed + ψ 2,i Q k,i This is similar to the accidental design combination, with the design value of the seismic action (A Ed ) replacing the accidental action (optionally multiplied by an importance factor γ I ) and with the quasi-permanent values used for all the variable actions (ψ 2,i Q k,i ). Partial factors for the ultimate limit states For buildings, the recommended partial factors for the permanent and variable situation in EN1990 are γ G =1 35 and γ Q = 1 5, but these may be altered by the national annex. EN1990 also provides recommended partial factors for the verification of ultimate limit states for the other design situations. 12 C I V I L E N G I N E E R I N G

6 EN1990 EUROCODE BASIS OF STRUCTURAL DESIGN structure for the limit state being considered. Actions that cannot occur simultaneously, for example due to physical reasons, should not be considered together in combination. Layout of EN1990 EN1990 has the following sections general sections, applicable to all structures within the fields of application of the structural Eurocodes, defining requirements and criteria separate application parts derived from the general sections, specific for each structural type (buildings, bridges, towers and masts, etc.). three annexes annex B: management of structural reliability for construction works annex C: basis for partial factor design and reliability analysis annex D: design assisted by testing a national annex, including the γ and ψ factors to be adopted nationally and the choice of the appropriate method for the ultimate limit state verification GEO. Appendix 2: verifying serviceability limit states For the serviceability limit states verification EN1990 stipulates that E d C d where C d is the limiting design value of the relevant serviceability criterion and E d is the design value of the effects of actions specified in the serviceability criterion, determined on the basis of the relevant combination. Combination of actions for the serviceability limit states For the serviceability limit states verification, EN1990 requires three combinations to be investigated rare, frequent and quasi-permanent. The characteristic (rare) combination, which is used mainly in those cases when exceeding a limit state causes a permanent local damage or permanent unacceptable deformation, is represented by G k,j + P + Q k,1 + ψ 0,i Q k,i This is the total of all permanent actions (G k,j ), the prestressing action (P), the dominant variable action (Q k,1 ) and the combination values of all other variable actions (ψ 0,i Q k,i ). The layout is designed to ensure that the document is user-friendly and to allow application parts of various types of structure can be added at future dates without amendments to the main part of the document. Conclusion EN1990 is a very innovative code. It is the head Eurocode and will be used with all the other Eurocodes for design. It is the first operational material-independent design code and has achieved a very important goal. In EN1990, the basic principles of structural design have been harmonised for European Community member states and, more importantly, for a large number of materials (concrete, steel, masonry, timber, aluminium) and disciplines (fire, geotechnics, earthquake, bridge design, etc.). In addition EN1990 is very innovative in the field of reliability and risk management. This operational code should provide civil engineers with many opportunities for innovation. The frequent combination, which is used mainly in those cases when exceeding a limit state causes local damage, large deformations or temporary vibrations, is represented by G k,j + P + ψ 1,1 Q k,1 + ψ 2,i Q k,i This uses the frequent value of the dominant variable action (ψ 1,i Q k,i ) and the quasi-permanent values of all other variable actions (ψ 2,i Q k,i ). The quasi-permanent combination which considers all quasi-permanent values of variable actions (ψ 2,i Q k,i ) is use mainly when long-term effects are of importance. This is represented by G k,j + P + ψ 2,i Q k,i Partial factors for serviceability limit states Unless otherwise stated (e.g. in Eurocodes 2 9), the partial factors for serviceability limit states are equal to 1 0. References 1. CEN. pren1990 Eurocode: Basis of structural Design Stage 34, April 2001, CEN, GULVANESSIAN H. Eurocodes a new environment for structural design (this issue). 3. BREITSCHAFT G.The conceptual approach of the structural Eurocodes. Proceedings of Structural Eurocodes International Conference, Davos, IABSE Reports, 65, COMMISSION FOR THE EUROPEAN COMMUNITIES. Eurocode No.1: Common unified rules for different types of construction and material. EUR 8847, GULVANESSIAN H. EN1991 Eurocode 1: Actions on structures (this issue). 6. CEN. ENV : Eurocode 1: Basis of design and actions on structures part 1, basis of design. CEN, GULVANESSIAN H. and HOLICKY M. Handbook for Eurocode 1: Basis of Design. Thomas Telford Publications, London. 8. CALGARO J.-A. and GULVANESSIAN H. Management of Reliability and Risk in the Eurocode System. Proceedings of International Conference, Malta, March OSTLUND L.Actions. Proceedings of IABSE Conference on Basis of Design and Actions on Structures, Delft, IABSE, SAVIDU A., GULVANESSIAN H. and PAVLOVIC M. Geotechnical limit states EN 1991 Actions on structures. Pisa University, pp DRISCOLL R. M. C. and SIMPSON B. EN1997 Eurocode 7: Geotechnical design (this issue). What do you think? If you would like to comment on this paper, please up to 500 words to the editor at simon.fullalove@ice.org.uk. If you would like to write a paper up to 2000 words about your own experience in this or any related area of civil engineering, the editor will be happy to provide any help or advice you need. C I V I L E N G I N E E R I N G 13