Basis of structural design - Decoding Eurocode 7

1 Chapter 2 Basis of structural design EN 1990 .. is the head document of the Eurocode suite and describes theprinciples and requirements for safety, serviceability and durability ofstructures of the EurocodeEurocode Basis of structural design2 is divided into six sections and fourannexes (A-D), as shown in Figure 12. In this diagram, the size of eachsegment of the pie is proportional to the number of paragraphs in therelevant 1990 describes the Basis for the design and verification of buildings andcivil engineering works, including geotechnical aspects, and gives guidancefor assessing their structural reliability (see Figure 13).

2 It covers the designof repairs and alterations to existing construction and assessing the impactof changes in use. Because of their special nature, some construction works(such as nuclear installations and dams) may need to be designed toprovisions other than those given in EN 12. Contents of the Eurocode28 Decoding Eurocode basic requirements of a structure are to sustain all likely actions andinfluences, to remain fit for purpose, and to have adequate structuralresistance, durability, and serviceability. These requirements must be met forthe structure s entire design working life, including 13. Scope of EN 1990, Basic of structural designBasis of structural design29 The structure must not suffer disproportionate damage owing to adverseevents, such as explosions, impact, or human error.

3 The events to be takeninto account are those agreed with client and relevant addition, the design must avoid or limit potential damage by reducing,avoiding, or eliminating hazards. This can be achieved by tying structuralmembers together, avoiding collapse without warning ( by employingstructural redundancy and providing ductility), and designing for theaccidental removal of a structural design working life is theassumed period for which a structure or part of it is to be used for its intendedpurpose with anticipated maintenance but without major repair beingnecessary.[EN 1990 ]Figure 14 compares the design working life of various structures accordingto EN 1990 (dark lines) with the modifications made to these time periods bythe UK National Annex to EN 1990 (lighter lines).

4 The most significantchange is the extension of Category 5 to 120 years although this only reallyaffects fatigue 14. design working life of various structures30 Decoding Eurocode 7 Figure 15. design according to the EurocodeBasis of structural 1990 makes important assumptions about the way structures aredesigned and executed (see Figure 15).It is assumed that people with appropriate qualifications, skill, andexperience will choose the structural system, design the structure, andconstruct the works. It is also assumed that construction will be adequatelysupervised and quality controlled; and the structure will be adequatelymaintained and used in accordance with the design they vary from country to country, EN 1990 gives no guidance as towhat appropriate qualifications are needed to perform these tasks.

5 Likewise adequate supervision and control is not further defined in the and Application RulesA distinctive feature of the structural Eurocodes is the separation ofparagraphs into Principles and Application Rules (see Figure 15). design which employs the Principles and Application Rules is deemed tomeet the requirements provided the assumptions given in EN 1990 to EN1999 are satisfied.[EN 1990 (1)]Principles identified by the letter P after their paragraph numbers aregeneral statements and definitions that must be followed, requirements thatmust be met, and analytical models that must be used. The English verb thatappears in Principles is shall.

6 [EN 1990 (2) & (3)]Application Rules identified by the absence of a letter after their paragraphnumbers are generally recognized rules that comply with the Principlesand satisfy their requirements. English verbs that appear in ApplicationRules include may , should , can , etc.[EN 1990 (4)] of limit state designThe structural Eurocodes are based on limit state principles, in which adistinction is made between ultimate and serviceability limit limit states are concerned with the safety of people and thestructure. Examples of ultimate limit states include loss of equilibrium,excessive deformation, rupture, loss of stability, transformation of thestructure into a mechanism, and Eurocode 7 Serviceability limit states are concerned with the functioning of the structureunder normal use, the comfort of people, and the appearance of theconstruction works.

7 Serviceability limit states may be reversible ( ) or irreversible ( yield).Limit state design involves verifying that relevant limit states are notexceeded in any specified design situation (see Section ). Verifications areperformed using structural and load models, the details of which areestablished from three basic variables: actions, material properties, andgeometrical data. Actions are classified according to their duration andcombined in different proportions for each design 16 illustrates the relationship between these various elements of limitstate situationsDesign situations are conditions in which the structure finds itself at differentmoments in its working normal use, the structure is in a persistent situation; under temporaryconditions, such as when it is being built or repaired, the structure is in atransient situation.

8 Under exceptional conditions, such as during a fire orexplosion, the structure is in an accidental situation or (if caused by anearthquake) a seismic situation.[EN 1990 (2)P]Society is willing to accept that fires and explosions may lead to buildingdamage necessitating repair whereas snow and wind should not. Noneof these events must lead to collapse. The structural Eurocodes define partialfactors for accidental and seismic situations ( exceptional conditions whichare unlikely to occur) that are typically These factors are considerablylower than those specified for persistent and transient situations (conditionswhich are more likely to occur), typically The development ofdifferent design situations helps to determine what level of reliability thedesign requires and what actions need to b e c o n s i d e r e d a s p a r t o f t h a t d e s i g of structural design33 Figure 16.

9 Overview of limit state design34 Decoding Eurocode limit statesUltimate limit states (ULSs) are concerned with the safety of people and thestructure.[EN 1990 (1)P]EN 1990 identifies three ULSs that must be verified where relevant: loss ofequilibrium (EQU); failure by excessive deformation, transformation into amechanism, rupture, or loss of stability (STR); and failure caused by fatigueor other time-related effects (FAT). (Limit states GEO, UPL, and HYD, whichare relevant to geotechnical design , are discussed in Chapters 6 and 7.) Thesethree letter acronyms are used throughout the Eurocodes as shorthand forthe limit states, which, for structures, are defined more fully as state EQUL imit state EQU, dealing with static equilibrium, is defined as:Loss of static equilibrium of the structure.

10 Considered as a rigid body, whereminor variations in the [actions or their distribution].. are significant, andthe strengths of .. materials .. are generally not governing.[EN 1990 (1)P(a)]Limit state EQU does not occur when the destabilizing design effects ofactions Ed,dst are less than or equal to the stabilizing design effects Ed,stb:[EN 1990 exp ( )] ,,EEddstdstbas illustrated in Figure example, consider a motorway gantry (see Figure 18) subjected to adesign horizontal wind load Pd = 250kN acting at a height h = above thebase of the gantry. The design destabilizing ( overturning) moment aboutthe toe of the structure MEd,dst is given by:= = = ,MPhkN mkNmEd dstdIf the design self-weight of the gantry is Wd = 1600kN and its base width B= , then the design stabilizing ( restoring) moment about the toeMEd,stb is:Figure 17.