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tInferred value (not given explicitly in EN 1997-1)

Values in [brackets] from UK National Annex to BS EN 1997-1

Underlined values from UK National Annex to BS EN 1990

*Values to be determined according to ultimate limits states STR/GEO

-Not applicable

7.3 Loss of static equilibrium

Ultimate limit state EQU is defined as:

[loss] of equilibrium of the structure or the ground considered as a rigid body, [where... minor variations... in actions... are significant, andf] in which the strengths of structural materials and the ground are insignificant in providing resistance. [en 1990 §6.4.1(1)P] and [en 1997-1 §2.4.7.1(1)P]

Verification of static equilibrium is expressed in the Eurocodes by the inequality:

Ed,dst - EdMb (+Rd ) [EN 1990 exp (6.7)] and [EN 1997-1 exp (2.4,modified)]

in which Ed,dst = the design effect of destabilizing actions, Edstb = the design effect of stabilizing actions, and Rd = any design resistance that helps to stabilize the structure. The term in brackets (i.e. Rd) is absent in EN 19904

EN 1997-1 notes that 'if any shearing resistance ... is included [in EQU], it should be of minor importance'. The inclusion of Rd in the inequality above appears to contradict this note and the latter part of the definition of EQU ('strengths ... are insignificant in providing resistance'). If resistance is insignificant, why is it included in the inequality? If resistance is significant, then limit states STR and GEO (see Chapter 6) ought to control the design.

EN 1997-1 also notes that EQU:

... is mainly relevant in structural design. In geotechnical design, EQU verification will be limited to rare cases, such as a rigid foundation bearing on rock. [EN 1997-1 §2.4.7.2(2)P NOTE 1]

Consider, therefore, the mass concrete dam bearing on rock that is shown in Figure 7.4, which would qualify as a rare case where EQU is relevant in geotechnical design. For simplicity, we will treat the dam as a rectangular block of height H and breadth B, whereas in reality its shape would be more complicated.

fthe words in brackets are omitted in Eurocode 7's definition.

*The symbol Td is used in Eurocode 7, but this is likely to be changed to Rd in a future corrigendum.

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