Page 15 - Dynamic Loading and Design of Structures

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function, gs( · ), in which case equation (1.1) can be expressed as

(1.2)

where s and r represent subsets of the basic variable vector, usually called loading and
resistance variables respectively.

1.2.2 Partial factors and code formats
Within present limit state codes, loading and resistance variables are treated as deterministic.
The particular values substituted into eqns (1.1) or (1.2)—the design values—are based on
past experience and, in some cases, on probabilistic modelling and reliability calibration.
In general terms, the design value x of any particular variable is given by
di

(1.3a)

(1.3b)

where xki is a characteristic (or representative) value and γ i is a partial factor. Equation (1.3a)
is appropriate for loading variables whereas eqn (1.3b) applies to resistance variables, hence
in both cases γ i has a value greater than unity. For variables representing geometric quantities,
the design value is normally defined through a sum (rather than a ratio) (i.e. x =x ± x,
di
ki
where x represents a small quantity).
A characteristic value is strictly defined as the value of a random variable which has a
prescribed probability of not being exceeded (on the unfavourable side) during a reference
period. The specification of a reference period must take into account the design working life
and the duration of the design situation.
The former (design working life) is the assumed period for which the structure is to be used
for its intended purpose with maintenance but without major repair. Although in many cases it
is difficult to predict with sufficient accuracy the life of a structure, the concept of a design
working life is useful for the specification of design actions (wind, earthquake, etc.), the
modelling of time-dependent material properties (fatigue, creep) and the rational comparison
of whole life costs associated with different design options. In Eurocode 1 (European
Standard, 2000), indicative design working lives range between 10 to 100 years, the two
limiting values associated with temporary and monumental structures respectively.
The latter (design situation) represents the time interval for which the design will
demonstrate that relevant limit states are not exceeded. The classification of design situations
mirrors, to a large extent, the classification of actions according to their time variation (see
Section 1.5). Thus, design situations may be classified as persistent, transient or accidental
(ISO, 1998). The first two are considered to act with certainty over the design working life.
On the other hand, accidental situations occur with relatively low probability over the design
working life. Clearly, whether certain categories of actions (snow, flood, earthquake) are
deemed to