Page 40 - Dynamic Loading and Design of Structures

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are characterized by a (usually) random magnitude and an occurrence rate. For many
accidental actions, statistical information is scarce. Hence, in practice, nominal values are
often used and sometimes values are agreed for individual projects.
Insofar as seismic actions are concerned, the design values are determined on the basis of a
return period of approximately 475 years for use in ultimate limit states and a return period of
about 50 years for serviceability limit states (see also Chapter 4).

1.5.3 Other representative values

For variable and accidental actions (i.e. for those actions whose time variation is significant),
there is a need to specify a few more representative values, in addition to the characteristic
value, for use in codified limit state design. These are briefly reviewed in the following and
are schematically shown in Figure 1.11.

Combination value ( 0 Q )
k
This value is chosen so that the probability that the action effects caused by any particular
load combination will be exceeded is approximately the same as by the characteristic value of
an individual action. In other words, the combination value is introduced to take account of
the reduced probability of the simultaneous occurrence of the most unfavourable values of
two or more independent variable actions. The combination value may be expressed as a
fraction of the characteristic value through a combination factor 0 (<1). The combination
value is used in load combinations pertaining to the ultimate limit state or to irreversible
serviceability limit states.
Using structural reliability theory, values for the combination factor 0 have been derived
for load combinations comprising two independent variable actions starting from either FBC
load processes or using Turkstra’s rule. Expressions for 0 for different probability
distributions can be found in code documents (ISO,

Figure 1.11 Definition of representative values of a variable action.