Page 42 - Dynamic Loading and Design of Structures

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mentioned, a number of pragmatic reasons will also influence the final selection of values and
all these are normally considered by the code drafting committees.

1.5.4 Duration of actions

The knowledge, and specification, of a maximum action effect individually or in combination
is essential for safety checking. In some cases, especially where the sustained live load is high,
the duration characteristics, and in particular any intermittencies, may also be of interest. In
such a case, the components of the stochastic model would increase and may, for example,
include an interarrival duration density in addition to a variable describing the number of
magnitude changes (e.g. a jump rate which quantifies the number of amplitude changes in a
specified period).
In Eurocode 1 (European Standard, 2000; Eurocode 1.1 Project Team, 1996), the frequent
and quasi-permanent values of a variable action may also be defined in terms of duration. For
example, the frequent value may be specified as that which is exceeded for 5 per cent of the
reference period considered; the corresponding percentage for the quasi-permanent value may
be 50 per cent.

1.6 CONCLUDING REMARKS

Structural reliability theory provides a rational basis for the description and quantification of
loads and resistances in structural engineering. It enables consistent comparisons to be made
between alternative hazards to which structures are exposed during their service life, and is an
indispensable tool for rational decision making in the presence of uncertainty. Whether this
uncertainty stems from objective (e.g. future realizations of natural events) or subjective (e.g.
limited knowledge of actual material properties in an existing structure) sources, the use of
structural reliability theory and the allied battery of probabilistic methods has led to very
significant contributions towards an improved design philosophy for structures, both large and
small, ordinary or extraordinary.
In the ensuing chapters of this book, the load effects arising from different natural or man-
made actions will be described in some detail, with regard to their nature and their treatment
in codes. Clearly, the best models for any particualr action and its effect will have to take into
account the principal characteristics of the generating phenomenon, as well as the detailed
features which come into play as the action interacts with different structural types and forms.
Nonetheless, there are generic features associated with actions and their effects, as well as
their consequences. This chapter has attempted to present, within a reasonable length, these
generic features, how they might be modelled using probabilistic concepts, what is their
significance in terms of the way in which the reliability of the structure may be estimated, and
finally how these issues are dealt with in modern codes of practice. The presentation herein
has been brief and, hopefully,