Page 16 - Dynamic Loading and Design of Structures

Page 4

give rise to transient or accidental situations, will depend on local conditions. Typically, the
load combination rules are not the same for transient and accidental situations, and also a
degree of local damage at ultimate limit state is more widely accepted for accidental situations.
Hence, the appropriate load classification is a very important issue in structural design.
In treating time varying loads, values other than the characteristic may be introduced. These
so-called representative values are particularly useful when more than a single time varying
load acts on the structure. For material properties a specified or nominal value is often used as
a characteristic value, and since most material properties are assumed to be time independent,
the above comments are not relevant. For geometrical data, the characteristic values usually
correspond to the dimensions specified in design.
Partial factors account for the possibility of unfavourable deviations from the characteristic
value, inaccuracies and simplifications in the assessment of the resistance or the load effect,
uncertainties introduced due to the measurement of actual properties by limited testing, etc.
The partial factors are an important element in controlling the safety of a structure designed to
the code but there are other considerations to help achieve this objective. Note that a
particular design value xdi may be obtained by different combinations of x ki and γ i.
The process of selecting the set of partial factors to be used in a particular code could be
seen as a process of optimization such that the outcome of all designs undertaken to the code
is in some sense optimal. Such a formal optimization process is not usually carried out in
practice; even in cases where it has been undertaken, the values of the partial factors finally
adopted have been adjusted to account for simplicity and ease of use. More often, partial
factor values are based on a long experience of building tradition. However, it is nowadays
generally accepted that a code should not be developed in a way that contradicts the principles
of probabilistic design and its associated rules.
Equation (1.2), lends itself to the following deterministic safety checking code format

(1.4)

where F , f and a are design values of basic variables representing loading, resistance and
d
d
d
geometrical variables respectively, which can be obtained from characteristic/representative
values and associated partial factors, and γ sd, γ Rd are partial factors related to modelling
uncertainties (loading and resistance functions, respectively).
As can be seen, the safety checking equation controls the way in which the various clauses
of the code lead to the desirable level of safety of structures designed to the code. It relates to
the number of design checks required, the rules for load combinations, the number of partial
factors and their position in design equations, as well as whether they are single or multiple
valued, and the definition of characteristic or representative values for all design variables.