Fixed or free
Static or dynamic.
whereby their nature in regard to variability of magnitude in time, variability in regard to
position in time and their nature in regard to their effect on the engineering system may be
characterised. Knowing these characteristics is a prerequisite for the probabilistic modelling.
As an example consider the case where the reliability in regard to ultimate collapse of a
reservoir structure is analysed. The load acting on the structure considered is the hydrostatic
pressure due to the water contained in the reservoir. As the hydro static pressure varies with
the water level in the reservoir, which is dependent on the amount of rainwater flowing into
the reservoir the loading must be considered to be variable in time. Furthermore due to the
characteristics of the hydrostatic pressure loading the loading is assumed fixed in space.
Finally as the reliability of the reservoir structure is considered in regard to an ultimate
collapse failure mode and is dynamically insensitive the loading may be idealised as acting
static.
Having identified the characteristics of the considered load the probabilistic modelling may
proceed by:
specifying the definition of the random variables used to represent the uncertainties in the
loading,
selecting a suitable distribution type to represent the random variable,
assigning the distribution parameters of the selected distribution.
In practical applications the first point is usually the most important issue to solve and
requires a clear definition of the reliability problem to be considered. For the example
considered the random variable should e.g. only be concerned about the modelling of the
random variations in time of the reservoir water level.
The second point can as mentioned in previously not be performed on the basis of data alone
but moreover requires the simultaneous consideration of data, experience and physical
understanding.
In the following sections the probabilistic modelling of loads acting on buildings is given. The
presentation closely follows the Probabilistic Model Code by the JCSS [17] but some aspects
have been simplified. The resulting probabilistic load model can thus be considered as a
“light” version of the Probabilistic Model Code.
Loads on Buildings – the JCSS Probabilistic Model Code “Light”
The “Light” version of the recommended load model for loads on buildings by the JCSS
(2001) considers the loads illustrated in Figure 7.1.