7.1 Introduction
In risk and reliability assessing the risk and reliability of infrastructure and building the
probabilistic modelling of loads and resistances play a key role. Not only is it of great
importance to represent all relevant uncertainties in full consistency with available
information, it is also decisive that such models are standardized and as a minimum
requirement cover the design and/or assessment situations addressed by governing codes of
standards.
Only if reliability assessments are performed on a standardized basis is it possible to compare
reliability analysis results. Furthermore, only in this case is it possible to compare results with
given requirements to the minimum acceptable reliability. This fact has been realized already
35-40 years ago and this was part of the motivation for initiating the Joint Committee on
Structural Safety (JCSS). Since the last 35 years the JCSS has been working partly on
establishing a standardized probabilistic framework for performing probabilistic design of
structures and partly on establishing standardized probabilistic models for the representation
of uncertainties associated with the most commonly types of loads and for the representation
of the resistances of the most commonly applied building materials.
At the present time a large selection of load and resistance models have been developed by the
JCSS and these are publicly available as the JCSS Probabilistic Model Code on
http://www.jcss.ethz.ch. In the present lecture probabilistic models for loads and resistances are
introduced shortly in general terms and thereafter simplified but fully compatible versions of
some of the models provided by the JCSS are outlined as a first basis for structural reliability
evaluations.
7.2 Probabilistic Load Modelling
In the following the term load will be related to forces acting on structural components and
systems, but the notions and concepts introduced will to a large extent be valid for other types
of influences, such as temperature, aggressive chemicals and radiation “acting from the
outside” of the engineering system of consideration.
Loads and/or load effects are uncertain due to:
Random variations in space and time
Model uncertainties
Statistical uncertainties.
Whereas the model uncertainties associated with the physical model used to represent the
loads and/or load effects in the reliability analysis may be represented by random variables as
explained in Lecture 2 the loads themselves are usually time and space varying quantities and
thus are best modelled by stochastic processes.
It is often helpful to categorise loads according to the following descriptors:
Permanent or variable