Page 13 - Dynamic Loading and Design of Structures

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Chapter 1

Probabilistic basis and code format for loading

Morios K.Chryssanthopoulos

1.1 INTRODUCTION

In the last 30 years, practical probabilistic and reliability methods have been developed to
help engineers tackle the analysis, quantification, monitoring and assessment of structural
risks, undertake sensitivity analysis of inherent uncertainties and make rational decisions
about the performance of structures over their working life. These tasks may be related to a
specifie structure, a group of similar structures or a larger population of structures built to a
code of practice. Within a time framework, the structures may be at the design stage, under
construction or in actual use. Hence, the methods may be required to back calculate
performance and compare with earlier perceptions and observations, or to predict future
performance in order to plan a suitable course of action for continued safety and functionality.
Clearly, uncertainty is present through various sources and can propagate through the decision
making process, thus rendering probabilistic methods a particularly useful tool.
The purpose of this chapter is to summarize the principles and procedures used in
reliability-based design and assessment of structures, placing emphasis on the requirements
relevant to loading. Starting from limit state concepts and their application to codified design,
the link is made between unacceptable performance and probability of failure. This is then
developed further in terms of a general code format, in order to identify the key parameters
and how they can be specified through probabilistic methods and reliability analysis. The
important distinction between time invariant and time variant (or time dependent)
formulations is discussed, and key relationships allowing the treatment of time varying loads
and load combinations are presented. In subsequent sections, an introduction to the theories of
extreme statistics and stochastic load combinations is presented in order to elucidate the
specification of characteristic, representative and design values for different types of actions.
This chapter is neither as broad nor as detailed as a number of textbooks on probabilistic
and reliability methods relevant to structural engineering. A list of such books is given at the
end of the chapter. The reader should also be aware of recent documents produced by ISO
(International Organization for Standardization)