12.1 Introduction
When a structure is designed the knowledge about the structure 'as built' is associated with
uncertainty regarding geometry, material properties, loading and environmental conditions.
This uncertainty is in part due to inherent variation of e.g. material properties and loading
characteristics, but a substantial part of the uncertainty arise from lack of information. In this
way the uncertainty related to e.g. material properties in the design phase contains a
significant contribution from the fact that the materials manufacturer may not be known and
because the material batch characteristics may not be known.
Hence, the probabilistic models used in design and assessment of a structure merely reflect the
imperfect knowledge about the structure and this knowledge may be updated as soon as the
structure has been realised.
Given that the requirements regarding the present and future use of a structure are specified
the reassessment process is a decision process with the purpose of identifying the measures,
which will lead to the most economical fulfilment of these requirements.
Such measures may be to inspect and collect information regarding the geometry of the
structure, the material properties, the degree of deterioration of the structure, the static and
dynamic behaviour of the structure and the loading on the structure. Measures may also be
taken to repair or strengthen the structure or even to change the “intended use” of the
structure. Whatever measure is taken, it must be evaluated and compared to alternative
measures in terms of its consequence on safety and monetary value throughout the required
service life.
The following lecture summarizes the basic philosophy for the assessment of existing
structures. The purpose is to suggest a direction of thinking in assuring an appropriate
performance of existing structures over their residual service life. More elaborated accounts
on the same issue are available from the JCSS (2001), where also most of the required
theoretical concepts are explained in detail. However, the present text also introduces new
perspectives to assessment of existing structures which have emerged over the last few years.
12.2 General Philosophy for Reassessment
Structures are planned, designed, constructed and operated subject to a number of
requirements, specifications and assumptions.
Requirements to the use of structures are typically specified in regard to:
Purpose/use.
Safety to users.
Reliability in fulfilment of purpose/use.
Service life.
Durability subject to normal maintenance.