Titel_SS06

Also the local experience of the performance of structures executed in the same cement, same
concrete composition, same producer and so on should be included in the planning.

12.3 Theoretical Framework for Reassessment

Assessment of existing structures using methods of modern reliability theory should be seen
as a successive process of model building, consequence evaluation and model updating by
introduction of new information, by modification of the structure or by changing the use of
the structure. The principle may be illustrated schematically as shown in Figure 12.4.

Uncertainty

modelling

Probabilistic modelling

Limit state equation Consequence

Actions

Change use of

structure

Introduce new

information

Modify

"design"

Figure 12.4: Bayesian probabilistic assessment of structures.

As an example consider the reassessment of an existing steel bar subject to tension loading. It
is required that the loading on the steel bar is increased by 10%. The yield stress of the bar is
uncertain and if the steel bar fails the consequences will be a significant loss of value and a
potential loss of human lives. The requirements to the safety of the steel bar are given in terms
of the maximum acceptable annual probability of failure.

In engineering terms the assessment problem is reduced to deciding whether the steel bar can
be claimed “fit for purpose” considering the required increase of loading. If this is not the case
it must be decided whether actions should be taken to improve the knowledge about the yield
stress of the steel and /or if the steel bar should be strengthened.

With reference to Figure 12.4 the uncertainty model of the yield stress fy may be formulated

on the basis of more or less specific knowledge about the material properties of the steel bar at
hand. At the time when the bar is designed the only information available about the steel is a
defined steel grade and characteristic value for the yield stress. On the basis of such relatively
imprecise information a so-called a-priori probabilistic model for the ultimate yield strength
may be formulated.

The limit state function relevant for the assessment of the safety of the steel bar may be
defined in terms of the uncertain yield capacity of the steel bar r and the uncertain annual
maximum tensile load s.