In assessment of structures it is usually most effective in a first step to collect additional
information about the structure. The collection of information then provides the basis for
deciding on the required and relevant structural reassessments and modifications. Two
important issues must be considered when planning inspections and assessing inspection
results, namely:
development of a hypothesis in regard to the phenomena being inspected and
the significance of the inspected indicators in regard to the hypothesized phenomena.
The first issue is important for the purpose of selecting appropriate inspection methods and
procedures. The ability of different inspection methods, visual as well as NDE methods, to
detect damages or conditions otherwise not complying with design assumptions is very much
dependent on the types of damages at hand. Whenever it is possible to develop hypothesis
about the possible causes for damages it is also possible to devise inspection strategies, i.e.
where to inspect with which method and how often. However, inspections should also be
performed at regular intervals in regard to possible unforeseen or simply unknown phenomena.
Such phenomena could in the beginning of the life of a structure be related to errors during
construction and at later stages to damages due to accidents. For this type of phenomena
visual inspections covering the total extent of the structure is normally useful performed in
connection with standard maintenance activities such as cleaning, painting etc.
Regarding the second issue it is of utmost importance that the ability of the inspection method
to detect the type of damage which is hypothesized is provided in quantitative terms. In effect
the inspection results can in general only be considered as indicators of the real condition of
the structure. The issue here is to which degree the indication of a certain condition is related
to the real condition. For this purpose the concept of the Probability of Detection is very
useful. The Probability of Detection provides a quantification of the quality of an inspection
method through the probability of detection of a damage of a given size or extent.
Structural Performance Assessment
Fundamentally the difference between a structure being designed and a structure subject to a
reassessment is the available or collectable information about the structure, related to
exposure, vulnerability and robustness as well as the costs associated with improvements of
the structural performance characteristics.
New structures are designed according to design codes whereby appropriate performance is
ensured by a prescribed design safety format. At the time of the design the assumed
uncertainties associated with the design variables are assessed through the given specifications
for the manufacturing and construction of the structure together with generally available
information in regard to the statistical characteristics of loads and other environmental factors.
For more details please refer to Lecture 11.
An existing structure can be measured, inspected, tested, instrumented and proof-loaded. In
principle all information relevant for assessing the condition and performance of an existing
structure can be collected, however, at a cost. In addition to the information which may be
collected at time of the assessment also information such that the structure has survived a