Lifetime-Oriented Structural Design Concepts

Lifetime-Oriented Structural Design Concepts

Structures deteriorate during their lifetimes, e.g. their original quality decreases.
In terms of structural safety, this reduces the original safety margin,
a process, which also can be described as an increase of structural damage.
If, in such deterioration, the safety parameter decreases below the admissible
safety limit, or the structural damage parameter increases beyond the admissible
damage limit, then the structural service life will be terminated. If the
failure safety value or the structural damage parameter both reach unity, the
structure (theoretically) will fail.

The initial structural properties must have sufficient reserves, in order to
compensate future reductions of safety against failure and of safety against
reaching the limits of serviceability. The final structural properties, at the end
of the service life or at the end of the relevant inspection interval, respectively,
must include a minimum resistance safety, a minimum serviceability level, and
other minimum qualities.
Lifetime-related deteriorations can happen in various forms and can consist
of various components. For example for structural concrete.

Deteriorations therein can be induced mechanically, e.g. by load cycles leading
to fatigue effects. They can also be induced non-mechanically, e.g. by
corrosion or other chemical processes.
In combination, deterioration effects can be superimposed by addition, if
there is no interaction between them. In case of interactions, the superposition

can be more than additive because of amplification effects due to influences

In the beginning of the structural lifetime, the safety against failure and
against losses of other important structural qualities must have sufficient reserves.
In the early lifetime, maybe e.g. concrete post-hardening may improve
the safety situation for a while, but later on, deteriorations lead to safety
reductions. At the end of the planned service period, a remaining minimum
safety is still required.

This time variant safety problem or reliability problem, respectively, is presented
in Figure 1.3, in principle, where the time histories of resistances and actions,
together with their statistical distributions, are plotted in relation to each
other.Degradations of the resistances andmaybe certain increases of the actions
effect time-dependent safety losses. For analytical predictions of these developments,
methods for time-dependent stochastic calculations are needed.

Current design standards do not provide a satisfactory basis or procedure to
ensure expected structural lifetimes. These may vary from only a few years—
for temporary structures—to more than a century for tunnels, dams of water
reservoirs, or nuclear repositories. There is an urgent demand for handling
this wide spectrum of lifetimes, in structural design and maintenance.
An appropriate differentiation of the design service lives of different building

classes is necessary.


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