OS-C501

Offshore Standard DNV-OS-C501, November 2013
Sec.2 Design philosophy and design principles – Page 21
Guidance note:
The following uncertainties are usually considered:
- Uncertainties in the loads, caused by natural variability, which is usually a temporal variability
- Uncertainties in the material properties, caused by natural variability, which is usually a spatial variability
- Uncertainties in the geometrical parameters, caused by
- deviations of the geometrical parameters from their characteristic (normal) value
- tolerance limits
- cumulative effects of a simultaneous occurrence of several geometrical variation
- Uncertainties in the applied engineering models
- uncertainties in the models for representation of the real structure or structural elements
- uncertainties in the models for prediction of loads, owing to simplifications and idealisations made
- uncertainties in the models for prediction of resistance, owing to simplifications and idealisations made
- effect of the sensitivity of the structural system (under- or over-proportional behaviour)
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3.6.2 Partial safety factors are applied in design inequalities for deterministic design as shown by examples in
[3.6.6]. The partial safety factors are usually or preferably calibrated to a specified target reliability by means
of a probabilistic analysis. Sometimes the design inequalities include model factors or bias correction factors
as well. Such model or bias correction factors appear in the inequalities in the same manner as the partial safety
factors, but they are not necessarily to be interpreted as partial safety factors as they are used to correct for
systematic errors rather than accounting for any variability or uncertainty. Model factors and bias correction
factors are usually calibrated experimentally.
3.6.3 The following two types of partial safety factors are used in this standard:
— Partial load effect factors, designated in this standard by γ F .
— Partial resistance factors, designated in this standard by γ M .
3.6.4 In some cases it is useful to work with only one overall safety factor. The uncertainties in loads and
resistance are then accounted for by one common safety factor denoted g FM . The following simple relationship
between this common safety factor on the one hand and the partial load and resistance factors on the other are
assumed here corresponding to the general design inequality quoted in [3.6.6]:
γ FM = γ F x γ M
3.6.5 The following two types of model factors are used in this Standard:
— Load model factors, designated in this Standard by γ Sd .
— Resistance model factors, designated in this Standard by γ Rd .
Guidance note:
- Partial load effect factors γ F are applicable to the characteristic values of the local response of the structure. They
account for uncertainties associated with the variability of the local responses of the structure (local stresses or
strains). The uncertainties in the local response are linked to the uncertainties on the loads applied to the structure
through the transfer function.
- Partial resistance factors γ M account for uncertainties associated with the variability of the strength.
- Load model factors γ Sd account for inaccuracies, idealisations, and biases in the engineering model used for
representation of the real response of the structure, e.g. simplifications in the transfer function (see Sec.9). For
example, wind characterised by a defined wind speed will induce wind loads on the structure, and those loads will
induce local stresses and strains in the structure. The load model factor account for the inaccuracies all the way
from wind speed to local response in the material.
- Resistance model factors γ Rd account for differences between true and predicted resistance values, e.g. differences
between test and in-situ materials properties (size effects), differences associated with the capability of the
manufacturing processes (e.g. deviations of the geometrical parameters from the characteristic value, tolerance
limits on the geometrical parameters), and differences owing to temporal degradation processes.
- Uncertainties or biases in a failure criterion are accounted for by the resistance model factor.
- Geometrical uncertainties and tolerances should be included in the load model factor.
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3.6.6 A factored design load effect is obtained by multiplying a characteristic load effect by a load effect
factor. A factored design resistance is obtained by dividing the characteristic resistance by a resistance factor.
The structural reliability is considered to be satisfactory if the following design inequalities are satisfied:
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