OS-C501

Offshore Standard DNV-OS-C501, November 2013
Sec.8 Safety-, model- and system factors – Page 135
SECTION 8 SAFETY-, MODEL- AND SYSTEM FACTORS
1 Overview of the various factors used in the standard
1.1 General
1.1.1 The safety factor methodology used in this standard is presented in section Sec.2 [3.6].
1.1.2 The Table 8-1 shows the various safety factors, model factors and system factors used in this standard.
Table 8-1 Overview of the various factors used in the standard
Symbol Designation Reference Type
γ F Partial load effect factor Sec.8 [2] safety factor
γ M Partial resistance factor Sec.8 [2] safety factor
γ FM (= γ F × γ M ) Combined load effect and resistance factor Sec.8 [2] safety factor
γ Sd Load model factor Sec.8 [3] model factor
γ Rd Resistance model factor Sec.8 [3] model factor
γ S System factor Sec.8 [4] model factor
γ fat Partial factor for fatigue analysis Sec.8 [5] safety factor
2 Partial load effect and resistance factors
2.1 General
2.1.1 The following two partial safety factors are defined in this standard (see Sec.2 [3.6]):
— partial load effect factors, designated by γ F
— partial resistance factors, designated by γ M .
2.1.2 In some cases it is useful to work with only one overall safety factor. The uncertainties in load effects
and resistance are then accounted for by one common safety factor, the combined load effect and resistance
factor, denoted γ FM , which in many cases comes about as the product of γ F and γ M :
γ FM = γ F x γ M
2.1.3 Partial load effect factors γ F are applicable to the local response of the structure. They account for
uncertainties associated with natural variability in the local responses of the structure (local stresses or strains)
from its characteristic values. The factors are selected based on the distribution type and coefficient of variation
(COV).
2.1.4 The distribution type and COV of the local response are linked to the uncertainties in the loads applied
to the structure, the transfer function and the type of structural analysis that was carried out. If the transfer
function and structural analysis are linear, the local load effect distribution type and COV will be the same as
those of the globally applied loads. If non-linearity is involved in either the transfer function or the analysis,
the distribution type and or the COV may change. In such a case, the distribution and the COV shall be
determined for the local response.
2.1.5 Partial resistance factors g M account for uncertainties associated with variability of the strength.
2.1.6 The partial load effect and resistance factor γ FM. = γ F x γ M in this standard is calibrated against different
target reliabilities. These target reliabilities correspond to annual probabilities of failure. The calibration has
been performed under the assumption of a design rule equal to the one given in Sec.2 [3.6.6], for which the
requirement to the partial safety factors in order to meet a specified reliability requirement is a requirement to
their product.
2.2 How to select the partial safety factors
2.2.1 The safety factor γ FM depends on the following:
— target reliability level, expressed in terms of annual probability of failure
— characteristic values for load effects and resistance, in this standard, there is only one option for definition
of characteristic load effect (see Sec.3 [9.4]) and one for the definition of characteristic resistance (see Sec.4
[1.6])
— type of distribution function for load effects and resistance, in this standard a normal distribution is assumed
for resistance, whereas several options are given for the load effect distribution type.
— coefficient of variation (COV) for load effect and for resistance.
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