Introduction - Assessment and Refurbishment of Steel Structures - page 19

considered as a spatial system. The influence of failures and damage is substantially more
significant in the models of resistance than in those of global analysis. Alterations in the
geometric parameters of cross-sections (corrosion losses) or elements (deviations from
direct alignments) considerably affect the reliability of structural elements and their
cross-sections in a very distinct way. Methodologically, the procedure is the same as
when considering the initial imperfections of systems and their elements in designing a
new structure. The basic difference is the possibility of specifying the damage and
introducing it as the appropriate imperfection in calculation models of global analysis and
structure reliability.

Modification of values of partial safety factors for existing structures
Adaptations of the reliability level of an existing structures in the method of partial
factors can be projected into the design values for load effects and resistance of the
structure. As these are determined by means of the characteristic values and appropriate
partial safety factors, this adaptation will be expressed in a change in values for the
partial safety factors of load effects and resistance of the structure.
In checking the reliability of building structures, the European pre-standard [1] has
introduced two values for the partial safety factors of loads for persistent and transient
design situations expressed in terms of ultimate limit states. In particular

• For adverse effects of permanent loads: γ FG =1.35
• For adverse effects of variable loads: γ GQ =1.50.

These values for partial safety factors are also essential in checking the reliability of
existing structures. In addition to them, the European pre-standard specifies namely states
[1] other values for these factors, applicable for checking the static balance of the
structure or, as the case may be, to check the reliability of its foundations, as well as the
values of these factors for accidental design situations. When applying an adjusted
reliability level for existing structures, the values of the above-mentioned partial safety
factors of load effects will be modified. Provided the value of the coefficient of variation
of permanent load effects is unaltered which is recommended in [1] as ν G =0.1, the
following values may be set depending on the planned residual lifetime of the structure t r :
• For t r >20 years γ FG @ 1.30

• For 10< t r ≤20 years γ FG @ 1.30

• For 5< t r ≤10 years γ FG @ 1.30

• For t r ≤5 years γ FG @ 1.25.

Similarly, for the partial safety factors of variable load effects, given on unaltered value
of the variation coefficient v Q of variable load effects, values may be set again depending
on the planned residual lifetime of the structure:
• For t r >20 years

• For 10< t r ≤20 years

• For 5< t r ≤10 years