Structural Steel
For the probabilistic modelling of the material properties of rolled structural steel sections a
multivariate Lognormal model is proposed in accordance with JCSS (2001). This model is
appropriate for structural steel with yield stresses up to 380MPa and is thus suitable the steel
categories S235, S275 and S355 used within SIA 263.
Description Variable Type EX VX
Yield stress fy Lognormal
,
uVfy
fysp 5 eC 0.07
Ultimate stress fu Lognormal BEf u 0.04
Modulus of elasticity E Lognormal Esp 0.03
Poisson’s ratio ^ Lognormal sp 0.03
Ultimate strain 'u Lognormal 'usp, 0.06
Table 7.6: Probabilistic model for rolled steel material properties.
The coefficients of variation as given in Table 7.6 have been assessed on the basis of
European studies from 1970 onwards. In Table 7.6 the index sp stands for a “specified” or
“nominal” value as provided by the code. The factor 5 is a spatial position factor which in
Normal cases may be assumed to be equal to one. Only for webs this factor increases to 1.05.
The factor u depends on the quantile value which corresponds to the nominal value and
normally lies in the interval - 2.0 to -1.5.
C is a reduction term accounting for a systematic difference between the yield stress obtained
by usual mill tests and the static yield stress as obtained by tensile experiments. According to
JCSS (2001) C may be taken as 20 MPa. The factor B is normally in the interval 1.1 - 1.5
depending on the considered type of steel (JCSS (2001)). For normal construction steels B
can be set equal to 1.5. If the steel material characteristics for a given batch are considered,
the coefficients of variation may be reduced by a factor of four and the variability of the
modulus of elasticity and the Poisson’s ratio can be neglected.
The spatial variability of the material characteristics parameters along the length of a rolled
steel section is in general rather small and can be neglected. The correlation matrix of the
parameters is given in Table 7.7.
(^) fy fu E (^) 'u
fy 1 0.75 0 0 -0.45
fu 1 0 0 -0.60
E 1 0 0
Symmetry 1 0
'u 1
Table 7.7: Correlation matrix of the properties for structural steel.