How strong is S355 really?

How strong is S355 really

S355 steel is a European standard structural steel grade which has a minimum yield strength of ReH,min = 355 N/mm2 for the smallest thickness range covered by the product standard. This is the nominal value of the yield stress, as opposed to the actual measured yield strength (which is usually higher). Steel structures designed to Eurocode 3 adopt nominal values of the steel properties as characteristic values in design calculations.

The new Annex E of prEN 1993-1-1:202X gives statistical data of properties for materials currently available on the European market, satisfying the relevant European product standards. For steel grade S355, the mean value Xm is 1.20ReH,min = 426 N/mm2 with a coefficient of variation of 5%. The values currently adopted for partial factors γM are calibrated on this statistical data, considering a reliability index of 3.8 for a reference period of 50 years. This means that for design purposes, the nominal value cannot simply be substituted by, for example, a mean value without proper re-calibration of the partial factors in order to satisfy the reliability requirements from Eurocode 0.

S355 steel is a European standard structural steel grade which has a minimum yield strength of ReH,min = 355 N/mm2 for the smallest thickness range covered by the relevant standard – i.e. BS EN 10025. This is the nominal value of the yield stress, as opposed to the actual measured yield strength.

Steel structures designed to Eurocode 3 (British Standard) adopt nominal values of the steel properties as characteristic values in design calculations. Eurocode 0 defines characteristic value as “the value of a material property having a prescribed probability of not being attained in a hypothetical unlimited test series. This value generally corresponds to a specified fractile of the assumed statistical distribution of the particular property of the material.” Clause 4.2(3) of BS EN 1990:2002+A1:2005 further states that:

  • where a low value of material or product property is unfavourable, the characteristic value should be defined as the 5% fractile value,
  • where a high value of material or product property is unfavourable, the characteristic value should be defined as the 95% fractile value.

Typically, the actual yield strength is higher than the nominal value. The new Annex E of prEN 1993-1-1:202X gives statistical data of properties for materials currently available on the European market, satisfying the relevant European product standards. For steel grade S355, the mean value Xm is 1.20ReH,min = 426 N/mm2 with a coefficient of variation of 5%. Annex E also gives an upper reference value, X5%, and a lower reference value, X0.12%. They refer to non-exceedance probabilities of 5% and 0.12%, respectively. For S355:

X5% = 1.11ReH,min = 394 N/mm2
X0.12% = 1.03ReH,min = 366 N/mm2

It should be stressed that the values currently adopted for partial factors γM are calibrated on this statistical data, considering a reliability index taken equal to 3.8 for a reference period of 50 years.

This means that for design purposes, the nominal value cannot simply be substituted by, for example, a mean value without proper re-calibration of the partial factors in order to satisfy the reliability requirements from Eurocode 0.