On Problems of Characteristic and Nominal Values of Steel Yield Strength and Their Influence on Design Resistance of Ste
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On Problems of Characteristic and Nominal Values of Steel Yield Strength and Their Influence on Design Resistance of Steel Structural Members Marcela Karmazínová1 and Jindrich J. Melcher1 1
Brno University of Technology, Faculty of Civil Engineering, Brno, Czech Republic
ABSTRACT The paper deals with the problems of mechanical properties of structural steels used for loadcarrying structures of civil engineering constructions. The attention is mainly paid to the values of yield strength from the viewpoint of the design concept of European Standards, i.e. Eurocode 3 for the design of steel structures, with respect to the structural reliability given by the failure probability. In the case of other materials, concrete or timber for example, characteristic and design values of strength given by the corresponding failure probability are used in the design procedures, where the partial safety factor of material is γM > 1.0. However, in the case of steel, the nominal values instead of characteristic values of steel yield strength are utilized in the structural design procedure, with the partial safety factor for material of γM = 1.0. These values have been derived based on the statistical and probabilistic evaluation of the data sets of actual material properties obtained from various recent steel producers. The paper presents the brief information on the results of the study oriented to the influence of the nominal or characteristic value of yield strength on the design resistance, which is shown on the examples of the member subjected to tension and bending moment, respectively. The influence mentioned is shown using the examples of structural steels used in constructions of the age of about 40 and 100 years. INTRODUCTION According to European Standards (see e.g. [1], [3]), structural material properties and other parameters, especially strengths are given by characteristic and design values in the case of the most materials. The characteristic strength is statistically defined (with only some exceptions) as 5 % fractile and the design value as 0.01 % fractile, with respect to the required resulting structural reliability which is derived from the allowable failure probability. This principle is usually used in the case of concrete, timber, steel reinforcement, for example, and so on. But for the yield strength of structural steels used for load-carrying structures of civil engineering constructions the nominal value instead of characteristic value is used mostly. PROBLEM FORMULATION In usual cases the design value of material property Xd can be calculated according to the general formula of
Xd
Xk , JM
(1)
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where Xk is the characteristic value of the property and γM is the partial safety factor for material and as a rule γM ≥ 1. But the design value of steel yield strength fyd is in the most cases given as fy f yd , (2)
JM
where fy is not the characteristic yield strength, but its nominal value, and partial safety factor is γM = 1. It is caused just by the utilized value of fy, which does not correspond to 5
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