Technical delivery standards like EN 10025, EN 10210, EN 12019 and EN 10225 clearly show a strong relation between the minimum yield strength and the thickness of a material. This effect is due to the rolling process and the chemical composition of the steel. In this article, the relations between material toughness and strength and the wall thickness and environmental temperature of steel structures are demonstrated. Further on, recommendations for structural engineers concerning the adequate choice of steel grade are given. Here, the use of high-strength steel and the consequences on a proper static and fatigue design is also concerned. To demonstrate the mentioned influences a calculation example of a circular hollow section steel column is given at the end of the article.
During the manufacturing of rolled products of structural steel, in the center of material of thicker plates and profiles less cold work hardening occurs due to the low rolling activity. The samples for the determination of the material properties are not taken from the edge, but from the inside of the steel plate for safety reasons, Figure 1. So, conditional on manufacturing thicker plates smaller yield strengths are provided. Generally, this fact seems to be known, but nevertheless this is frequently not taken into account in daily practice. Also for fatigue loads a reduction of the fatigue strength for large wall thicknesses has to be applied depending on the weld detail. Different reasons are held responsible for the so called size effect. Further on, the operation temperature of a structure influences the mechanical properties of steel and should therefore be considered during the choice of steel grade also.
That means, that the steel denomination does not give automatically the yield strength for all rolled products made of this steel grade.
