# Mechanical Properties of Bridge Steel from the Late 19 th Century
## 4 Discussion
On the other hand, in comparison to type S235 steel, which is widely used in construction and bridge engineering, the analysed steel has a similar yield point. Standard EN 10025-1 [30] requires this type of steel to have a yield point of the value of 235 MPa. Simultaneously, the mean value of yield point determined in the course of static tensile tests amounts to 320 MPa; therefore, the strength of the analysed bridge steel amounts to approximately 75% of this value. This comparison is even less favourable when set against the main type of steel used in bridge engineering--i.e., type S355 steel--which has a much higher yield point defined in the course of experimental tests that amounts to 355 MPa. Moving on to the discussion of parameters which determine the plasticity of the analysed bridge steel, it can be observed that a minor difference between the \(A\) and \(Z\) characteristics was noted in longitudinal and transverse directions of rolling. This is clearly visible in the combined graph illustrated in Figure 7. The obtained mean values of elongation of \(A=37.5\%\) and \(A=35.6\%\) for the longitudinal and transverse directions, respectively, classify the analysed material as highly ductile steel. This level is similar to that demonstrated by S235 steel types. It should also be noted that both \(A\) and \(Z\) deformation parameters obtained for the tested bridge are far higher than the values reported in [14; 20; 21] for similar bridge steels. The lowest values of \(A=6\%\) and \(Z=5.6\%\) were identified for the \"Most pod Cytadela\" bridge [20] in the transverse direction to the rolling. On a different note, both of these values are generally very low for structural steels. This proves the phenomenon of similar strength with significant differences in ductility at the same time for compared steels. With reference to impact strength, the first observation is the fact that its value varied significantly depending on the direction of rolling. In the case of the longitudinal direction, the average impact strength stood at \(U=62.9\) J/cm\({}^{2}\), while in the case of the transverse direction it was \(U=107.3\) J/cm\({}^{2}\). Thus, we can see that the ratio of these values gives a relative difference of about 70%. When comparing the obtained results with impact strength values defined for other types of bridge steel of a similar age, it can be observed that this value is higher in comparison with them. Charpy \(K_{v}\) energy required for a failure of steel determined in the temperature 0 \({}^{\circ}\)C for bridges constructed in Germany between 1901 and 1919 was  
Figure 7: Illustration of values of elongation and area reduction defined on bridge steel samples cut out in longitudinal and transverses directions of rolling. 18 J [31]. For bridge steel considered in this study, this is 50.3 J for longitudinal direction to the rolling. This is almost a three times higher Charpy energy than that estimated for German bridges. Upon the analysis of existing requirements for steel in accordance with standard EN 1993-1-10 [32], it can be observed that the analysed bridge steel easily fulfils them. According to [32], the minimum energy needed to fracture steel types S235/S275/S355, which are comparable with the analysed steel in this regard, is \(CVN_{min}=27\) J at a the temperature of +20 \({}^{\circ}\)C. In the case of the analysed steel, the minimum energy needed to fracture it was observed in the case of one sample cut out in longitudinal direction of rolling, and amounted to 44 J, at a test temperature of +23 \({}^{\circ}\)C. Therefore, in working conditions and in ambient temperature, the analysed bridge steel fulfils the requirements of existing standards.