# Mechanical Properties of Bridge Steel from the Late 19 th Century
## 4 Discussion  
With reference to strength testing, it was primarily observed that each of the tested samples demonstrated a certain scope of plastic flow and a clear yield point. This can be observed in the examples of engineering stress-strain curves shown in Figure 4, both in longitudinal and transverse directions of rolling. Therefore, it was relatively straightforward to establish the strength of this bridge steel, which is defined by its yield point. In this case, according to standard EN 10025-1 [30], we assumed the lowest value recorded for the tested sample population. For both directions, longitudinal and transverse to rolling, this was \(R_{CH}=241\) MPa. If we assume that structural elements are made of material of the orientation longitudinal to the direction of rolling, the value of \(R_{CH}=241\) MPa should be considered as the strength of the analysed steel. In the course of analysis of results obtained in terms of yield stress, it was generally observed that tests for samples of both orientations were very homogeneous. This applies both to upper and lower yield strength, as well as ultimate strength. The obtained standard deviations are relatively low, because their maximum value is 3.79 MPa for upper yield strength, whereas minimum value is only 1.0 MPa for lower yield strength for transverse direction. Thus, it can be observed that the analysed material is very homogeneous in terms of its strength. Naturally, this also applies to mean values of individual strength parameters, which have very similar values. In terms of ultimate strength, minimum values defined for both the longitudinal and transverse directions are \(R_{m}=367\) MPa. Mean values of these parameters are \(R_{m}=368.7\) MPa and \(R_{m}=368.3\) MPa. As in the case of the yield point, very homogeneous test results were also obtained for ultimate strength for both directions of samples individually, and in combination. This homogeneity can be very clearly observed in the graph illustrated in Figure 6, which demonstrates the values of individual parameters and their mean values for samples cut out in the longitudinal and transverse direction of rolling. On the basis of the above, the definition of yield point of the analysed steel at \(R_{CH}=241\) MPa means that this steel can be classified as medium-strength steel. This value is lower than yield point values defined for bridge steel types of a similar age. For example, according to data provided in [9], steel taken from a bridge built in 1909 in China demonstrated a yield point of the value of \(R_{s}=275.7\) MPa. These values differ by about 14%, which is not a significant amount. In comparison to other examples, the differences are higher. An example is the Eiffel bridge, which was built in Portugal in 1878--i.e., in the similar period as the considered bridge. The strength of the puddled steel extracted from the Eiffel bridge was \(f_{y}=292\) MPa [21], which is about 21% higher than strength of the tested bridge steel. A similar comparison can be made with reference to data listed in [7] for steel bridges built in Sweden before 1901. In this case, the mean yield point may  
Figure 6: Illustration of values of yield and ultimate strength defined in bridge steel samples cut out in longitudinal and transverse directions of rolling. be assumed as \(f_{y}=295\) MPa. This value is 22% higher than \(R_{eH}=241\) MPa obtained for the steel of the analysed bridge. In turn, for a bridge built in 1896 in the US, the yield strength for rolled sections was estimated as 220 MPa [6]. This is almost 10% lower than the yield stress of steel considered in this study. The same difference applies to the Fao Bridge, for which the steel strength was identified as \(\sigma_{0.2\%}=219.9\) MPa [14]. However, in relation to the \"Most pod Cytadela\" bridge erected in 1873 in Warsaw in Poland, the strength is practically the same. The yield tensile strength for this structure was determined as equal to 250 and 245 MPa [20], for longitudinal and transverse directions, respectively.