PSI - Issue 22

Stéphane Sire et al. / Procedia Structural Integrity 22 (2019) 64–69 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

66

3

metric tons and wagons of 16 metric tons each. It should be noted that from 1915 onwards, axle loads were 20 metric tons.

Fig. 2. Evolution of overloads for French railway bridges calculations.

The mechanical characteristics of metals have also increased over time. The evolution of the production processes of cast irons, wrought irons and steels has improved the static characteristics of these metals. In particular, for wrought irons and steels, materials used in railway bridge construction, the permissible tensile or compressive stresses are shown in Table 1 (for more details, see Strres, 2014). It should be noted that steel, not yet used in bridge construction, is not mentioned in the 1877 regulation and that wrought iron is no longer used as a construction material from the 1915 regulation. Table 1. Evolution of allowable static stresses (tension or compression) for the 1877, 1891, 1915 and 1925 French regulations Wrought irons (MPa) Steels (MPa)

Regulation of 09/07/1877 Regulation of 29/08/1891 Regulation of 08/01/1915 Regulation of 10/05/1927

60

-

65 or 85

85 or 115 80 to 135 130 or 140

- -

Considering cyclic loads, many simple calculation formulas have been proposed to take into account (working) stress variations in iron and steel elements of railway bridges. The 1891 French regulation introduced for the first time resistance criteria for wrought iron and steel parts that took cyclic loads into account. With this regulation, the Ministère des Travaux Publics in France indicates, for instance, that for wrought iron bridges the working stress limits (in kg/mm²), in case of variable loadings are described in Eq. (1) for compression or tension loadings and in Eq. (2) for alternative tension and compression loadings. σ = 6 + 3 (1) A and B are respectively the minimum and the maximum loads to which the studied part is subjected. σ = 6 − 3 (2) B and C are respectively the maximum load (absolute value) supported by the studied part, and the maximum load in the opposite direction.