For the purpose of this chapter, secondary effects are understood to include dynamic stresses as well as those which arise from the nature of construction details, especially the rigidity of joints (connections) in triangulated trusses (bridge girders). The term secondary stress is usually associated with these latter, following the initiative of Professor Asimont of the Munich Polytechnikum in 1877. In that year a prize was offered by the Polytechnikum (as noted in Chapter 1) for a method of calculating those stresses in trusses (termed Sekundarspannung by Asimont, to distinguish them from the stresses due to the axial or primary forces in the bars, that is, Hauptspannung). Dynamic stresses, on the other hand, became the subject of research in the nineteenth century, due to the failure of a number of iron railway bridges, which were caused by the passage of trains.

Secondary stresses

According to Grimm (1908), Asimont formulated the problem of secondary stresses in rigidly-jointed trusses and suggested that, since the resultants no longer pass through the panel points, a solution might be afforded by ‘Euler's equation of the elastic line’. In the event, the prize was awarded in 1879 to Professor Manderla and his solution was published soon afterwards (1880), although an approximate solution by Engesser in which chords were treated as continuous and web members as pin-jointed, appeared a year earlier (1879).