Influence of Residual Stresses Induced by Cold Curving on the Resistance of I-section Steel Members

Cold curving is today the cheapest and more common way to produce curved steel members by bending or curving of straight members. This is a repetitive process during which the straight member passes several times through a machine with bending rolls imposing progressively the desire curvature (see for example the curving machine of figure 1). The non-reversible deformations introduced by the process are associated with additional self-equilibrated stresses locked in the section. Inevitably, these locked-in stresses, also called residual stresses, will have an influence on the amount of stresses that the curved member can resist during service. Actually, the presence of residual stresses is not specific to arches and such stresses also exist in hot-rolled straight members due to thermal effects as the section is cooled down when exiting the mill. It is well known that these stresses have non negligible influence on the behaviour of the member (especially on beam-columns) and this influence has been abundantly studied, quantified and incorporated in the design methodology and all standards for steel construction. Yet the way these stresses influence the member behaviour depends on their distribution in the cross section, which depends, in turn, on their physical origin. As those origins are very different in the case of curved members than in straight members, it seemed important to provide a better understanding of these residual stresses and of their influence on the overall behaviour of arches.