Why curved is not the same as straight
Safe and effective construction of curved steel for use in bridges requires a thorough understanding of the stresses induced by bending and compression – stresses that differ for a curved member relative to a straight one. In fact, current European standards are lacking, given that they do not evaluate straight and curved members differently. The ‘Development of structural design recommendations for steel arches’ (Derestar) project was designed to address the current limitations in European design of steel arches for bridges. The researchers focused on two stresses that affect curved steel, one induced during the manufacturing process and one experienced during use. Specifically, curved members are typically formed by bending straight ones in a process called cold curving. The process locks in stresses in the curved sections that then influence the amount of stress the construction can withstand during use. Secondly, curved components are subject to radial pressures that straight members are not. Imagine standing on a wooden plank versus standing on a curved wooden arch. Standing on a plank induces only normal stress, or stress perpendicular to the plank. Standing on the curved arch induces transverse stress in addition to perpendicular stress, caused by radial pressure related to the curvature of the arch. The researchers developed analytical and numerical studies that elucidated specific size and shape parameters that influence residual and transverse stresses, enabling identification of thresholds for these parameters above which the straight member model no longer is applicable. Thus, the Derestar project resulted in design recommendations for implementation into Eurocode 3 (EC3) standards that currently consider only straight members. Future European steel arch bridges may now be more durable, leading to significant cost and safety benefits for European citizens and manufactures alike.
