Structural optimization of a bridge beam section subjected to instability of equilibrium

It is well known that in the design of thin-walled structures, extreme attention must be made to the instability of equilibrium problem. This phenomenon involves a performance characteristics loss of the element in which it is manifested, almost instantaneous. If the is a single structural element it can be called local instability”; If the problem is a full structure it can be called global instability”. In both cases, the failure usually occurs suddenly with extreme danger. In this work, starting from a typical steel bridge, the design optimization target was in relation to the instability of equilibrium phenomenon about the web of the main beams. The starting structure is characterized by two main beams with 40 m of span and 2m of height. The beams are obtained by the union of three segments (one central and two lateral). The thicknesses of the flanges and web of the segments are modified, depending on the project needs. The two main beams, following a classic constructive scheme, are internally connected together through a series of reticular substructures. These transverse stiffening elements, in addition to stabilizing the beams during assembly, can also be used as reinforcement against the possible flexural-torsional equilibrium-type instability. The web of the beams are further reinforced with welded ribbings. These panel types have variable sizes (typically smaller in the zones under a shear stress state joined to axial compression) depending by needs.