Second-order Effects in Steel-Concrete Composite Columns of the Unbraced Frames

Abstract

An unbraced frame is regarded as the one without a bracing system to avoid the horizontal displacement of its nodes. These displacements progressively increase due to their interaction with axial load (second-order effects) reducing the column's resistant capacity and can lead to the global frame instability. At the present work the Eurocodes criteria related to the way to approach its design will be displayed. Different iterative processes that take into account second-order effects will be exposed as well as a non-linear procedure based on an incremental load application. This procedure will be applied over an unbraced frame under gravity and wind loads with reinforced concrete beams and steel-concrete composite columns. Two geometrical models will be used: one of them starts from geometry without deformation and the other one takes into account the initial imperfections. The results comparison regarding first-order linear analysis will show that bending moments at the critical section of the columns will increase by nearly 50% when their initial imperfections are taken into account.