Cover plates are often added to built-up steel columns as an effective retrofit method for increasing strength. For retrofitted columns, the original member carries all loads, whereas the added cover plates resist only the loads applied after their addition. As a result, the original member may yield before the added cover plates. Although some specifications provide guidance for evaluating the capacity of retrofitted columns, the approaches are conservative and do not address the behavior of retrofitted columns if portions of the original cross section yield first because of preload. In this paper, the behavior and strength of retrofitted built-up columns, as well as the influence of the cover plates, are evaluated using two analysis approaches: (1) three-dimensional finite-element analysis and (2) nonlinear inelastic column buckling analysis. Both approaches include the effects of geometric imperfections, column slenderness, cyclic loading, and temperature gradient on the behavior and strength of retrofitted columns. The failure modes of the retrofitted columns depend on the column slenderness ratio (KL/r). For short columns, the failure mode consists of compression yielding of the entire cross section, where the existing section yields first, followed by the cover plates. For slender columns, the failure mode consists of inelastic flexural buckling about the weak axis. The existing section yields first, significantly reducing the flexural stiffness of the retrofitted section.
