Retrofit of Steel Columns: Parametric Studies and Design

Cover plates are often added to existing (loaded) steel columns as an effective retrofit method for increasing their compressive design strength. Current bridge design specifications provide equations for calculating the compressive design strength of steel columns. However, these equations cannot be extended to calculate the design strength of retrofitted columns because they do not account for the effects of cover plates added to the existing loaded members. The authors previously investigated the fundamental behavior of retrofitted built-up steel double-box columns using the finite-element analysis approach and also developed and benchmarked a nonlinear inelastic column buckling (NICB) analysis approach that accounts for the mechanics of existing (loaded) built-up steel columns retrofitted by added cover plates. This paper presents the results of comprehensive parametric studies conducted using the benchmarked NICB analysis approach to evaluate the behavior and strength of retrofitted steel columns. The parameters included are the geometric imperfection magnitude, locked-in dead-load ratio, cover-plate properties (yield stress, width, and thickness), and column-slenderness ratio (KL/r). Results from the parametric studies are used to propose revisions to the existing design equation. The proposed equation can conservatively estimate the axial compressive strength of retrofitted steel columns with (i) doubly symmetric sections and (ii) singly symmetric sections bending about the axis of symmetry. This is confirmed by using the equation to predict the axial compressive strength of retrofitted steel columns with different cross-sectional shapes. Finally, a design example is presented to illustrate the implementation of the proposed design equation.