Nonlinear Backbone Modeling of Concrete Columns Retrofitted with Fiber-Reinforced Polymer or Steel Jackets

The use of nonlinear analysis procedures in seismic retrofitting of existing concrete structures has become commonplace in practice. Backbone curves are often used to capture the nonlinear response of components in a simplified but sufficiently accurate way. Procedures to construct curves for existing components of frames (beams, joints, and columns) have received considerable attention and have been modified over the years. In contrast, recommendations to construct these backbone curves for retrofitted components are largely lacking. This paper presents recommendations that can be used to construct backbone curves of circular and rectangular retrofitted columns using jacketing materials within the context of ASCE/SEI 41-13 and ACI 369R-11. The recommendations are developed using a sectional model for force parameters and a statistical study of load-deformation results from a database of retrofitted columns for drift parameters when sufficient experimental data are available. Key points in the backbone response of jacketed columns are summarized in tabular form consistent with ASCE/SEI 41-13 to facilitate possible adoption in future updates of the standard. A probabilistic model is introduced to allow selection of drift values that correspond to selected exceedance probabilities.