A mechanistic model and design procedure for composite-confined concrete columns

Federal Highway (FHWA) design procedures for seismic retrofit of bridge columns using composite jackets in the United States utilize an empirical design model derived from full scale tests. The model employed uses an energy balance concept that ignores a major component of strain energy and does not predict the radial strain in the confining wrap. An alternate approach to the design of confined concrete columns for both retrofit and new construction is proposed. The proposed approach uses a series of design models derived from basic mechanistic principals.; An existing moment-curvature model, which incorporates the “crack path” material model, is combined with new force-displacement and shear models to predict both the curvature and total radial strain as a function of moment, shear, axial load and wrap stiffness. Unlike the FHWA procedure, the proposed procedure accounts for the effect of the axial strain gradient on the degree of confinement provided by the wrap.; The new force-displacement model accounts for the relative stiffness of the foundation, the change in stiffness at the column-foundation interface due to localized damage and bond slip, and the variation between the initial and cracked stiffness of the concrete under cyclic loading. Displacement ductility and overall column stability can be determined from the force-displacement relationship. The new shear model is used to predict the additional wrap confinement stress due to the application of a lateral load combined with constant axial load.; Experiments were performed using cantilever concrete columns wrapped with an E-Glass composite wrap. Experimental results from the tests show good correlation between the predicted and experimental wrap confinement stress over a range of wrap stiffness, axial loads and diameter to height ratios. The experimental results, used in conjunction with the analytical models, provide a basis for new design equations to calculate to the total stress in the confining composite wrap. The experimental work also demonstrated that additional failure limit states, such as minimum wrap stiffness, low-cycle fatigue, and shear sliding should be included in any design procedure for external composite confinement.