Innovative approaches to column confinement.

Reinforced concrete columns subjected to strong earthquakes may experience inelastic deformations. Inelastic deformability of these columns is of utmost importance for overall strength and stability of structures. Column deformability may be increased through confinement of core concrete. Conventional confinement reinforcement for square mid rectangular columns consist of closely spaced perimeter hoops, overlapping hoops, and crossties. The confinement steel requirements of current building codes often result in high volumetric ratios of transverse reinforcement which may lead to the congestion of column cages, which may result in concrete placement problems. An alternative to conventional transverse reinforcement is to use welded reinforcement steel grids (WRG) prefabricated to the required size and amount of steel, double headed studs (DHS) made of high-strength steel with conventional perimeter hoops, or Fiber Reinforced Plastic (FRP) grids. An experimental investigation was conducted to evaluate physical and mechanical properties of the new confinement reinforcement. Column tests were conducted to investigate structural performance of concrete columns reinforced with WRG, DHS, or FRP grids. Thirty-one large-scale columns with different volumetric ratios, spacings, and arrangements of confinement reinforcement were tested under simulated seismic loading. Test results indicate that the welded reinforcement grids can be used effectively as confinement reinforcement provided that the steel used has sufficient ductility and the welding process employed does not alter the strength and elongation characteristics of steel. The experimental program met these requirements and showed 7% to 10% strains prior to failure. Concrete columns confined with double head snub and conventional perimeter hoops also showed ductile behavior, developing lateral drifts of 4% to 6% prior to a significant loss in moment capacity, depending on the degree of confinement provided. The failure of these columns was characterized by the sudden opening of the 135-degree hooks of perimeter hoops followed by the crushing of core concrete and buckling of longitudinal bars. In addition, test results indicated also that concrete columns properly confined with fiber reinforced plastic grids showed ductile response and behaved as well as the companion columns confined with welded steel grids. The research program also included analysis of columns tested in the experimental phase. A rational analytical model was developed for buckling of longitudinal reinforcement in columns. The model was incorporated in the analytical procedure to compute the ultimate deformation capacity of concrete columns, many of which failed due to bar buckling. (Abstract shortened by UMI.