Capacity Design and Fatigue Analysis of Confined Concrete Columns
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The capacity design philosophy requires the identification of all potential failure mechanisms. A preferred failure mechanism is chosen and efforts are made, through design detailing, to suppress all other undesirable failure modes. The undesirable failure modes that must be suppressed by design are three: concrete failure due to lack of confinement; buckling of the longitudinal reinforcement; and shear failures both within and outside the plastic hinge zone. This report examines of these primary failure modes and establishes a theoretical basis for suppressing their occurrence. First, based on energy balance requirements, the required amount of transverse confinement reinforcement to inhibit hoop fracture resulting from reversed cyclic (low cycle fatigue) loading is derived. Secondly, the required amount of transverse reinforcement to inhibit buckling of the longitudinal compression reinforcement is considered and a new approach to the inelastic buckling problem based on plastic analysis is presented. The theory distinguishes between local buckling and global buckling. This approach bar to stability analysis is validated against experimental results. The third undesirable failure mode involves shear resistance. For this, following a review of present state-of-the-art and state-of-the-practice recommendations, a new rational method of shear resistance is proposed. Finally, design recommendations are presented in the form of simple equations that require determination of a volumetric ratio of transverse reinforcement.