| Research Abstract |
Eight full-scale simply supported beams, four half-scale beams under load reversals, and four columns were tested in 1992,1993, and 1994, respectively, to gather experimental data about the effect of confinement stress on bond behavior in longitudinal bars. The specimens were designed to fail in bond splitting along the longitudinal bars. The variables of specimens were the number and the diameter of test bars, the spacing of lateral reinforcement, the use of sub ties, and axial stress ratio. From the test results, maximum bond stress was observed to improve as the amount of lateral reinforcement increases, and the effectiveness of sub-ties, which confine intermediate bars, was observed. Bond stress was observed to be governed by the confinement stress provided by lateral reinforcement. To evaluate the confinement stresses acting on the splitting plane, an analytical study was carried out in 1995. A simple analytical model consisted of three components : (1) beam which represents cover concrete ; (2) steel columns connected to the beam with pin connection, which represents hoops and sub-ties ; (3) crack spring which represent tensile force and crack opening of concrete in the splitting plane. From the analystical study of the confinement stresses provided by cover concrete, hoops and sub-ties, the concrete between bars does not contribute to the confinement at ultimate state. Analytically predicted ultimate bond stress agreed with the test results.
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