Lateral Force €“ Story Drift Relationship of RC Columns With Side Walls Failing in Flexure

Abstract eng:
Practical use of secondary walls such as side wall is expected because contributions of secondary walls for stiffness or strength have been recognized. In this report hysteresis models of longitudinal reinforcing bars considering buckling and concrete considering confinement by not only transverse reinforcement but rigid base stub are proposed. Using these proposed models flexural analyses are conducted to simulate the moment curvature relations of two RC column specimens with a side wall. Authors have conducted static loading tests of 4 RC column specimens with an opened side wall failing in flexure and examined effects of openings on strength and deformation capacity of RC columns with a side wall. Specimens were subjected to constant axial load by two vertical jacks and lateral load reversals were applied at the top of the upper girder of the specimen. A flexural component among the lateral drift was evaluated using 6 sets of axial deformation obtained by vertical transducers installed at both sides of the specimen divided into 6 regions. Representative moment - curvature relationship, which would be compared with analysis in this study, was that obtained in the bottom region of the specimen. Ramberg Osgood hysteresis model is used for stress-strain relationship of longitudinal reinforcing bars with some modification. The reversed Ramberg Osgood function is applied for stress-strain relationship after buckling. On the other hand a buckling model for longitudinal reinforcing bars of RC columns has been proposed taking buckling mode into account. In general buckling length of longitudinal reinforcement in RC members extends over several times of the spacing of the transverse reinforcement. The buckling mode represents the number of hoop spacing along one buckling wave form. Authors have proposed a model of stress-strain relationship of concrete confined by hoop reinforcement. However authors have also reported that real flexural behaviors of columns could not be simulated with enough accuracy using the concrete model confined by hoop reinforcement only. This was because the compressive failure zone of concrete was limited locally near critical sections in case of columns subjected to moment and shear force and the confinement for concrete from rigid base stubs could not be ignored. From this view point confinement from rigid base stub is considered. The analytical cases ignoring both buckling of main bars and confinement from rigid base stub indicate that the analytical results cannot simulate the test results well from following two view points; i.e. (i)calculated restoring force after maximum strength decreases rapidly comparing to test in the positive loading direction where side wall is subjected to compressive force. (ii)Calculated hysteresis energy becomes high comparing to test in the negative loading direction where side wall is subjected to tensile force. On the other hand the cases considering both buckling of main bars and confinement from rigid base stub indicate that the analytical results can simulate the test results qualitatively well from following two view points; i.e. slope after maximum strength in the positive loading direction and hysteresis energy in the negative loading direction.

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