Confinement of rectangular reinforced concrete column with non-seismic detailing

Abstract

In Hong Kong, reinforced concrete structures are traditionally designed without seismic provisions. In particular, one commonly used local detailing for columns allows large spacing of transverse reinforcements, use of 90º hooks and transverse reinforcements not necessary tied to the main reinforcements. By end of the last century, Hong Kong has been classified as a region with moderate seismic hazard. There is in need of assessing structural behavior of columns with local detailing when subjected to cyclic loading. Reinforced concrete columns are normally designed in a ductile manner to resist seismic load. It is necessary to assess the confinement action provided by transverse reinforcements based on local detailing. Objectives of this study are to assess the seismic resistance of and to develop mathematical models for a class of columns with local detailing. In order to achieve the above, (a) confinement action of transverse reinforcements under axial load; and (b) hysteresis behavior of columns with local detailing were examined experimentally. Confinement of columns with local detailing was assessed by conducting axial loading tests on 12 -scaled specimens. Detailing of transverse reinforcements consisted of reinforcement hoops with 90º end hooks, long crossties and short crossties. This is a common type of local detailing for columns. The test results indicated that columns with local detailing have limited confinement action with up to 80% reduction in load carrying capacity. Stress-strain behavior of confined concrete with local detailing is developed by performing statistical analysis on the test results. In particular, ascending branch of stress-strain relationship of confined concrete is similar to Propovics' model while descending branch assumes a linear relationship similar to Hoshikuma's model. Hysteretic response of columns with local detailing was investigated by conducting cyclic-loading tests on 12 0.4-scaled specimens. Test parameters include volumetric transverse reinforcement ratio, axial load ratio and type of details. Specimens with high and low volumetric transverse reinforcement ratio failed distinctively, namely by flexural-shear mechanism and shear failure respectively. A hysteresis model is proposed based on the test results to predict the cyclic behavior of columns with local detailing. A shear damage model related to the lateral displacement of column is developed. It is recommended to limit the spacing of transverse reinforcements to 160mm and to apply the local detail when axial load ratio is very small and ductility demand is less than 4.