Behavior Of Steel Reinforced High Strength Concrete Columns

Due to the full usage of high strength concrete and structural steel, as well as the excellent seismic behavior resulting form the confinement of structural steel to concrete under large axial load, steel reinforced high strength concrete (SRHC) columns have the potential to be widely used in tall buildings in seismic regions. However, the values of the limited axial load ratio proposed by existing experimental researches were too small. The maximum allowable axial load was not enhanced a lot when SRHC columns were used instead of steel reinforced normal strength concrete (SRNC) columns, because of the small values of the limited axial load ratio. Moreover, the cost of high strength concrete is much higher than normal strength concrete, so it is uneconomical to reduce the values of the limited axial load ratio to ensure the seismic ductility of SRHC columns. So, this paper aims to (1) investigate the effects of design parameters on flexural behavior, axial compressive property, and seismic behavior of SRHC columns;(2) take measures to improve the behavior of SRHC columns, and most importantly, the seismic behavior of SRHC columns subjected to large axial load;(3) develop rational design formulas for axial compression strength, flexural strength and shear strength of SRHC columns. The main contents and conclusions are summarized as follows:(1) A program is compiled to analyze moment-curvature behavior of steel reinforced concrete column sections based on the fiber element method. The effects of buckling on strength of longitudinal bars and structural steel are considered in stress-strain relations, and confinement of stirrups and structural steel to concrete is also considered. Based on this method and program, a parametric study is conducted to investigate the effects of various factors on curvature ductility of steel reinforced high strength concrete column sections. At the same time, a rational cross section is proposed based on the above research.(2) An experimental investigation on the axial load performance of steel reinforced high strength concrete short columns is carried out. The performance of a total of ten specimens was investigated, including the failure pattern, load-strain response, compressive strength and ductility. The parameters considered in this study are stirrup and structural steel arrangements. Based on the experimental research, it is found that the effect of structural steel on ductility is more obvious when effective confinement index is larger. If the proposed cross section is adopted, the axial compressive property is excellent. At last, design formulas for axial compression strength, common suggestions to improve ductility of axially loaded SRHC short columns are proposed. (3) Results from experimental study aimed at investigating the seismic behavior of SRHC columns are presented. A total of21SRHC column specimens with λ=3.0are tested up to failure under lateral cyclic load and constant axial load. The behavior of SRHC columns, including failure modes, hysteretic behavior, energy dissipation and deformation capacity, stiffness and strength degradation, are investigated. The parameters considered are axial load levels, stirrup details, structural steel arrangement and studs, and effects of the parameters on behavior of SRHC columns were studied. It is found that SRHC columns confined properly may have excellent seismic behavior with sufficient energy dissipation and deformation capacity even if under large axial load. Based on the above, rational formulas for evaluation flexural strength based on the assumption that the initial plane sections remain plane after bending are proposed.(4) A total of6SRHC short column specimens with λ=2.0are tested up to failure under lateral cyclic load and constant axial load. The behavior of SRHC short columns, including failure modes, hysteretic behavior, energy dissipation and deformation capacity, stiffness and strength degradation, are investigated. The parameters considered are axial load level and stirrup details. Based on the experimental research, it is found that SRHC short columns with proper confinement exhibits excellent seisimic behavior even if subjected to large axial load with P/fcmAg=0.38. At last, values of limited axial load ratio of SRHC columns, length of critical region for confinement stirrups and stirrups requirements for certain ductility demands in the critical region are suggested based on the ductility based seismic design.(5) An effective model based on modified compression field theory is presented for estimation shear strength and revelation shear mechanism of steel reinforced high strength concrete columns. Centroidal strain of the end cross-section is employed for considering the axial-flexure-shear interaction and effects of shear span ratio, axial force and stirrups on shear strength of columns are considered through the angle of inclination of principal stress and the average strain in longitudinal direction according to the proposed model. The proposed model is validated by comparing predictions of the model with experimental results of steel reinforced high strength concrete columns subjected to constant axial force and reversed cyclic horizontal load. The results indicate that, the proposed model estimates the shear strength reasonably, and is suitable for shear analysis and design of steel reinforced high strength concrete columns.