Study On Seismic Performance Of Steel Reinforced Ultra High Strength Concrete Frame Connection

Recently, ultra high strength concrete (UHSC) is more and more widely used in the high-rise and long-span buildings because of its higher bearing capability and better durability. But with the increasement of concrete grade, UHSC becomes more and more brittle. The design of beam-to-column connection is the key to prevent building from collapsing in the earthquake. In China, the concrete strength grade is C15～C80 in the new applying "Code for design of concrete structures" (GB 50010-2002), it is C30～C60 in the "Technical specification for steel reinforced concrete composite structures" (JGJ 138-2001), and it is C30～C80 in the "Technical specification of steel-reinforced concrete structures"(YB9082-2006). However, now there is very little of correlative research on seismic performance of steel reinforced ultra high strength concrete frame connection, so the contradiction between engineering requirement and current code or specification becomes more and more serious. In order to solve the above-mentioned problems, the following aspects are carried out in this thesis:(1) Based on the experiments on SRUHSC column to RC beam connections under low cyclic reversed load, influences of axial load ratio, volumetric stirrup ratio and structural steel shape are analyzed on failure mode, hysteretic behavior, ductility, energy dissipation, stiffness or strength degradation and relation between load and strain, et al. The experimental results shows that SRUHSC column to RC beam connections have plumper hysteretic curves, bigger laminal area, better ductility and stronger energy dissipation compared to RC connection, and that ductility becomes poorer, energy dissipated becomes less, stiffness or strength degrades faster with an increase of axial load ratio or decrease of volumetric stirrup ratio. At the same time, the connections with encased + shaped structural steel have better ductility, stronger energy dissipation and slower stiffness or strength degradation compared to ones with encased I shaped structural steel.(2) The SRUHSC column to SRC beam connections used widely are tested under low cyclic reversed load to investigate their seismic performance, and the experimental parameters consists of axial load ratio, volumetric stirrup ratio and structural steel shape. Based on the measured curves of load to displacement, failure modes and crack development, damage process, failure mechanism, stiffness or strength degradation, ductility and energy dissipation are studied. The experimental results shows that the ductility of SRUHSC column to SRC beam connections are better and their mechanical behaviors are influenced obviously by experimental parameters. Besides, the ductility and energy dissipation capacity become poor with an increase of axial load ratio or decrease of volumetric stirrup ratio, and are improved due to encase + shaped structural steel.(3) Based on the experiments of SRUHSC column to RC beam connections and SRUHSC column to SRC beam connections, mechanical behavior and respective contribution to shear capacity are analyzed overall. Calculation methods of shear and crack resistance are presented. The results shows that ultra high strength concrete bears most part of shear capacity in the course of experiment with 88% for SRUHSC column to RC beam connections and 73% for SRUHSC column to SRC beam connections, that shear capacity is possible improved with an increase of axial load ratio and volumetric stirrup ratio, and that structure steel shape has less influence on shear capacity of connection. The ratio of calculated values to experimental values ranges from 0.91 to 1.02 for shear capacity and the ratio of calculated values to experimental values ranges from 0.89 to 1.05 for crack capacity.(4) Based on the failure mode, damage variables of steel reinforced ultra high strength concrete frame connection are determined and damage model consisted of deformation and strength degradation is presented. Damage development and relation between damage index and displacement are analyzed by damage accumulation and damage evolution in the course of loading. The results show that damage model describes failure mode of steel reinforced ultra high strength concrete frame connection under the low cyclic reversal load and evaluates damage behavior objectively. The damage development is influenced obviously by experimental parameters on the stage of medium-term and final-term loading. With a decrease of axial load ratio or an increase of volumetric stirrup ratio, the damage index becomes smaller, damage develops slower, and + shaped structural steel slows damage development of the connections.(5) The finite element analysis of steel reinforced ultra high strength concrete frame connection is carried on by ANSYS-APDL software based on the experiment. Boundary restraint and load are applied on the FE model according to the loading program. The mechanical behavior of connection is described by reasonable element and meshing under low cyclic reversal load. After FE calculation, hysteretic curves and skeletal curves are obtained, and comparison between FE analysis and experimental results is analyzed. Finally, ductility and energy dissipation capacity are studied according to displacement ductility coefficients and equivalent damping coefficients.