Research On Flexural Stiffness Of Concrete-filled Square Steel Tubular Column

Concrete-filled square steel tubular column forms an innovative type of composite structure. It combines the benefits of concrete and steel together, so that the concrete in the three-state pressure improves core strength and deformation capacity of concrete. In recent years, steel concrete structure in China has developed rapidly. Especially in the last decade it has been widely used in many seismic regions. In the seismic design and calculation process, the structure of the deformation is a very important indicator. Deformation calculation is bound to involve the flexural stiffness of concrete filled steel tube. In recent decades, domestic and foreign scholars have done a lot of research on mechanical properties of concrete filled steel tube. However, most of the research dedicates to the calculation of flexural capacity. Little work focus on the bending stiffness.Currently many countries use a seismic standard which targets no damage under minor earthquake r, repairable under moderate earthquake r, no collapsing under major earthquake. This standard is based on the scale of the earthquake and cannot estimate the structural deformation and the real damage after a strong earthquake. In this context, performance-based seismic design concept has attracted international attention. Elastoplastic analysis of structures has become an indispensable part of performance-based seismic design. In elastoplastic analysis, how to determine the location of plastic hinges and to study the flexural stiffness of concrete filled steel tube is particularly important. This paper introduces the algorithm to calculate the cross section of concrete filled steel tubes bending rigidity and bending stiffness. It provides a reference to steel tube structure and performance-based seismic design.In order to study the plastic hinge deformation and flexural stiffness section on the section of concrete filled square steel tubular column, we use numerical integration methods. FORTRAN based program is used to do nonlinear analysis on section of the square steel tubular column (M-Φprogram). Existing experimental data is used to verify the program. Measured values and theoretical calculation from the problem show good agreement, indicating the program is reliable to be used for steel square tubular column cross-section stress analysis. M-Φcurve calculated from the yield corresponding to the equivalent flexural rigidity EI ff and stiffness E s I s + Ec Icof concrete filled steel tube is used as the ratio of theoretical cross-section steel tube bending stiffness reduction factor. Based on the M-Φprogram, numerical integration method is used to develop procedures to analysis load of square steel tube concrete stress (P-Δprogram). In addition, based on the program which analysis steel square tubular column procedure, numerical integration method is used to develop another program for calculating the preparation of concrete filled steel tubes bending stiffness reduction factor (m-a program).In this paper, M-Φprogram and m-a program are used to calculate and analysis three sections in the form of square steel tubular column sections and components, the four concrete strength, three kinds of steel ratio, four steel strength, four axial load levels, four slenderness ratio under the deformation properties. Result shows that the cross sections of square steel tubular column corresponding to the elastic hinge flexural rigidity is much smaller than the initial cross-section flexural stiffness. Its value is not only related to the cross-section form, materials properties, steel ratio, but also related to the axial compression ratio. Finally, data fitting gives the formula for calculating the flexural rigidity EI ff. Moreover, the study also found that the the core concrete of concrete-filled square steel tubular column has small impact on the bending stiffness of the column on the use stage. Through calculation and analysis, this paper proposed material nonlinearity and geometric nonlinearity characteristics of concrete-filled square steel tubular column flexural stiffness reduction factor.