The reduced stiffness method had been adopted to consider material nonlinearity characteristics of reinforced concrete structures in concrete structures standards in the United States and Canada. The concrete structures design code of China also accepted the reduced stiffness method as a supplementary method of considering the second-order effects problem when necessary. However, the concrete structures with specially shaped columns code of China still uses amplified coefficients of eccentricity to consider nonlinearity characteristics of reinforced concrete structures with special shape columns. The flexural stiffness reduction factor of reinforced concrete columns with specially shaped sections considering characters of material nonlinearity and geometrical nonlinearity lacks corresponding research. Based on the numerical integral method, the change law of flexural stiffness for existing test models of reinforced concrete columns with specially shaped sections under different axial load levels and different levels of seismic action is analyzed, and results are reported. It is concluded that the theoretical values are in agreement with the test values. As a result, a flexural stiffness reduction factor is proposed to consider characteristics of material nonlinearity and geometrical nonlinearity of reinforced concrete columns with equal L specially shaped sections.
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