In order to accurately compute the axial stiffness of bolted members

the compressive stress of each compressive section perpendicular to the bolt axis is assumed as a quartic equation of the radial size. Following this assumption

new formulas for member stiffness are derived. Finite element analysis and its interpolation algorithms are conducted to calculate the stiffness of members with different materials and geometrical sizes. The obtained member stiffness is used to inversely calculate the corresponding semi-apex angle. Moreover

the real semi-apex angle is obtained from the contact pressure data of the member interface which is measured with the I-Scan system. The experimental results show good coincidence with the analytical semi-apex angles

and the corresponding error is within 2°. On the basis of the above derived formulas for the semi-apex angle and the member stiffness

the axial stiffness is calculated for bolted members with different materials and geometrical sizes. Compared with the finite element analysis

the errors of obtained member stiffness get less than 6%

so the member stiffness can be accurately calculated with the proposed method.