Experimental Research on the Calculating Method of Stress Concentration Factor for CFST K-Joint

The concrete-filled steel tubular (CFST) K-joint was taken as the research object, full-scale specimen tests and finite-element (FE) analysis of seven CFST K-joints and one steel tubular (ST) K-joint were carried out. The accuracy of CFST K-joint stress concentration coefficient (SCF) calculation using the existing K-joint SCF formula was discussed. Based on the effect of core concrete on the chord deformation mechanism, the releasing coefficient of CFST K-joint SCF was proposed. According to the relationship between releasing coefficient and stripping arc length, the calculation method of releasing coefficient was established by theoretical derivation. The calculation method of CFST K-joint SCF was then established based on the releasing coefficient and ST K-joint SCF. The obtained results show that the maximum SCF of the K-joint appears in the position of the intersecting weld between tension brace and chord near the crown toe of the chord. The maximum SCFs of the CFST and ST K-joints in order are obtained as 3.98 and 6.08, which indicates that the core concrete can reduce the SCF of the CFST K-joint by about 34.5%. The existing methods for calculating the SCF of CFST and ST K-joints fail to fully consider the influence of core concrete constraints and the coupling relationship between geometric parameters; therefore, the accuracy of CFST K-joint SCF calculation using the existing K-joint SCF calculation formula is low. By employing the proposed SCF calculation method, the calculation error of the CFST K-joint SCF can be reduced from 58.0% to 19.6%.