Ultimate Resistance of Unstiffened Multiplanar Tubular TT- and KK-Joints

Using a database containing test results of 58 specimens, simple mathematical expressions that explain the resistance of multiplanar tube-to-tube joints with circular hollow sections under axial brace loading are sought. The joints concerned are multiplanar TT- and KK-joints consisting of two uniplanar T- or K-joints that lie in different planes. The uniplanar joints have an identical configuration and loading of the braces. Multiple regression analysis is performed to obtain prediction equations. The proposed ultimate strength equations are shown to be in good agreement with the test results. The prediction models are based on a simple mechanical model of a ring that enables an extrapolation of the geometric variables' application ranges. The proposed ultimate strength equations are compared with existing prediction formulas. For the proposed validity ranges of the ultimate strength equations, the multiplanar coefficient , the factor by which the uniplanar joint strength has to be multiplied to obtain the multiplanar joint strength, ranges from 0.5 to 1.3, indicating that multiplanar effects are indeed significant.