Current design codes, used to predict the ultimate static load of uniplanar and multiplanar X-joints in circular hollow sections, are mainly based on extensive tests on simple uniplanar joints. Very few test results on multiplanar joints are available for verification. Therefore, a test series, consisting of 3 uniplanar and 9 multiplanar X-joints in circular hollow sections, have been carried out for determination of the initial stiffness, the ultimate static load and the deformation capacity. The objective of this study is to investigate the influence of loaded and unloaded out-of-plane braces on the behaviour of statically loaded X-joints under axial load, bending in-plane or bending out-of plane. A clear multiplanar effect is observed for stiffness, strength and deformation capacity of multiplanar X-joints in comparison to uniplanar Xjoints. Finally, the test results are compared with the values of the ultimate loads obtained from several design codes and recommendations. It appeared that the AWS code, which is the only code taking into account the multiplanar effects, gives conservative values for ultimate loads for axially loaded multiplanar joints.
In offshore structures it is common practice to analyse and design multiplanar joints with braces in different planes as being uniplanar. Initial investigations (Paul (1989), van der Vegte (1989» have shown that, depending on the geometry and the loading, this may result in actual strengths which are either 30% lower, or in some cases even 100% higher than the ultimate strengths for uniplanar joints. Furthermore, the stiffness and the deformation capacity of multiplanar joints is largely influenced by multiplanar loading effects. From the existing codes, only the AWS (1988) takes the multiplanar effects into account. The AWS, as well as other codes are based on an extensive series of tests on uniplanar joints. Very few test results on multiplanar joints are available.
